CADEX Batteries User Manual

Part One

Battery Basics Everyone Should Know

Author's Note

Battery user groups have asked me to write an edited version of Batteries in a Portable World.
The first edition was published in 1997. Much has changed since then.

My very first publication in book form was entitled Strengthening the Weakest Link. It was, in
part, a collection of battery articles which I had written. These articles had been published in
various trade magazines and gained the interest of many readers. This goes ba ck to the late
1980s and the material covered topics such as the memory effect of NiCd batteries and ho w
to restore them.

In the early 1990s, attention moved to the nickel-metal hydride (NiMH) and the articles
compared the classic nickel cadmium (NiCd) with the NiMH, the new kid on the block. In
terms of longevity and ruggedness, the NiMH did not perform so well when placed against the
NiCd and I was rather blunt about it. Over the years, however, the NiMH improved and today
this chemistry performs well for mobile phones and other applications.

Then came the lithium-ion (Li-ion), followed by the lithium-ion polymer (Li-ion polymer). Each
of these new systems, as introduced, claimed better performance, freedom fro m the memory
effect and longer runtimes than the dated NiCd. In many cases, the statements made by the
manufacturers about improvements were true, but not all users were convinced.

The second edition of Batteries in a Portable World has grown to more than three times the
size of the previous version. It describes the battery in a broader scope and includes the
latest technologies, such as battery quick test.

Some new articles have also been woven in and some redundancies cannot be fully avoided.
Much of this fresh material has been published in trade magazines, both in North America
and abroad.

In the battery field, there is no black and white, but many shades of gray. In fact, the battery
behaves much like a human being. It is mystical, unexplainable and can never be fully
understood. For some users, the battery causes no problems at all, for others it is nothing but
a problem. Perhaps a comparison can be made with the aspirin. For some, it works to remedy
a headache, for others the headache gets worse. And no one knows exactly why.

Batteries in a Portable World is written for the non-engineer. It addresses the use of the
battery in the hands of the general public, far removed from the protected test lab
environment of the manufacturer. Some information contained in this book was obtained
through tests performed in Cadex laboratories; other knowledge was gathered by simply
talking to diverse groups of battery users. Not all views and opinions expressed in the book
are based on scientific facts. Rather, they follow opinions of the general public, who use
batteries. Some difference of opinion with the reader cannot be avoided. I will accept the
blame for any discrepancies, if justified.

Readers of the previous edition have commented that I favor the NiCd over the NiMH.
Perhaps this observation is valid and I have taken note. Having been active in the mobile
radio industry for many years, much emphasis was placed on the longevity of a battery, a
quality that is true of the NiCd. Today’s battery has almost become a disposable item. This is

especially true in the vast mobile phone market where small size and high energy density
take precedence over longevity.

Manufacturers are very much in tune with customers’ demands and deliver on maximum
runtime and small size. These attributes are truly visible at the sales counter and catch the
eye of the vigilant buyer. What is less evident is the shorter service life. However, with rapidly
changing technology, portable equipment is often obsolete by the time the battery is worn out.
No longer do we need to pamper a battery like a Stradivarius violin that is being handed down
from generation to generation. With mobile phones, for example, upgrading to a new handset
may be cheaper than purchasing a replacement battery. Small size and reasonable runtime
are key issues that drive the consumer market today. Longevity often comes second or third.

In the industrial market such as public safety, biomedical, aviation and defense, requirements
are different. Longevity is given preference over small size. To suit particular applications,
battery manufacturers are able to adjust the amount of chemicals and active materials that go
into a cell. This fine-tuning is done on nickel-based as well as lead and lithium-based batteries.

In a nutshell, the user is given the choice of long runtime, small size or high cycle count. No
one single battery can possess all these attributes. Battery technology is truly a compromise.

Introduction

During the last few decades, rechargeable batteries have made only moderate improvement s
in terms of higher capacity and smaller size. Compared with the vast advancements in areas
such as microelectronics, the lack of progress in battery technology is apparent. Consider a
computer memory core of the sixties and compare it with a modern microchip of the same
byte count. What once measured a cubic foot now sits in a tiny chip. A comparable size
reduction would literally shrink a heavy-duty car battery to the size of a coin. Since batteries
are still based on an electrochemical process, a car battery the size of a coin may not be
possible using our current techniques.

Research has brought about a variety of battery chemistries, each offering distinct
advantages but none providing a fully satisfactory solution. With today’s increased selection,
however, better choices can be applied to suit a specific user application.

The consumer market, for example, demands high energy densities and small sizes. This is
done to maintain adequate runtime on portable devices that are becoming increasingly more
powerful and power hungry. Relentless downsizing of portable equipment has pressured
manufacturers to invent smaller batteries. This, however, must be done without sacrificing
runtimes. By packing more energy into a pack, other qualities are often compromised. One of
these is longevity.

Long service life and predictable low internal resistance are found in the NiCd family.
However, this chemistry is being replaced, where applicable, with systems that provide longer
runtimes. In addition, negative publicity about the memory phenomenon and concerns of
toxicity in disposal are causing equipment manufacturers to seek alternatives.

Once hailed as a superior battery system, the NiMH has also failed to provide the universal
battery solution for the twenty-first century. Shorter than expected service life remains a major
complaint.

The lithium-based battery may be the best choice, especially for the fast-moving commercial
market. Maintenance-free and dependable, Li-ion is the preferred choice for m any because it
offers small size and long runtime. But this battery system is not without problems. A relatively
rapid aging process, even if the battery is not in use, limits the life to between two and three

years. In addition, a current-limiting safety circuit limits the discharge current, rendering the Li­ion unsuitable for applications requiring a heavy load. The Li-ion polymer exhibits similar
characteristics to the Li-ion. No major breakthrough has been achieved with this system. It
does offer a very slim form factor but this quality is attained in exchange for slightly less
energy density.

With rapid developments in technology occurring today, battery systems that use neither
nickel, lead nor lithium may soon become viable. Fuel cells, which enable uninterrupted
operation by drawing on a continuous supply of fuel, may solve the portable energy needs in
the future. Instead of a charger, the user carries a bottle of liquid energy. Such a battery
would truly change the way we live and work.

This book addresses the most commonly used consumer and industrial batteries, which are
NiCd, NiMH, Lead Acid, and Li-ion/polymer. It also includes the reusable alkaline for
comparison. The absence of other rechargeable battery systems is done for reasons of clarity.
Some weird and wonderful new battery inventions may only live in experimental labs. Others
may be used for specialty applications, such as military and aerospace. Since this book
addresses the non-engineer, it is the author’s wish to keep the matter as simple as possible.

Chapter 1: When was the battery invented?

One of the most remarkable and novel discoveries in the last 400 years has been electricity.
One may ask, “Has electricity been around that long?” The answer is yes, and perhaps much
longer. But the practical use of electricity has only been at our disposal since the mid-to late
1800s, and in a limited way at first. At the world exposition in Paris in 1900, for example, one
of the main attractions was an electrically lit bridge over the river Seine.

The earliest method of generating electricity occurred by creating a static charge. In 1660,
Otto von Guericke constructed the first electrical machine that consisted of a large sulphur
globe which, when rubbed and turned, attracted feathers and small pieces of paper. Gu ericke
was able to prove that the sparks generated were truly electrical.

The first suggested use of static electricity was the so-called “electric pistol”. Invented by
Alessandro Volta (1745-1827), an electrical wire was placed in a jar filled with methane gas.
By sending an electrical spark through the wire, the jar would explode.

Volta then thought of using this invention to provide long distance communications, albeit only
addressing one Boolean bit. An iron wire supported by wooden poles was to be st rung from
Como to Milan, Italy. At the receiving end, the wire would terminate in a jar filled with methane
gas. On command, an electrical spark is sent by wire that would detonate the electric pistol to
signal a coded event. This communications link was never built.

Figure 1-1: Alessandro Volta, inventor of the electric battery.

Volta’s discovery of the decomposition of water by an electrical current laid the foundation of electrochemistry.
©Cadex Electronics Inc.

In 1791, while working at Bologna University, Luigi Galvani discovered that the muscle of a
frog contracted when touched by a metallic object. This phenomenon became known as
animal electricity — a misnomer, as the theory was later disproven. Prompted by these
experiments, Volta initiated a series of experiments using zinc, lead, tin or iron as positive
plates. Copper, silver, gold or graphite were used as negative plates.

Volta discovered in 1800 that a continuous flow of electrical force was generated when using
certain fluids as conductors to promote a chemical reaction between the metals or electrodes.
This led to the invention of the first voltaic cell, better know as the battery. Volta discovered
further that the voltage would increase when voltaic cells were stacked on top of each other.

Figure 1-2: Four variations of Volta’s electric battery.

Silver and zinc disks are separated with moist paper. ©Cadex Electronics Inc.

In the same year, Volta released his discovery of a continuous source of electricity to the
Royal Society of London. No longer were experiments limited to a brief display of sparks that
lasted a fraction of a second. A seemingly endless stream of electric current was now
available.

France was one of the first nations to officially recognize Volta’s discoveries. At the time,
France was approaching the height of scientific advancements and new ideas we re
welcomed with open arms to support the political agenda. By invitation, Volta addressed the
Institute of France in a series of lectures at which Napoleon Bonaparte was present as a
member of the Institute.

Figure 1-3: Volta’s experimentations at the French National Institute.

Volta’s discoveries so impressed the world that in November 1800, he was invited by the French National Institute to
lectures in which Napoleon Bonaparte participated. Later, Napoleon himself helped with the experiments, drawing
sparks from the battery, melting a steel wire, discharging an electric pistol and decomposing water into its elements.
©Cadex Electronics Inc.

New discoveries were made when Sir Humphry Davy, inventor of the miner’s safety lamp,
installed the largest and most powerful electric battery in the vaults of the Royal Institution of
London. He connected the battery to charcoal electrodes and produced the first electric light.
As reported by witnesses, his voltaic arc lamp produced “the most brilliant ascending arch of
light ever seen.”

Davy's most important investigations were devoted to electrochemistry. Following Galvani's
experiments and the discovery of the voltaic cell, interest in galvanic electricity had become
widespread. Davy began to test the chemical effects of electricity in 1800. He soon found that
by passing electrical current through some substances, these substances decomposed, a
process later called electrolysis. The generated voltage was directly related to the reactivity of
the electrolyte with the metal. Evidently, Davy understood that the actions of electrolysis and
the voltaic cell were the same.

In 1802, Dr. William Cruickshank designed the first electric battery capable of mass
production. Cruickshank had arranged square sheets of copper, which he soldered at their
ends, together with sheets of zinc of equal size. These sheets were placed into a long
rectangular wooden box that was sealed with cement. Grooves in the box held the metal
plates in position. The box was then filled with an electrolyte of brine, or watered down acid.

The third method of generating electricity was discovered relatively late — electricity through
magnetism. In 1820, André-Marie Ampère (1775-1836) had noticed that wires carrying an
electric current were at times attracted to one another while at other times they were repelled.

In 1831, Michael Faraday (1791-1867) demonstrated how a copper disc was able to provide a
constant flow of electricity when revolved in a strong magnetic field. Faraday, assisting Davy
and his research team, succeeded in generating an endless electrical force as long as the
movement between a coil and magnet continued. The electric generator was invented. This
process was then reversed and the electric motor was discovered. Shortly thereafter,

transformers were developed that could convert electricity to a desired voltage. In 1833,
Faraday established the foundation of electrochemistry with Faraday's Law, which describes
the amount of reduction that occurs in an electrolytic cell.

In 1836, John F. Daniell, an English chemist, developed an improved battery which produced
a steadier current than Volta's device. Until then, all batteries had been composed of primary
cells, meaning that they could not be recharged. In 1859, the French physicist Gaston Planté
invented the first rechargeable battery. This secondary battery was based on lead acid
chemistry, a system that is still used today.

Figure 1-4: Cruickshank and the first flooded battery.

William Cruickshank, an English chemist, built a battery of electric cells by joining zinc and copper plates in a wooden
box filled with electrolyte. This flooded design had the advantage of not drying out with use and provided more
energy than Volta’s disc arrangement. ©Cadex Electronics Inc.

Toward the end of the 1800s, giant generators and transformers were built. Transmission
lines were installed and electricity was made available to humanity to produce light, heat and
movement. In the early twentieth century, the use of electricity was further refined. The
invention of the vacuum tube enabled generating controlled signals, amplifications and sound.
Soon thereafter, radio was invented, which made wireless communication possibl e.

In 1899, Waldmar Jungner from Sweden invented the nickel-cadmium battery, which used
nickel for the positive electrode and cadmium for the negative. Two years later, Edison

produced an alternative design by replacing cadmium with iron. Due to high material costs
compared to dry cells or lead acid storage batteries, the practical applications of the nickel­cadmium and nickel-iron batteries were limited.

It was not until Shlecht and Ackermann invented the sintered pole plate in 1932 that large
improvements were achieved. These advancements were reflected in highe r load currents
and improved longevity. The sealed nickel-cadmium battery, as we know it toady, became
only available when Neumann succeeded in completely sealing the cell in 1947.

From the early days on, humanity became dependent on electricity, a product without which
our technological advancements would not have been possible. With the increased need for
mobility, people moved to portable power storage — first for wheeled applications, then for
portable and finally wearable use. As awkward and unreliable as the early batteries may have
been, our descendants may one day look at today’s technology in a similar way to how we
view our predecessors’ clumsy experiments of 100 years ago.

History of Battery Development

1600
1791
1800
1802
1820
1833
1836
1859
1868
1888
1899
1901
1932
1947
Mid 1960
Mid 1970
1990
1992
1999
2001

Gilbert (England) Establishment electrochemistry study
Galvani (Italy) Discovery of ‘animal electricity’
Volta (Italy) Invention of the voltaic cell
Cruickshank (England) First electric battery capable of mass production
Ampère (France) Electricity through magnetism
Faraday (England) Announcement of Faraday’s Law
Daniell (England) Invention of the Daniell cell
Planté (France) Invention of the lead acid battery
Leclanché (France) Invention of the Leclanché cell
Gassner (USA) Completion of the dry cell
Jungner (Sweden) Invention of the nickel-cadmium battery
Edison (USA) Invention of the nick el-iron battery
Shlecht & Ackermann (Germany) Invention of the sintered pole plate
Neumann (France) Successfully sealing the nickel-cadmium battery
Union Carbide (USA) Development of primary alkaline battery
Development of valve regulated lead acid battery
Commercialization nickel-metal hydride battery
Kordesch (Canada) Commercialization reusable alkaline battery
Commercialization lithium-ion polymer
Anticipated volume production of proton exchange membrane

fuel cell

Figure 1-5: History of battery development.

The battery may be much older. It is believed that the Parthians who ruled Baghdad (ca. 250 bc) used batteries to
electroplate silver. The Egyptians are said to have electroplated antimony onto copper over 4300 years ago.

Chapter 2: Battery Chemistries

Battery novices often argue that advanced battery systems are now available that offer very
high energy densities, deliver 1000 charge/discharge cycles and are paper thin. These
attributes are indeed achievable — unfortunately not in the same battery pack. A given
battery may be designed for small size and long runtime, but this pack would have a limited
cycle life. Another battery may be built for durability, but it would be big and bulky. A third
pack may have high energy density and long durability, but would be too expensive for the
commercial consumer.

Battery manufacturers are well aware of customer needs and have responded by offering
battery packs that best suit the specific application. The mobile phone industry is an example
of this clever adaptation. For this market, the emphasis is placed on small size and high
energy density. Longevity comes in second.

The mention of NiMH on a battery pack does not automatically guarantee high energy density.
A prismatic NiMH battery for a mobile phone, for example, is made for slim geometry and may
only have an energy density of 60Wh/kg. The cycle count for this battery would be limited to
around 300. In comparison, a cylindrical NiMH offers energy densities of 80Wh/kg and higher.
Still, the cycle count of this battery will be moderate to low. High durability NiMH batteries,
which are intended for industrial use and the electric vehicle enduring 1000 discharges to
80 percent depth-of discharge, are packaged in large cylindrical cells. The energy density on
these cells is a modest 70Wh/kg.

Similarly, Li-ion batteries for defense applications are being produced that far exceed the
energy density of the commercial equivalent. Unfortunately, these super-high capacity Li-ion
batteries are deemed unsafe in the hands of the public. Neither would the general public be
able to afford to buy them.

When energy densities and cycle life are mentioned, this book refers to a middle-of-the-road
commercial battery that offers a reasonable compromise in size, energy density, cycle life and
price. The book excludes miracle batteries that only live in controlled environments.

Chemistry Comparison

Let's examine the advantages and limitations of today’s popular battery systems. Batteries
are scrutinized not only in terms of energy density but service life, load characteristics,
maintenance requirements, self-discharge and operational costs. Since NiCd remains a
standard against which other batteries are compared, let’s evaluate alternative chemist rie s
against this classic battery type.

Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density.
The NiCd is used where long life, high discharge rate and economical price are important.
Main applications are two-way radios, biomedical equipment, professional video cameras and
power tools. The NiCd contains toxic metals and is not environmentally friendly.

Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the
expense of reduced cycle life. NiMH contains no toxic metals. Applications include mobile
phones and laptop computers.

Lead Acid — most economical for larger power applications where weight is of little concern.
The lead acid battery is the preferred choice for hospital equipment, wheelchairs, emerg en cy
lighting and UPS systems.

Lithium Ion (Li-ion) — fastest growing battery system. Li-ion is used where high-energy
density and light weight is of prime importance. The Li-ion is more expensive than other
systems and must follow strict guidelines to assure safety. Applications include notebook
computers and cellular phones.

Lithium Ion Polymer (Li-ion polymer) — a potentially lower cost version of the Li-ion. This
chemistry is similar to the Li-ion in terms of energy density. It enables very slim geometry and
allows simplified packaging. Main applications are mobile phones.

Reusable Alkaline — replaces disposable household batteries; suitable for low-power
applications. Its limited cycle life is compensated by low self-discharge, making this battery
ideal for portable entertainment devices and flashlights.

Figure 2-1 compares the characteristics of the six most commonly used rechargeable battery
systems in terms of energy density, cycle life, exercise requirements and cost. The figures are
based on average ratings of commercially available batteries at the time of publication. Exotic
batteries with above average ratings are not included.

Gravimetric Energy
Density

(Wh/kg)

Internal Resistance

(includes peripheral circuits)
in mW

Cycle Life (to 80% of

initial capacity)

Fast Charge Time
Overcharge Tolerance
Self-discharge /

Month (room temperature)
Cell Voltage (nominal)
Load Current

- peak

- best result

Operating
Temperature

only)

(discharge

Maintenance
Requirement

Typical Battery Cost

(US$, reference only)

Cost per Cycle (US$)
Commercial use since

NiCd NiMH Lead Acid Li-ion Li-ion

polymer

45-80 60-120 30-50 110-160 100-130 80 (initial)

100 to 2001
6V pack

2

1500

1h typical 2-4h 8-16h 2-4h 2-4h 2-3h
moderate low high very low low moderate

4

20%

6

1.25V

20C
1C

-40 to
60°C

30 to 60 days 60 to 90 days 3 to 6

$50
(7.2V)

11

$0.04 $0.12 $0.10 $0.14 $0.29 $0.10-0.50
1950 1990 1970 1991 1999 1992

200 to 3001
6V pack

300 to 500

30%

1.25V

5C

0.5C or lower

-20 to
60°C

$60
(7.2V)

2,3

4

6

<1001
12V pack

200 to

2

300

5% 10%

2V 3.6V 3.6V 1.5V

7

5C

0.2C

-20 to
60°C

months
$25

(6V)

150 to 2501

7.2V pack

500 to 10003300 to

5

>2C
1C or lower

-20 to
60°C

not req. not req. not req.

9

$100
(7.2V)

200 to 3001

7.2V pack

500

5

~10%

>2C
1C or lower

0 to
60°C

$100
(7.2V)

Reusable
Alkaline

200 to 20001
6V pack

503
(to 50%)

0.3%

0.5C

0.2C or lower
0 to

65°C

$5
(9V)

Figure 2-1: Characteristics of commonly used rechargeable batteries.

The figures are based on average ratings of batteries available commercially at the time of publication; experimental
batteries with above average ratings are not included.

1. Internal resistance of a battery pack varies with cell rating, type of protection circuit
and number of cells. Protection circuit of Li-ion and Li-polymer adds about 100mW.

2. Cycle life is based on battery receiving regular maintenance. Failing to apply periodic
full discharge cycles may reduce the cycle life by a factor of three.

3. Cycle life is based on the depth of discharge. Shallow discharges provide more
cycles than deep discharges.

4. The discharge is highest immediately after charge, then tapers off. The NiCd capacity
decreases 10% in the first 24h, then declines to about 10% every 30 days thereafter.
Self-discharge increases with higher temperature.

5. Internal protection circuits typically consume 3% of the stored energy per month.

6. 1.25V is the open cell voltage. 1.2V is the commonly used value. There is no
difference between the cells; it is simply a method of rating.

7. Capable of high current pulses.

8. Applies to discharge only; charge temperature range is more confined.

9. Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.

10. Cost of battery for commercially available portable devices.

11. Derived from the battery price divided by cycle life. Does not include the cost of
electricity and chargers.

Observation: It is interesting to note that NiCd has the shortest charge time, delivers the
highest load current and offers the lowest overall cost-per-cycle, but has the most demanding
maintenance requirements.

The Nickel Cadmium (NiCd) Battery

Alkaline nickel battery technology originated in 1899, when Waldmar Jungner in vented the
NiCd battery. The materials were expensive compared to other battery types available at the
time and its use was limited to special applications. In 1932, the active materials were
deposited inside a porous nickel-plated electrode and in 1947, research began on a sealed
NiCd battery, which recombined the internal gases generated during charge rather than
venting them. These advances led to the modern sealed NiCd battery, which is in use today.

The NiCd prefers fast charge to slow charge and pulse charge to DC charge. All other
chemistries prefer a shallow discharge and moderate load currents. The Ni Cd is a strong and
silent worker; hard labor poses no problem. In fact, the NiCd is the only battery type that
performs best under rigorous working conditions. It does not like to be pampered by sitting in
chargers for days and being used only occasionally for brief periods. A periodic full discharge
is so important that, if omitted, large crystals will form on the cell plates (also referred to as
'memory') and the NiCd will gradually lose its performance.

Among rechargeable batteries, NiCd remains a popular choice for applications such as two­way radios, emergency medical equipment, professional video cameras and power tools.
Over 50 percent of all rechargeable batteries for portable equipment are NiCd. However, the
introduction of batteries with higher energy densities and less toxic metals is causing a
diversion from NiCd to newer technologies.

Advantages

Advantages and Limitations of NiCd Batteries

Fast and simple charge — even after prolonged storage.

High number of charge/discharge cycles — if properly maintained,
the NiCd provides over 1000 charge/discharge cycles.

Good load performance — the NiCd allows recharging at low
temperatures.

Long shelf life – in any state-of-charge.

Simple storage and transportation — most airfreight companies
accept the NiCd without special conditions.

Good low temperature performance.

Forgiving if abused — the NiCd is one of the most rugged
rechargeable batteries.

Economically priced — the NiCd is the lowest cost battery in terms of
cost per cycle.

Available in a wide range of sizes and performance options — most
NiCd cells are cylindrical.

Limitations

Relatively low energy density — compared with newer systems.

Memory effect — the NiCd must periodically be exercised to prevent
memory.

Environmentally unfriendly — the NiCd contains toxic metals. Some
countries are limiting the use of the NiCd battery.

Has relatively high self-discharge — needs recharging after storage.

Figure 2-2: Advantages and limitations of NiCd batteries.

The Nickel-Metal Hydride (NiMH) Battery

Research of the NiMH system started in the 1970s as a means of discovering how to store
hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used
for satellite applications. They are bulky, contain high-pressure steel canisters and cost
thousands of dollars each.

In the early experimental days of the NiMH battery, the metal hydride alloys were unstable in
the cell environment and the desired performance characteristics could not be achieved. As a
result, the development of the NiMH slowed down. New hydride alloys were developed in the
1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily
improved, mainly in terms of energy density.

The success of the NiMH has been driven by its high energy density and the use of
environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy
density compared to NiCd. There is potential for yet higher capacities, but not without some
negative side effects.

Both NiMH and NiCd are affected by high self-discharge. The NiCd loses about 10 percent of
its capacity within the first 24 hours, after which the self-discharge settles to about 10 percent
per month. The self-discharge of the NiMH is about one-and-a-half to two times greater
compared to NiCd. Selection of hydride materials that improve hydrogen bonding and reduce
corrosion of the alloy constituents reduces the rate of self-discharge, but at the cost of lower
energy density.

The NiMH has been replacing the NiCd in markets such as wireless communications and
mobile computing. In many parts of the world, the buyer is encouraged to use NiMH rather
than NiCd batteries. This is due to environmental concerns about careless disposal of the
spent battery.

The question is often asked, “Has NiMH improved over the last few years?” Experts agree
that considerable improvements have been achieved, but the limitations remain. Most of the
shortcomings are native to the nickel-based technology and are shared with the NiCd battery.
It is widely accepted that NiMH is an interim step to lithium battery technology.

Initially more expensive than the NiCd, the price of the NiMH has dropped and is now almost
at par value. This was made possible with high volume production. With a lower demand for
NiCd, there will be a tendency for the price to increase.

Advantages

Limitations

Advantages and Limitations of NiMH Batteries

30 – 40 percent higher capacity over a standard NiCd. The NiMH has
potential for yet higher energy densities.

Less prone to memory than the NiCd. Periodic exercise cycles are
required less often.

Simple storage and transportation — transportation conditions are
not subject to regulatory control.

Environmentally friendly — contains only mild toxins; profitable for
recycling.

Limited service life — if repeatedly deep cycled, especially at high
load currents, the performance starts to deteriorate after 200 to 300
cycles. Shallow rather than deep discharge cycles are preferred.

Limited discharge current — although a NiMH battery is capable of
delivering high discharge currents, repeated discharges with high
load currents reduces the battery’s cycle life. Best results are
achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of
the rated capacity).

More complex charge algorithm needed — the NiMH generates more
heat during charge and requires a longer charge time than the NiCd.
The trickle charge is critical and must be controlled carefully.

High self-discharge — the NiMH has about 50 percent higher self­discharge compared to the NiCd. New chemical additives improve
the self-discharge but at the expense of lower energy density.

Performance degrades if stored at elevated temperatures — the
NiMH should be stored in a cool place and at a state-of-charge of
about 40 percent.

High maintenance — battery requires regular full discharge to
prevent crystalline formation.

About 20 percent more expensive than NiCd — NiMH batteries
designed for high current draw are more expensive than the regular
version.

Figure 2-3: Advantages and limitations of NiMH batteries

The Lead Acid Battery

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable
battery for commercial use. Today, the flooded lead acid battery is used in automobiles,
forklifts and large uninterruptible power supply (UPS) systems.

During the mid 1970s, researchers developed a maintenance-free lead acid

battery, which could operate in any position. The liquid electrolyte was
transformed into moistened separators and the enclosure was sealed. Safety valves were
added to allow venting of gas during charge and discharge.

Driven by diverse applications, two designations of batteries emerged. They are the sealed
lead acid (SLA), also known under the brand name of Gelcell, and the valve regulated lead
acid (VRLA). Technically, both batteries are the same. No scientific definition exists as to
when an SLA becomes a VRLA. (Engineers may argue that the word ‘sealed lead acid’ is a
misnomer because no lead acid battery can be totally sealed. In essence, all are valve
regulated.)

The SLA has a typical capacity range of 0.2Ah to 30Ah and powers portable and wheeled
applications. Typical uses are personal UPS units for PC backup, small emergency lighting
units, ventilators for health care patients and wheelchairs. Because of low cost, dependable
service and minimal maintenance requirements, the SLA battery is the preferred choice for
biomedical and health care instruments in hospitals and retirement homes.

The VRLA battery is generally used for stationary applications. Their capacities range from
30Ah to several thousand Ah and are found in larger UPS systems for power backup. Typical
uses are mobile phone repeaters, cable distribution centers, Internet hubs and utilities, as well
as power backup for banks, hospitals, airports and military installations.

Unlike the flooded lead acid battery, both the SLA and VRLA are designed with a low over­voltage potential to prohibit the battery from reaching its gas-generating potential during
charge. Excess charging would cause gassing and water depletion. Consequently, the SLA
and VRLA can never be charged to their full potential.

Among modern rechargeable batteries, the lead acid battery family has the lowest energy
density. For the purpose of analysis, we use the term ‘sealed lead acid’ to describe the lead
acid batteries for portable use and ‘valve regulated lead acid’ for stationary application s.
Because of our focus on portable batteries, we focus mainly on the SLA.

The SLA is not subject to memory. Leaving the battery on float charge for a prolonged time
does not cause damage. The battery’s charge retention is best among recharge able batteries.
Whereas the NiCd self-discharges approximately 40 percent of its stored energy in three
months, the SLA self-discharges the same amount in one year. The SLA is relatively
inexpensive to purchase but the operational costs can be more expensive than the NiCd if full
cycles are required on a repetitive basis.

The SLA does not lend itself to fast charging — typical charge times are 8 to 16 hours. The
SLA must always be stored in a charged state. Leaving the battery in a discharged condition
causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.

Unlike the NiCd, the SLA does not like deep cycling. A full discharge causes extra strain and
each discharge/charge cycle robs the battery of a small amount of capacity. This loss is very
small while the battery is in good operating condition, but becomes more acute once the
performance drops below 80 percent of its nominal capacity. This wear-down characteristic
also applies to other battery chemistries in varying degrees. To prevent the battery from being
stressed through repetitive deep discharge, a larger SLA battery is recommended.

Depending on the depth of discharge and operating temperature, the SLA provides 200 to
300 discharge/charge cycles. The primary reason for its relatively short cycle life is grid
corrosion of the positive electrode, depletion of the active material and expansion of the
positive plates. These changes are most prevalent at higher operating temperatures. Applying
charge/discharge cycles does not prevent or reverse the trend.

There are some methods that improve the performance and prolong the life of the SLA. The
optimum operating temperature for a VRLA battery is 25°C (77°F). As a rule of thumb, every
8°C (15°F) rise in temperature will cut the battery life in half. VRLA that would last for
10 years at 25°C would only be good for 5 years if operated at 33°C (95°F). The same battery
would endure a little more than one year at a temperature of 42°C (107°F).

Advantages and Limitations of Lead Acid Batteries

Advantages

Inexpensive and simple to manufacture — in terms of cost per watt
hours, the SLA is the least expensive.

Mature, reliable and well-understood technology — when used
correctly, the SLA is durable and provides dependable service.

Low self-discharge —the self-discharge rate is among the lowest in
rechargeable batterysystems.

Low maintenance requirements — no memory; no electrolyte to fill.

Capable of high discharge rates.

Limitations

Cannot be stored in a discharged condition.

Low energy density — poor weight-to-energy density limits use to
stationary and wheeled applications.

Allows only a limited number of full discharge cycles — well suited for
standby applications that require only occasional deep discharges.

Environmentally unfriendly — the electrolyte and the lead content can
cause environmental damage.

Transportation restrictions on flooded lead acid — there are
environmental concerns regarding spillage in case of an accident.

Thermal runaway can occur with improper charging.

Figure 2-4: Advantages and limitations of lead acid batteries.

The SLA has a relatively low energy density compared with other rechargeable batteries,
making it unsuitable for handheld devices that demand compact size. In addition,
performance at low temperatures is greatly reduced.

The SLA is rated at a 5-hour discharge or 0.2C. Some batteries are even rated at a slow
20 hour discharge. Longer discharge times produce higher capacity readings. The SLA
performs well on high pulse currents. During these pulses, discharge rates well in excess of
1C can be drawn.

In terms of disposal, the SLA is less harmful than the NiCd battery but the high lead content
makes the SLA environmentally unfriendly. Ninety percent of lead acid-based batteries are
being recycled.

The Lithium Ion Battery

Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the
early 1970s that the first non-rechargeable lithium batteries became commercially available.
Attempts to develop rechargeable lithium batteries followed in the 1980s, but failed due to
safety problems.

Lithium is the lightest of all metals, has the greatest electrochemical potential and provides
the largest energy density per weight. Rechargeable batteries using lithium metal anodes
(negative electrodes) are capable of providing both high voltage and excellent capacity,
resulting in an extraordinary high energy density.

After much research on rechargeable lithium batteries during the 1980s, it was found that
cycling causes changes on the lithium electrode. These transformations, which are part of
normal wear and tear, reduce the thermal stability, causing potential thermal
runaway conditions. When this occurs, the cell temperature quickly approaches the melting
point of lithium, resulting in a violent reaction called ‘venting with flame’. A large quantity of
rechargeable lithium batteries sent to Japan had to be recalled in 1991 after a battery in a
mobile phone released flaming gases and inflicted burns to a person’s face.

Because of the inherent instability of lithium metal, especially during charging, research
shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy
density than lithium metal, the Li-ion is safe, provided certain precautions are met when
charging and discharging. In 1991, the Sony Corporation commercialized the first Li-ion
battery. Other manufacturers followed suit. Today, the Li-ion is the fastest growing and most
promising battery chemistry.

The energy density of the Li-ion is typically twice that of the standard NiCd. Improvements in
electrode active materials have the potential of increasing the energy density close to three
times that of the NiCd. In addition to high capacity, the load characteristics are reasonably
good and behave similarly to the NiCd in terms of discharge characteristics (similar shape of
discharge profile, but different voltage). The flat discharge curve offers effective utilization of
the stored power in a desirable voltage spectrum.

The Li-ion is a low maintenance battery, an advantage that most other chemistries cannot
claim. There is no memory and no scheduled cycling is required to prolong the battery’s life.
In addition, the self-discharge is less than half compared to NiCd and NiMH, making the Li-ion
well suited for modern fuel gauge applications.

The high cell voltage of Li-ion allows the manufacture of battery packs consisting of only one
cell. Many of today’s mobile phones run on a single cell, an advantage that simplifies battery
design. Supply voltages of electronic applications have been heading lower, which in turn
requires fewer cells per battery pack. To maintain the same power, however, higher currents
are needed. This emphasizes the importance of very low cell resistance to allow unrestricted
flow of current.

Chemistry variations — During recent years, several types of Li-ion batteries have emerged
with only one thing in common — the catchword 'lithium'. Although strikingly similar on the
outside, lithium-based batteries can vary widely. This book addresses the lithium-based
batteries that are predominantly used in commercial products.

Sony’s original version of the Li-ion used coke, a product of coal, as the negative electrode.
Since 1997, most Li-ions (including Sony’s) have shifted to graphite. This electrode provides a
flatter discharge voltage curve than coke and offers a sharp knee bend at the end of
discharge (see Figure 2-5). As a result, the graphite system delivers the stored energy by only
having to discharge to 3.0V/cell, whereas the coke version must be discharged to 2.5V to get
similar runtime. In addition, the graphite version is capable of delivering a higher discharge
current and remains cooler during charge and discharge than the coke version.

For the positive electrode, two distinct chemistries have emerged. They are cobalt and
spinel (also known as manganese). Whereas cobalt has been in use longer, spinel is
inherently safer and more forgiving if abused. Small prismatic spinel packs for mobile phones
may only include a thermal fuse and temperature sensor. In addition to cost savings on a
simplified protection circuit, the raw material cost for spinel is lower than that of cobalt.

Figure 2-5: Li-ion discharge characteristics.

The graphite Li-ion only needs to discharge to 3.0V/cell, whereas the coke version must be discharged to 2.5V/cell to
achieve similar performance.

As a trade-off, spinel offers a slightly lower energy density, suffers capacity loss at
temperatures above 40°C and ages quicker than cobalt. Figure 2-6 compares the advantages
and disadvantages of the two chemistries.

Energy density

(Wh/kg)

Safety

Temperature

Aging

Life Expectancy
Cost

Cobalt Manganese (Spinel)

1

140

On overcharge, the cobalt electrode provides
extra lithium, which can form into metallic lithium,
causing a potential safety risk if not protected by
a safety circuit.

Wide temperature range. Best suited for
operation at elevated temperature.

Short-term storage possible. Impedance
increases with age. Newer versions offer longer
storage.

300 cycles, 50% capacity at 500 cycles. May be shorter than cobalt.
Raw material relatively high; protection circuit

adds to costs.

1

120

On overcharge, the manganese electrode runs
out of lithium causing the cell only to get warm.
Safety circuits can be eliminated for small 1 and
2 cell packs.

Capacity loss above +40°C. Not as durable at
higher temperatures.

Slightly less than cobalt. Impedance changes
little over the life of the cell. Due to continuous
improvements, storage time is difficult to predict.

Raw material 30% lower than cobalt. Cost
advantage on simplified protection circuit.

Figure 2-6: Comparison of cobalt and manganese as positive electrodes.

Manganese is inherently safer and more forgiving if abused but offers a slightly lower energy density. Manganese
suffers capacity loss at temperature above 40°C and ages quicker than cobalt.

Based on present generation 18650 cells. The energy density tends to be lower for prismatic
cells.

The choice of metals, chemicals and additives help balance the critical trade-off between high
energy density, long storage time, extended cycle life and safety. High energy densities can
be achieved with relative ease. For example, adding more nickel in lieu of cobalt increases
the ampere/hours rating and lowers the manufacturing cost but makes the cell less safe.
While a start-up company may focus on high energy density to gain quick market acceptance,
safety, cycle life and storage capabilities may be compromised. Reputable manufacturers,
such as Sony, Panasonic, Sanyo, Moli Energy and Polystor place high importance on safety.
Regulatory authorities assure that only safe batteries are sold to the public.

Li-ion cells cause less harm when disposed of than lead or cadmium-b ased batteries. Among
the Li-ion family, the spinel is the friendliest in terms of disposal.

Despite its overall advantages, Li-ion also has its drawbacks. It is fragile and req uires a
protection circuit to maintain safe operation. Built into each pack, the protection circuit limits
the peak voltage of each cell during charge and prevents the cell voltage from dropping too
low on discharge. In addition, the maximum charge and discharge current is limited and the
cell temperature is monitored to prevent temperature extremes. With these precautions in
place, the possibility of metallic lithium plating occurring due to overcharge is virtually
eliminated.

Aging is a concern with most Li-ion batteries. For unknown reasons, battery manufacturers
are silent about this issue. Some capacity deterioration is noticeable after one year, whether
the battery is in use or not. Over two or perhaps three years, the battery frequently fails. It
should be mentioned that other chemistries also have age-related degenerative e ffects. This
is especially true for the NiMH if exposed to high ambient temperatures.

Storing the battery in a cool place slows down the aging process of the Li-ion (and other
chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the
battery should only be partially charged when in storage.

Extended storage is not recommended for Li-ion batteries. Instead, packs should be rotated.
The buyer should be aware of the manufacturing date when purchasing a replacement Li-ion

battery. Unfortunately, this information is often encoded in an encrypted serial number and is
only available to the manufacturer.

Manufacturers are constantly improving the chemistry of the Li-ion battery. Every six months,
a new and enhanced chemical combination is tried. With such rapid progress, it becomes
difficult to assess how well the revised battery ages and how it performs after long-term
storage.

Cost analysis — The most economical lithium-based battery in terms of cost-to-energy ratio
is a pack using the cylindrical 18650 cell. This battery is somewhat bulky but suitable for
portable applications such as mobile computing. If a slimmer pack is required (thinner than
18 mm), the prismatic Li-ion cell is the best choice. There is little or no gain in energy density
per weight and size over the 18650, however the cost is more than double.

If an ultra-slim geometry is needed (less than 4 mm), the best choice is Li-ion polymer. This is
the most expensive option in terms of energy cost. The Li-ion polymer does not offer
appreciable energy gains over conventional Li-ion systems, nor does it match the durability of
the 18560 cell.

Advantages and Limitations of Li-ion Batteries

Advantages

High energy density — potential for yet higher capacities.

Limitations

Relatively low self-discharge — self-discharge is less than half that of
NiCd and NiMH.

Low Maintenance — no periodic discharge is needed; no memory.
Requires protection circuit — protection circuit limits voltage and

current. Battery is safe if not provoked.

Subject to aging, even if not in use — storing the battery in a cool
place and at 40 percent state-of-charge reduces the aging effect.

Moderate discharge current.

Subject to transportation regulations — shipment of larger quantities
of Li-ion batteries may be subject to regulatory control. This
restriction does not apply to personal carry-on batteries.

Expensive to manufacture — about 40 percent higher in cost than
NiCd. Better manufacturing techniques and replacement of rare
metals with lower cost alternatives will likely reduce the price.

Not fully mature — changes in metal and chemical combinations
affect battery test results, especially with some quick test methods.

Figure 2-7: Advantages and limitations of Li-ion batteries.

Caution: Li-ion batteries have a high energy density. Exercise precaution when handling and

testing. Do not short circuit, overcharge, crush, drop, mutilate, penetrate, apply reverse
polarity, expose to high temperature or disassemble. Only use the Li-ion battery with the
designated protection circuit. High case temperature resulting from abuse of the cell could
cause physical injury. The electrolyte is highly flammable. Rupture may cause venting with
flame.

The Lithium Polymer Battery

The Li-polymer differentiates itself from other battery systems in the type of electrolyte used.
The original design, dating back to the 1970s, uses a dry solid polymer electrolyte only. This
electrolyte resembles a plastic-like film that does not conduct electricity but allows an
exchange of ions (electrically charged atoms or groups of atoms). The polymer electrolyte
replaces the traditional porous separator, which is soaked with electrolyte.

The dry polymer design offers simplifications with respect to fabrication, ruggedness, safety
and thin-profile geometry. There is no danger of flammability because no liquid or gelled
electrolyte is used.

With a cell thickness measuring as little as one millimeter (0.039 inches), equipment
designers are left to their own imagination in terms of form, shape and size. It is possible to
create designs which form part of a protective housing, are in the shape of a mat that can be
rolled up, or are even embedded into a carrying case or piece of clothing. Such innovative
batteries are still a few years away, especially for the commercial market.

Unfortunately, the dry Li-polymer suffers from poor conductivity. Internal resistance is too high
and cannot deliver the current bursts needed for modern communication devices and
spinning up the hard drives of mobile computing equipment. Although heating the cell to 60°C
(140°F) and higher increases the conductivity to acceptable levels, this requirement is
unsuitable in commercial applications.

Research is continuing to develop a dry solid Li-polymer battery that performs at room
temperature. A dry solid Li-polymer version is expected to be commercially available by 2005.
It is expected to be very stable; would run 1000 full cycles and would have higher energy
densities than today’s Li-ion battery.

In the meantime, some Li-polymers are used as standby batteries in hot climates. One
manufacturer has added heating elements that keeps the battery in the conductive
temperature range at all times. Such a battery performs well for the application intended
because high ambient temperatures do not affect the service life of this battery in the same
way it does the VRLA, for example.

To make a small Li-polymer battery conductive, some gelled electrolyte has been added.
Most of the commercial Li-polymer batteries used today for mobile phones are a hybrid and
contain gelled electrolyte. The correct term for this system is ‘Lithium Ion Polymer’. For
promotional reasons, most battery manufacturers mark the battery simply as Li-polymer.
Since the hybrid lithium polymer is the only functioning polymer battery for portable use today,
we will focus on this chemistry.

With gelled electrolyte added, what then is the difference between Li-ion and Li-ion polymer?
Although the characteristics and performance of the two systems are very similar, the Li-ion
polymer is unique in that it uses a solid electrolyte, replacing the porous separator. The gelled
electrolyte is simply added to enhance ion conductivity.

Technical difficulties and delays in volume manufacturing have deferred the introduction of
the Li-ion polymer battery. This postponement, as some critics argue, is due to ‘cashing in’ on
the Li-ion battery. Manufacturers have invested heavily in research, development and
equipment to mass-produce the Li-ion. Now businesses and sharehold ers want to see a
return on their investment.

In addition, the promised superiority of the Li-ion polymer has not yet been realized. No
improvements in capacity gains have been achieved — in fact, the capacity is slightly less

than that of the standard Li-ion battery. For the present, there is no cost advantage in using
the Li-ion polymer battery. The thin profile has, however, compelled mobile phone
manufacturers to use this promising technology for their new generation handsets.

One of the advantages of the Li-ion polymer, however, is simpler packaging because the
electrodes can easily be stacked. Foil packaging, similar to that used in the food industry, is
being used. No defined norm in cell size has been established by the industry.

Advantages and Limitations of Li-ion Polymer Batteries

Advantages

Very low profile — batteries that resemble the profile of a credit card
are feasible.

Flexible form factor — manufacturers are not bound by standard cell
formats. With high volume, any reasonable size can be produced
economically.

Light weight – gelled rather than liquid electrolytes enable simplified
packaging, in some cases eliminating the metal shell.

Improved safety — more resistant to overcharge; less chance for
electrolyte leakage.

Limitations

Lower energy density and decreased cycle count compared to Li-ion
— potential for improvements exist.

Expensive to manufacture — once mass-produced, the Li-ion
polymer has the potential for lower cost. Reduced control circuit
offsets higher manufacturing costs.

Figure 2-8: Advantages and limitations of Li-ion polymer batteries.

Reusable Alkaline Batteries

The idea of recharging alkaline batteries is not new. Although not endorsed by manufacturers,
ordinary alkaline batteries have been recharged in households for many years. Recharging
these batteries is only effective, however, if the cells have been discharged to less than
50 percent of their total capacity. The number of recharges depends solely on the depth of
discharge and is limited to a few at best. With each recharge, less capacity can be reclaim ed.
There is a cautionary advisory, however: charging ordinary alkaline batteries may generate
hydrogen gas, which can lead to explosion. It is therefore not prudent to charge ordinary
alkaline unsupervised.

In comparison, the reusable alkaline is designed for repeated recharge. It too loses charge
acceptance with each recharge. The longevity of the reusable alkaline is a direct function of
the depth of discharge; the deeper the discharge, the fewer cycles the battery can endure.

Tests performed by Cadex on ‘AA’ reusable alkaline cells showed a very high capacity
reading on the first discharge. In fact, the energy density was similar to that of a NiMH battery.
When the battery was discharged, then later recharged using the manufa cturer’s charger, the
reusable alkaline settled at 60 percent, a capacity slightly below that of a NiCd. Repeat
cycling in the same manner resulted in a fractional capacity loss with each cycle. In our tests,

the discharge current was adjusted to 200mA (0.2 C-rate, or one fifth of the rated capacity);
the end-of-discharge threshold was set to 1V/cell.

An additional limitation of the reusable alkaline system is its low load current capability of
400mA (lower than 400mA provides better results). Although adequate for portable AM/FM
radios, CD players, tape players and flashlights, 400mA is insufficient to power most mobile
phones and video cameras.

The reusable alkaline is inexpensive but the cost per cycle is high when compared to the
nickel-based rechargeables. Whereas the NiCd checks in at $0.04 per cycle based on
1500 cycles, the reusable alkaline costs $0.50 based on 10 full discharge cycles. For many
applications, this seemingly high cost is still economical when compared to the non-reusable
alkaline that has a one-time use. If the reusable alkaline battery is only partially discharged
before recharge, an improved cycle life is possible. At 50 percent depth of discharge,
50 cycles can be expected.

To compare the operating cost between the standard and reusable alkali ne, a study was done
on flashlight batteries for hospital use. The low-intensity care unit using the flashlights only
occasionally achieved measurable savings by employing the reusable alkaline. The high­intensity unit that used the flashlights constantly, on the other hand, did not attain the same
result. Deeper discharge and more frequent recharge reduced their se rvice life and offset any
cost advantage over the standard alkaline battery.

In summary, the standard alkaline offers maximum energy density whereas the reusable
alkaline provides the benefit of allowing some recharging. The compromise of the reu sable
alkaline is loss of charge acceptance after the first recharge.

Advantages and Limitations of Reusable Alkaline Batteries

Advantages

Inexpensive and readily available — can be used as a direct
replacement of non-rechargeable (primary) cells.

More economical than non-rechargeable – allows several recharges.

Low self-discharge — can be stored as a standby battery for up to
10 years.

Environmentally friendly — no toxic metals used, fewer batteries are
discarded, reduces landfill.

Maintenance free — no need for cycling; no memory.

Limitations

Limited current handling — suited for light-duty applications like
portable home entertainment, flashlights.

Limited cycle life — for best results, recharge before the battery gets
too low.

Figure 2-9: Advantages and limitations of reusable alkaline batteries.

The Supercapacitor

The supercapacitor resembles a regular capacitor with the exception that it offers very high
capacitance in a small size. Energy storage is by means of static charge. Applying a voltage
differential on the positive and negative plates charges the supercapacitor. This concept is
similar to an electrical charge that builds up when walking on a carpet. Touching an object at
ground potential releases the energy. The supercapacitor concept has been around for a
number of years and has found many niche applications.

Whereas a regular capacitor consists of conductive foils and a dry separator, the
supercapacitor is a cross between a capacitor and an electro-chemical battery. It uses special
electrodes and some electrolyte. There are three kinds of electrode materials suitable for the
supercapacitor, namely: high surface area activated carbons, metal oxide and conducting
polymers. The one using high surface area activated carbons is the most economical to
manufacture. This system is also called Double Layer Capacitor (DLC) because the energy is
stored in the double layer formed near the carbon electrode surface.

The electrolyte may be aqueous or organic. The aqueous electrolyte offers low internal
resistance but limits the voltage to one volt. In contrast, the organic electrolyte allows two and
three volts of charge, but the internal resistance is higher.

To make the supercapacitor practical for use in electronic circuits, higher voltages are needed.
Connecting the cells in series accomplishes this task. If more than three or four capacitors are
connected in series, voltage balancing must be used to prevent any cell from reaching over­voltage.
The amount of energy a capacitor can hold is measured in microfarads or µF. (1µF =

0.000,001 farad). Small capacitors are measured in nanofarads (1000 times smaller than 1µF)
and picofarads (1 million times smaller than 1µF). Supercapacitors are rated in units of 1
farad and higher. The gravimetric energy density is 1 to 10Wh/kg. This energy density is high
in comparison to the electrolytic capacitor but lower than batteries. A relatively low internal
resistance offers good conductivity.

The supercapacitor provides the energy of approximately one tenth that of the NiMH battery.
Whereas the electro-chemical battery delivers a fairly steady voltage in the usable energy
spectrum, the voltage of the supercapacitor is linear and drops from full voltage to zero volts
without the customary flat voltage curve characterized by most chemical batteries. Because of
this linear discharge, the supercapacitor is unable to deliver the full charge. The percentage of
charge that is available depends on the voltage requirements of the application.

If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the
supercapacitor reaches that threshold within the first quarter of the discharge time. The
remaining energy slips into an unusable voltage range. A DC-to-DC converter can be used to
increase the voltage range but this option adds costs and introduces inefficiencies of 10 to 15
percent.

The most common supercapacitor applications are memory backup and standby power. In
some special applications, the supercapacitor can be used as a direct replacement of the
electrochemical battery. Additional uses are filtering and smoothing of pulsed load currents.
A supercapacitor can, for example, improve the current handling of a battery. During low load
current, the battery charges the supercapacitor. The stored energy then kicks in when a high
load current is requested. This enhances the battery's performance, prolongs the runtime and
even extends the longevity of the battery. The supercapacitor will find a ready market for
portable fuel cells to compensate for the sluggish performance of some systems and enh ance
peak performance.

If used as a battery enhancer, the supercapacitor can be placed inside the portable
equipment or across the positive and negative terminals in the battery pack. If put into the

equipment, provision must be made to limit the high influx of current when the equipment is
turned on.

Low impedance supercapacitors can be charged in seconds. The charge characteristics are
similar to those of an electro-chemical battery. The initial charge is fairly rapid; the topping
charge takes some extra time. In terms of charging method, the supercapacitor resembles the
lead acid cell. Full charge takes place when a set voltage limit is reached. Unlike the electro­chemical battery, the supercapacitor does not require a full-charge detection circuit.
Supercapacitors can also be trickle charged.

Limitations Unable to use the full energy spectrum - depending on the application, not all
energy is available. Low energy density - typically holds one-fifth to one-tenth the energy of
an electrochemical battery. Cells have low voltages - serial connections are needed to obtain
higher voltages. Voltage balancing is required if more than three capacitors are connected in
series. High self-discharge - the self-discharge is considerably higher than that of an
electrochemical battery.

Advantages and Limitations of Supercapacitors

Advantages

Virtually unlimited cycle life - not subject to the wear and aging
experienced by the electrochemical battery.

Low impedance - enhances pulse current handling by paralleling with
an electrochemical battery.

Rapid charging - low-impedance supercapacitors charge in seconds.

Simple charge methods - voltage-limiting circuit compensates for self­discharge; no full-charge detection circuit needed.

Cost-effective energy storage - lower energy density is compensated
by a very high cycle count.

Limitations

Unable to use the full energy spectrum - depending on the
application, not all energy is available.

Low energy density - typically holds one-fifth to one-tenth the energy
of an electrochemical battery.

Cells have low voltages - serial connections are needed to obtain
higher voltages.

Voltage balancing is required if more than three capacitors are
connected in series.

High self-discharge - the self-discharge is considerably higher than
that of an electrochemical battery.

Figure 2-10: Advantages and limitations of supercapacitors.

By nature, the voltage limiting circuit compensates for the self-discharge. The supercapacitor
can be recharged and discharged virtually an unlimited number of times. Unlike the
electrochemical battery, there is very little wear and tear induced by cycling.

The self-discharge of the supercapacitor is substantially higher than that of the electro­chemical battery. Typically, the voltage of the supercapacitor with an organic electrolyte drops
from full charge to the 30 percent level in as little as 10 hours.

Other supercapacitors can retain the charged energy longer. With these designs, the capacity
drops from full charge to 85 percent in 10 days. In 30 days, the voltage drops to roughly 65
percent and to 40 percent after 60 days.

Chapter 3: The Battery Pack

In the 1700 and 1800s, cells were encased in glass jars. Later, larger batteries were
developed that used wooden containers. The inside was treated with a sealant to prevent
electrolyte leakage. With the need for portability, the cylindrical cell appeared. After World
War II, these cells became the standard format for smaller, rechargeable batteries.

Downsizing required smaller and more compact cell design. The button cell, which
gained popularity in the 1980s, was a first attempt to achieve a reasonably flat
geometry, or obtain higher voltages in a compact profile by stacking. The early 1990s
brought the prismatic cell, which was followed by the modern pouch cell.

This chapter addresses the cell designs, pack configurations and intrinsic safety
devices. In keeping with portability, this book addresses only the smaller cells used
for portable batteries.

The Cylindrical Cell

The cylindrical cell continues to be the most widely used packaging style. The
advantages are ease of manufacture and good mechanical stability. The cylinder has
the ability to withstand high internal pressures. While charging, the cell pressure of a
NiCd can reach 1379 kilopascals (kPa) or 200 pounds per square inch (psi). A venting
system is added on one end of the cylinder. Venting occurs if the cell pressure reaches
between 150 and 200 psi. Figure 3-1 illustrates the conventional cell of a NiCd battery.

Figure 3-1: Cross-section of a classic NiCd cell.

The negative and positive plates are rolled together in a metal cylinder. The positive plate is sintered and filled with
nickel hydroxide. The negative plate is coated with cadmium active material. A separator moistened with electrolyte
isolates the two plates. Design courtesy of Panasonic OEM Battery Sales Group, March 2001.

The cylindrical cell is moderately priced and offers high energy density. Typical
applications are wireless communication, mobile computing, biomedical instruments,
power tools and other uses that do not demand ultra-small size.

NiCd offers the largest selection of cylindrical cells. A good variety is also available
in the NiMH family, especially in the smaller cell formats. In addition to cylind rical
formats, NiMH also comes in the prismatic cell packaging.

The Li-ion batteries are only available in limited cells sizes, the most popular being
the 18650. ‘Eighteen’ denotes the diameter in millimeters and ‘650’ describes the
length in millimeters. The 18650 cell has a capacity of 1800 to 2000mAh. The larger
26650 cell has a diameter of 26 mm and delivers 3200mAh. Because of the flat
geometry of the Li-ion polymer, this battery chemistry is not available in a cylindrical
format.

Most SLA batteries are built in a prismatic format, thus creating a rectangle box that
is commonly made of plastic materials. There are SLA batteries, however, that take
advantage of the cylindrical design by using a winding technique that is similar to the
conventional cell. The cylindrical Hawker Cyclone SLA is said to offer improved cell
stability, provide higher discharge currents and have better temperature stability than
the conventional prismatic design.

The drawback of the cylindrical cell is less than maximum use of space. When
stacking the cells, air cavities are formed. Because of fixed cell size, the pack must be
designed around the available cell size.

Almost all cylindrical cells are equipped with a venting mechanism to expel excess
gases in an orderly manner. Whereas nickel-based batteries feature a resealable vent,
many cylindrical Li-ion contain a membrane seal that ruptures if the pressure exceeds
3448 kPa (500 psi). There is usually some serious swelling of the cell before the seal
breaks. Venting only occurs under extreme conditions.

The Button Cell

The button cell was developed to miniaturize battery packs and solve stacking
problems. Today, this architecture is limited to a small niche market. Non­rechargeable versions of the button cell continue to be popular and can be found in
watches, hearing aids and memory backup.

The main applications of the rechargeable button cell are (or were) older cordless
telephones, biomedical devices and industrial instruments. Although small in design
and inexpensive to manufacture, the main drawback is swelling if charged too rapidly.
Button cells have no safety vent and can only be charged at a 10 to 16 hour charge
rate. New designs claim rapid charge capability.

Figure 3-2: The button cell.

The button cell offers small size and ease of stacking but does not allow fast charging. Coin cells, which are similar in
appearance, are normally lithium-based and are non-rechargeable. Photograph courtesy of Sanyo Corporation;
design courtesy of Panasonic OEM Battery Sales Group, March 2001.

The Prismatic Cell

The prismatic cell was developed in response to consumer demand for thinner pack
sizes. Introduced in the early 1990’s, the prismatic cell makes almost maximum use of
space when stacking. Narrow and elegant battery styles are possible that suit today’s
slim-style geometry. Prismatic cells are used predominantly for mobile phone
applications. Figure 3-3 shows the prismatic cell.

Prismatic cells are most common in the lithium battery family. The Li-ion polymer is
exclusively prismatic. No universally accepted cell size exists for Li-ion polymer
batteries. One leading manufacturer may bring out one or more sizes that fit a certain
portable device, such as a mobile phone. While these cells are produced at high
volume, other cell manufacturers follow suit and offer an identical cell at a
competitive price. Prismatic cells that have gained acceptance are the 340648 and the

340848. Measured in millimeters, ‘34’ denotes the width, ‘06’ or ‘08’ the thickness
and ‘48’ the length of the cell.

Figure 3-3: Cross-section of a prismatic cell.

The prismatic cell improves space utilization and allows more flexibility in pack design. This cell construction is less
cost effective than the cylindrical equivalent and provides a slightly lower energy density. Design courtesy of Polystor
Corporation, March 2001.

Some prismatic cells are similar in size but are off by just a small fraction. Such is the
case with the Panasonic cell that measures 34 mm by 50 mm and is 6.5 mm thick. If a
few cubic millimeters can be added for a given application, the manufacturer will do
so for the sake of higher capacities.

The disadvantage of the prismatic cell is slightly lower energy densities compared to
the cylindrical equivalent. In addition, the prismatic cell is more expensive to
manufacture and does not provide the same mechanical stability enjoyed by the
cylindrical cell. To prevent bulging when pressure builds up, heavier gauge metal is
used for the container. The manufacturer allows some degree of bulging when
designing the battery pack.

The prismatic cell is offered in limited sizes and chemistries and runs from about
400mAh to 2000mAh and higher. Because of the very large quantities required for
mobile phones, special prismatic cells are built to fit certain models. Most prismatic
cells do not have a venting system. In case of pressure build-up, the cell starts to bulge.
When correctly used and properly charged, no swelling should occur.

The Pouch Cell

Cell design made a profound advance in 1995 when the pouch cell concept was
developed. Rather than using an expensive metallic cylinder and glass-to-metal
electrical feed-through to insulate the opposite polarity, the positive and negative
plates are enclosed in flexible, heat-sealable foils. The electrical contacts consist of
conductive foil tabs that are welded to the electrode and sealed to the pouch material.
Figure 3-4 illustrates the pouch cell.

The pouch cell concept allows tailoring to exact cell dimensions. It makes the most
efficient use of available space and achieves a packaging efficiency of 90 to
95 percent, the highest among battery packs. Because of the absence of a metal can,
the pouch pack has a lower weight. The main applications are mobile phones and
military devices. No standardized pouch cells exist, but rather, each m anufacturer
builds to a special application.

The pouch cell is exclusively used for Li-ion and Li-ion polymer chemistries. At the
present time, it costs more to produce this cell architecture and its reliability has not
been fully proven. In addition, the energy density and load current are slightly lower
than that of conventional cell designs. The cycle life in everyday applications is not
well documented but is, at present, less than that of the Li-ion system with
conventional cell design.

A critical issue with the pouch cell is the swelling that occurs when gas is generated
during charging or discharging. Battery manufacturers insist that Li-ion or Polymer
cells do not generate gas if properly formatted, are charged at the correct current and
are kept within allotted voltage levels. When designing the protective housing for a
pouch cell, some provision for swelling must be made. To alleviate the swelling issue
when using multiple cells, it is best not to stack pouch cells, but lay them side by side.

Figure 3-4: The pouch cell.

The pouch cell offers a simple, flexible and
lightweight solution to battery design. This new
concept has not yet fully matured and the
manufacturing costs are still high.
© Cadex Electronics Inc.

The pouch cell is highly sensitive to twisting. Point pressure must also be avoided.
The protective housing must be designed to protect the cell from mechanical stress.

Series and Parallel Configurations

In most cases, a single cell does not provide a high enough voltage and a serial
connection of several cells is needed. The metallic skin of the cell is insulated to
prevent the ‘hot’ metal cylinders from creating an electrical short circuit against the
neighboring cell.

Nickel-based cells provide a nominal cell voltage of 1.25V. A lead acid cell delivers
2V and most Li-ion cells are rated at 3.6V. The spinel (manganese) and Li-ion
polymer systems sometimes use 3.7V as the designated cell voltage. This is the reason
for the often unfamiliar voltages, such as 11.1V for a three cell pack of spinel
chemistry.

Nickel-based cells are often marked 1.2V. There is no difference between a 1.2 and

1.25V cell; it is simply the preference of the manufacturer in marking. Whereas
commercial batteries tend to be identified with 1.2V/cell, industrial, aviation and
military batteries are still marked with the original designation of 1.25V/cell.

A five-cell nickel-based battery delivers 6V (6.25V with 1.25V/cell marking) and a
six-cell pack has 7.2V (7.5V with 1.25V/cell marking). The portable lead acid comes
in 3 cell (6V) and 6 cell (12V) formats. The Li-ion family has either 3.6V for a single
cell pack, 7.2V for a two-cell pack or 10.8V for a three-cell pack. The 3.6V and 7.2V
batteries are commonly used for mobile phones; laptops use the larger 10.8V packs.

There has been a trend towards lower voltage batteries for light portable devices, such
as mobile phones. This was made possible through advancements in microelectronics.
To achieve the same energy with lower voltages, higher currents are needed. With
higher currents, a low internal battery resistance is critical. This presents a challenge
if protection devices are used. Some losses through the solid-state switches of
protection devices cannot be avoided.

Packs with fewer cells in series generally perform better than those with 12 cells or
more. Similar to a chain, the more links that are used, the greater the odds of one

breaking. On higher voltage batteries, precise cell matching becomes important,
especially if high load currents are drawn or if the pack is operated in cold
temperatures.

Parallel connections are used to obtain higher ampere-hour (Ah) ratings. When
possible, pack designers prefer using larger cells. This may not always be practical
because new battery chemistries come in limited sizes. Often, a parallel connection is
the only option to increase the battery rating. Paralleling is also necessary if pack
dimensions restrict the use of larger cells. Among the battery chemistries, Li-ion lends
itself best to parallel connection.

Protection Circuits

Most battery packs include some type of protection to safeguard battery and
equipment, should a malfunction occur. The most basic protection is a fuse that opens
if excessively high current is drawn. Some fuses open permanently and render the
battery useless once the filament is broken; other fuses are based on a Polyswitch™,
which resembles a resettable fuse. On excess current, the Polyswitch™ creates a high
resistance, inhibiting the current flow. When the condition normalizes, the resistance
of the switch reverts to the low ON position, allowing normal operation to resume.
Solid-state switches are also used to disrupt the current. Both solid-state switches and
the Polyswitch™ have a residual resistance to the ON position during normal
operation, causing a slight increase in internal battery resistance.

A more complex protection circuit is found in intrinsically safe batteries. These
batteries are mandated for two-way radios, gas detectors and other electronic
instruments that operate in a hazardous area such as oil refineries and grain elevators.
Intrinsically safe batteries prevent explosion, should the electronic devices
malfunction while operating in areas that contain explosive gases or high dust
concentration. The protection circuit prevents excessive current, which could lead to
high heat and electric spark.

There are several levels of intrinsic safety, each serving a specific hazard level. The
requirement for intrinsic safety varies from country to country. The purchase cost of
an intrinsically safe battery is two or three times that of a regular battery.

Commercial Li-ion packs contain one of the most exact protection circuits in the
battery industry. These circuits assure safety under all circumstances when in the
hands of the public. Typically, a Field Effect Transistor (FET) opens if the charge
voltage of any cell reaches 4.30V and a fuse activates if the cell temperature
approaches 90°C (194°F). In addition, a disconnect switch in each cell permanently
interrupts the charge current if a safe pressure threshold of 1034 kPa (150 psi) is
exceeded. To prevent the battery from over-discharging, the control circuit cuts off
the current path at low voltage, which is typically 2.50V/cell.

The Li-ion is typically discharged to 3V/cell. The lowest ‘low-voltage’ power cut-off
is 2.5V/cell. During prolonged storage, however, a discharge below that cut-off level

is possible. Manufacturers recommend a ‘trickle’ charge to raise such a battery
gradually back up into the acceptable voltage window.

Not all chargers are designed to apply a charge once a Li-ion battery has dipped
below 2.5V/cell. A ‘wake-up’ boost will be needed to first engage the electronic
circuit, after which a gentle charge is applied to re-energize the battery. Caution must
be applied not to boost lithium-based batteries back to life, which have dwelled at a
very low voltage for a prolonged time.

Each parallel string of cells of a Li-ion pack needs independent voltage monitoring. The more
cells that are connected in series, the more complex the protection circuit becomes. Fou r cells
in series is the practical limit for commercial applications.

The internal protection circuit of a mobile phone while in the ON position has a
resistance of 50 to 100 mW. The circuit normally consists of two switches connected
in series. One is responsible for high cut-off, the other for low cut-off. The combined
resistance of these two devices virtually doubles the internal resistance of a battery
pack, especially if only one cell is used. Battery packs powering mobile phones, for
example, must be capable of delivering high current bursts. The internal protection
does, in a certain way, interfere with the current delivery.

Some small Li-ion packs with spinel chemistry containing one or two cells may not
include an electronic protection circuit. Instead, they use a single component fuse
device. These cells are deemed safe because of small size and low capacity. In
addition, spinel is more tolerant than other systems if abused. The absence of a
protection circuit saves money, but a new problem arises. Here is what can happen:

Mobile phone users have access to chargers that may not be approved by the battery
manufacturer. Available at low cost for car and travel, these chargers may rely on the
battery’s protection circuit to terminate at full charge. Without the protection circuit,
the battery cell voltage rises too high and overcharges the battery. Apparently still
safe, irreversible battery damage often occurs. Heat buildup and bulging is common
under these circumstances. Such situations must be avoided at all times. The
manufacturers are often at a loss when it comes to replacing these batteries under
warranty.

Li-ion batteries with cobalt electrodes, for example, require full safety protection. A
major concern arises if static electricity or a faulty charger has destroyed the battery’s
protection circuit. Such damage often causes the solid-state switches to fuse in a
permanent ON position without the user’s knowledge. A battery with a faulty
protection circuit may function normally but does not provide the required safety. If
charged beyond safe voltage limits with a poorly designed accessory charger, the
battery may heat up, then bulge and in some cases vent with flame. Shorting such a
battery can also be hazardous.

Manufacturers of Li-ion batteries refrain from mentioning explosion. ‘Venting with
flame’ is the accepted terminology. Although slower in reaction than an explosion,

venting with flame can be very violent and inflicts injury to those in close proximity.
It can also damage the equipment to which the battery is connected.

Most manufacturers do not sell the Li-ion cells by themselves but make them
available in a battery pack, complete with protection circuit. This precaution is
understandable when considering the danger of explosion and fire if the battery is
charged and discharged beyond its safe limits. Most battery assembling houses must
certify the pack assembly and protection circuit intended to be used with the
manufacturer before these items are approved for sale.

Chapter 4: Proper Charge Methods

To a large extent, the performance and longevity of rechargeable batteries depends on the
quality of the chargers. Battery chargers are commonly given low priority, especially on
consumer products. Choosing a quality charger makes sense. Thi s is espe cially true when
considering the high cost of battery replacements and the frustration that poorly performing
batteries create. In most cases, the extra money invested is returned because the batteries
last longer and perform more efficiently.

All About Chargers

There are two distinct varieties of chargers: the personal chargers and the industrial chargers.
The personal charger is sold in attractive packaging and is offered with such pro ducts as
mobile phones, laptops and video cameras. These chargers are economically priced and
perform well when used for the application intended. The personal charger offers moderate
charge times.

In comparison, the industrial charger is designed for employee use and accommodates fleet
batteries. These chargers are built for repetitive use. Available for single or multi-bay
configurations, the industrial chargers are offered from the original equipment manufacturer
(OEM). In many instances, the chargers can also be obtained from third party manufacturers.
While the OEM chargers meet basic requirements, third party manufacturers often include
special features, such as negative pulse charging, discharge function for battery conditioning,
and state-of-charge (SoC) and state-of-health (SoH) indications. Many third party
manufacturers are prepared to build low quantities of custom chargers. Other benefits third
party suppliers can offer include creative pricing and superior performance.

Not all third party charger manufacturers meet the quality standards that the industry
demands, The buyer should be aware of possible quality and performance compromises
when purchasing these chargers at discount prices. Some units may not be rugged enough to
withstand repetitive use; others may develop maintenance problems such as burned or
broken battery contacts.

Uncontrolled over-charge is another problem of some chargers, es pecially those used to
charge nickel-based batteries. High temperature during charge and standby kills batteries.
Over-charging occurs when the charger keeps the battery at a temperature that is warm to
touch (body temperature) while in ready condition.

Some temperature rise cannot be avoided when charging nickel-ba s ed batteries. A
temperature peak is reached when the battery approaches full charge. The temperature must
moderate when the ready light appears and the battery has switched to trickle charge. The
battery should eventually cool to room temperature.

If the temperature does not drop and remains above room temperature, the charger is
performing incorrectly. In such a case, the battery should be removed as soon as possible
after the ready light appears. Any prolonged trickle charging will damage the battery. This
caution applies especially to the NiMH because it cannot absorb overcharge well. In fact, a
NiMH with high trickle charge could be cold to the touch and still be in a damaging overcharge
condition. Such a battery would have a short service life.

A lithium-based battery should never get warm in a charger. If this happens, the battery is
faulty or the charger is not functioning properly. Discontinue using this battery and/or charger.

It is best to store batteries on a shelf and apply a topping-charge before use rather than
leaving the pack in the charger for days. Even at a seemingly correct trickle charge, nickel­based batteries produce a crystalline formation (also referred to as ‘memory’) when left in the

charger. Because of relatively high self-discharge, a topping charge is needed before use.
Most Li-ion chargers permit a battery to remain engaged without inflicting damage.

There are three types of chargers for nickel-based batteries. They are:

Slow Charger — Also known as ‘overnight charger’ or ‘normal charger’, the slow-charger
applies a fixed charge rate of about 0.1C (one tenth of the rated capacity) for as long as the
battery is connected. Typical charge time is 14 to 16 hours. In most cases, no full-charge
detection occurs to switch the battery to a lower charge rate at the end of the charge cycle.
The slow-charger is inexpensive and can be used for NiCd batteries only. With the need to
service both NiCd and NiMH, these chargers are being replaced with more advanced units.

If the charge current is set correctly, a battery in a slow-charger remains lukewarm to the
touch when fully charged. In this case, the battery does not need to be removed immediately
when ready but should not stay in the charger for more than a day. The sooner the battery
can be removed after being fully charged, the better it is.

A problem arises if a smaller battery (lower mAh) is charged with a charger designed to
service larger packs. Although the charger will perform well in the initial charge phase, the
battery starts to heat up past the 70 percent charge level. Because there is no provision to
lower the charge current or to terminate the charge, heat-damaging over-charge will occur in
the second phase of the charge cycle. If an alternative charger is not available, the user is
advised to observe the temperature of the battery being charged and disconnect the battery
when it is warm to the touch.

The opposite may also occur when a larger battery is charged on a charger designed for a
smaller battery. In such a case, a full charge will never be reached. The battery remains cold
during charge and will not perform as expected. A nickel-based battery that is continuously
undercharged will eventually loose its ability to accept a full charge due to memory.

Quick Charger — The so-called quick-charger, or rapid charger, is one of the most popular.
It is positioned between the slow-charger and the fast-charger, both in terms of charging time
and price. Charging takes 3 to 6 hours and the charge rate is around 0.3C. Charge control is
required to terminate the charge when the battery is ready. The well designed quick-charger
provides better service to nickel-based batteries than the slow-charger. Batteries last longer if
charged with higher currents, provided they remain cool and are not overcharged. The quick­chargers are made to accommodate either nickel-based or lithium-based batteries. These two
chemistries can normally not be interchanged in the same charger.

Fast Charger — The fast-charger offers several advantages over the other chargers; the
obvious one is shorter charge times. Because of the larger power supply and the more
expensive control circuits needed, the fast-charger costs more than slower chargers, but the
investment is returned in providing good performing batteries that live longer.

The charge time is based on the charge rate, the battery’s SoC, its rating and the chemistry.
At a 1C charge rate, an empty NiCd typically charges in a little more than an hour. When a
battery is fully charged, some chargers switch to a topping charge mode governed by a timer
that completes the charge cycle at a reduced charge current. Once fully charged, the charger
switches to trickle charge. This maintenance charge compensates for the self-discharge of
the battery.

Modern fast-chargers commonly accommodate both NiCd and NiMH batteries. Because of
the fast-charger’s higher charge current and the need to monitor the battery during charge, it
is important to charge only batteries specified by the manufacturer. Some battery
manufacturers encode the batteries electrically to identify their chemistry and rating. The
charger then sets the correct charge current and algorithm for the battery intended. Lead Acid

and Li-ion chemistries are charged with different algorithms and are not compatible with the
charge methods used for nickel-based batteries.

It is best to fast charge nickel-based batteries. A slow charge is known to build up a crystalline
formation on nickel-based batteries, a phenomenon that lowers battery performance and
shortens service life. The battery temperature during charge should be moderate and the
temperature peak kept as short as possible.

It is not recommended to leave a nickel-based battery in the charger for more than a few days,
even with a correctly set trickle charge current. If a battery must remain in a charger for
operational readiness, an exercise cycle should be ap plied once every month.

Simple Guidelines

A charger designed to service NiMH batteries can also accommodate NiCd’s, but not the
other way around. A charger only made for the NiCd batteries could overcharge the NiMH
battery.

While many charge methods exist for nickel-based batteries, chargers for lithium-based
batteries are more defined in terms of charge method and charge time. This is, in part, due to
the tight charge regime and voltage requirements demanded by these batteries. There is only
one way to charge Li-ion/Polymer batteries and the so-called ‘miracle chargers’, which claim
to restore and prolong battery life, do not exist for these chemistries. Neither does a super­fast charging solution apply.

The pulse charge method for Li-ion has no major advantages and the voltage peaks wreak
havoc with the voltage limiting circuits. While charge times can be reduced, some
manufacturers suggest that pulse charging may shorten the cycle life of Li-ion batteries.

Fast charge methods do not significantly decrease the charge time. A charge rate over 1C
should be avoided because such high current can induce lithium plating. With most packs, a
charge above 1C is not possible. The protection circuit limits the amount of current the battery
can accept. The lithium-based battery has a slow metabolism and must take its time to absorb
the energy.

Lead acid chargers serve industrial markets such as hospitals and health care units. Charge
times are very long and cannot be shortened. Most lead acid chargers charge the battery in
14 hours. Because of its low energy density, this battery type is not used for small portable
devices.

In the following sections various charging needs and charging methods are studied. The
charging techniques of different chargers are examined to determine why some perform
better than others. Since fast charging rather than slow charging is the norm today, we look at
well-designed, closed loop systems, which communicate with the battery and terminate the
fast charge when certain responses from the battery are received.

Charging the Nickel Cadmium Battery

Battery manufacturers recommend that new batteries be slow-charged for 24 hours before
use. A slow charge helps to bring the cells within a battery pack to an equal charge level
because each cell self-discharges to different capacity levels. During long storage, the
electrolyte tends to gravitate to the bottom of the cell. The initial trickle charge helps
redistribute the electrolyte to remedy dry spots on the separator that may have developed.

Some battery manufacturers do not fully form their batteries before
shipment. These batteries reach their full potential only after the customer

has primed them through several charge/discharge cycles, either with a
battery analyzer or e cycles
are needed to fully form a nickel-based battery. Quality cells, such as those made by Sanyo

through normal use. In many cases, 50 to 100 discharge/charg

and Panasonic, are known to perform to full specification after as few as 5 to
7 discharge/charge cycles. Early readings may be inconsistent, but the capacity levels
become very steady once fully primed. A slight capacity peak is observed between 100 an

d

300 cycles.

Most rechargeable cells are equipped with a safety vent to release excess pressure if
incorrectly charged. The safety vent on a NiCd cell opens at 1034 to 1379 kPa (150 to
200 psi). In comparison, the pressure of a car tire is typically 240 kPa (35 psi). With a
resealable vent, no damage occurs on venting but some electrolyte is lost and the seal may
leak afterwards. When this happens, a white powder will accumulate over time at the vent
opening.

Commercial fast-chargers are often not designed in the best interests of the battery. This is
especially true of NiCd chargers that measure the battery’s charge state solely through
temperature sensing. Although simple and inexpensive in design, charge termination by
temperature sensing is not accurate. The thermistors used commonly exhibit broad
tolerances; their positioning with respect to the cells are not consistent. Ambient temperatures
and exposure to the sun while charging also affect the accuracy of full-charge detection. To
prevent the risk of premature cut-off and assure full charge under most conditions, charger
manufacturers use 50°C (122°F) as the recommended temperature cut-off. Although a
prolonged temperature above 45°C (113°F) is harmful to the battery, a brief temperature peak
above that level is often unavoidable.

More advanced NiCd chargers sense the rate of temperature increase, defined as dT/dt, or
the change in temperature over charge time, rather than responding to an absolute
temperature (dT/dt is defined as delta Temperature / delta time). This type of charger is kinder
to the batteries than a fixed temperature cut-off, but the cells still need to generate heat to
trigger detection. To terminate the charge, a temperature increase of 1°C (1.8°F) per minute
with an absolute temperature cut-off of 60°C (140°F) works well. Because of the relatively
large mass of a cell and the sluggish propagation of heat, the delta temperature, as this
method is called, will also enter a brief overcharge condition before the full-charge is detected.
The dT/dt method only works with fast chargers.

Harmful overcharge occurs if a fully charged battery is repeatedly inserted for topping charge.
Vehicular or base station chargers that require the removal of two-way radios with each use
are especially hard on the batteries because each reconnection ini tiates a fast-charge cycle.
This also applies to laptops that are momentarily disconnected and reconnected to perform a
service. Likewise, a technician may briefly plug the laptop into the power source to check a
repeater station or service other installations. Problems with laptop batteries have also been
reported in car manufacturing plants where the workers move the laptops from car to ca r,
checking their functions, while momentarily plugging into the external power source.
Repetitive connection to power affects mostly ‘dumb’ nickel-based batteries. A ‘dumb’ battery
contains no electronic circuitry to communicate with the charger. Li-ion chargers detect the
SoC by voltage only and multiple reconnections will not confuse the charging regime.

More precise full charge detection of nickel-based batteries can be achieved with the use of a
micro controller that monitors the battery voltage and terminates the charge when a certain
voltage signature occurs. A drop in voltage signifies that the battery has reached full charge.
This is known as Negative Delta V (NDV).

NDV is the recommended full-charge detection method for ‘open-lead’ NiCd chargers
because it offers a quick response time. The NDV charge detection also works well with a
partially or fully charged battery. If a fully charged battery is inserted, the terminal voltage
raises quickly, then drops sharply, triggering the ready state. Such a charge lasts only a few

minutes and the cells remain cool. NiCd chargers based on the NDV full charge detection
typically respond to a voltage drop of 10 to 30mV per cell. Chargers that respond to a very
small voltage decrease are preferred over those that require a larger drop.

To obtain a sufficient voltage drop, the charge rate must be 0.5C and higher. Lower than 0.5C
charge rates produce a very shallow voltage decrease that is often difficult to measure,
especially if the cells are slightly mismatched. In a battery pack that has mismatched cells,
each cell reaches the full charge at a different time and the curve gets distorted. Failing to
achieve a sufficient negative slope allows the fast-charge to continue, causing excessive heat
buildup due to overcharge. Chargers using the NDV must include other charge-termination
methods to provide safe charging under all conditions. Most chargers also observe the battery
temperature.

The charge efficiency factor of a standard NiCd is better on fast charge than slow charge. At a
1C charge rate, the typical charge efficiency is 1.1 or 91 percent. On an overnight slow
charge (0.1C), the efficiency drops to 1.4 or 71 percent.

At a rate of 1C, the charge time of a NiCd is slightly longer than 60 minutes (66 minutes at an
assumed charge efficiency of 1.1). The charge time on a battery that is partially discharged or
cannot hold full capacity due to memory or other degradation is shorter accordingly. At a 0.1C
charge rate, the charge time of an empty NiCd is about 14 hours, which relates to the charge
efficiency of 1.4.

During the first 70 percent of the charge cycle, the charge efficiency of a NiCd battery is close
to 100 percent. Almost all of the energy is absorbed and the battery remains cool. Currents of
several times the C-rating can be applied to a NiCd battery designed for fast charging without
causing heat build-up. Ultra-fast chargers use this unique phenomenon and charge a battery
to the 70 percent charge level within a few minutes. The charge continues at a lower rate until
the battery is fully charged.

Once the 70 percent charge threshold is passed, the battery gradually loses ability to accept
charge. The cells start to generate gases, the pressure rises and the temperature increases.
The charge acceptance drops further as the battery reaches 80 and 90 percent SoC. Once
full charge is reached, the battery goes into overcharge. In an attempt to gain a few extra
capacity points, some chargers allow a measured amount of overcharge. Figure 4-1 illustrates
the relationship of cell voltage, pressure and temperature while a NiCd is being charged.

Ultra-high capacity NiCd batteries tend to heat up more than the standard NiCd if charged at
1C and higher. This is partly due to the higher internal resistance of the ultra-high capacity
battery. Optimum charge performance can be achieved by applying higher current at the
initial charge stage, then tapering it to a lower rate as the charge acceptance decreases. This
avoids excess temperature rise and yet assures fully charged batteries.

Figure 4-1: Charge characteristics of a NiCd cell.

These cell voltage, pressure and temperature characteristics are similar in a NiMH cell.

Interspersing discharge pulses between charge pulses improves the charge acceptance of
nickel-based batteries. Commonly referred to as ‘burp’ or ‘reverse load’ charge, this charge
method promotes high surface area on the electrodes, resulting in enhanced performance
and increased service life. Reverse load also improves fast charging because it helps to
recombine the gases generated during charge. The result is a cooler and more effective
charge than with conventional DC chargers.

Charging with the reverse load method minimizes crystalline formation. The US Army
Electronics Command in Fort Monmouth, NJ, USA, had done extensive research in this field
and has published the results. (See Figure 10-1, Crystalline form ation on NiCd cell).
Research conducted in Germany has shown that the reverse load method adds 15 percent to
the life of the NiCd battery.

After full charge, the NiCd battery is maintained with a trickle charge to compensate for the
self-discharge. The trickle charge for a NiCd battery ranges between 0.05C and 0.1C. In an
effort to reduce the memory phenomenon, there is a trend towards lower trickle charge
currents.

Charging the Nickel-Metal Hydride Battery

Chargers for NiMH batteries are very similar to those of the NiCd system but the electronics is
generally more complex. To begin with, the NiMH produces a very small voltage drop at full
charge. This NDV is almost non-existent at charge rates below 0.5C and elevated
temperatures. Aging and cell mismatch works further against the already minute voltage delta.
The cell mismatch gets worse with age and increased cycle count, which makes the use of
the NDV increasingly more difficult.

The NDV of a NiMH charger must respond to a voltage drop of 16mV or less. Increasing the
sensitivity of the charger to respond to the small voltage drop often terminates the fast charge
by error halfway through the charge cycle. Voltage fluctuations and noise induced by the
battery and charger can fool the NDV detection circuit if set too precisely.

The popularity of the NiMH battery has introduced many innovative charging techniques. Most
of today’s NiMH fast chargers use a combination of NDV, voltage plateau, rate-of­temperature-increase (dT/dt), temperature threshold and timeout timers. The charger utilizes
whatever comes first to terminate the fast-charge.

NiMH batteries which use the NDV method or the thermal cut-off control tend to deliver higher
capacities than those charged by less aggressive methods. The gain is approximately
6 percent on a good battery. This capacity increase is due to the brief overcharge to which the
battery is exposed. The negative aspect is a shorter cycle life. Rather than expecting 350 to
400 service cycles, this pack may be exhausted with 300 cycles.

Similar to NiCd charge methods, most NiMH fast-chargers work on the rate-of-temperature­increase (dT/dt). A temperature raise of 1°C (1.8°F) per minute is commonly used to
terminate the charge. The absolute temperature cut-off is 60°C (140°F). A topping charge of

0.1C is added for about 30 minutes to maximize the charge. The continuous trickle charge
that follows keeps the battery in full charge state.

Applying an initial fast charge of 1C works well. Cooling periods of a few minutes are adde d
when certain voltage peaks are reached. The charge then continues at a lower current. When
reaching the next charge threshold, the current steps down further. This process is repeated
until the battery is fully charged.

Known as ‘step-differential charge’, this charge method works well with NiMH and NiC d
batteries. The charge current adjusts to the SoC, allowing high current at the beginning and
more moderate current towards the end of charge. This avoids excessive temperature build­up towards the end of the charge cycle when the battery is less capable of accepting charge.

NiMH batteries should be rapid charged rather than slow charged. The amount of trickle
charge applied to maintain full charge is especially critical. Because NiMH does not absorb
overcharge well, the trickle charge must be set lower than that of the NiCd. The
recommended trickle charge for the NiMH battery is a low 0.05C. This is why the original
NiCd charger cannot be used to charge NiMH batteries. The lower trickle charge rate is
acceptable for the NiCd.

It is difficult, if not impossible, to slow-charge a NiMH battery. At a C-rate of 0.1C and 0.3C,
the voltage and temperature profiles fail to exhibit defined characteristics to measure the full
charge state accurately and the charger must depend on a timer. Harmful overcharge can
occur if a partially or fully charged battery is charged on a charger with a fixed timer. The
same occurs if the battery has lost charge acceptance due to age and can only hold
50 percent of charge. A fixed timer that delivers a 100 percent charge each time without
regard to the battery condition would ultimately apply too much charge. Overcharge could
occur even though the NiMH battery feels cool to the touch.

Some lower-priced chargers may not apply a fully saturated charge. On these economy
chargers, the full-charge detection may occur immediately after a given voltage peak is
reached or a temperature threshold is detected. These chargers are commonly promoted on
the merit of short charge time and moderate price.

Figure 4-2 summarizes the characteristics of the slow charger, quick charger and fast charger.
A higher charge current allows better full-charge detection.

Slow
Charger

Quick
Charger

Fast
Charger

Charge
C-rate

0.1C 14h 0°C to 45°C

0.3-0.5C 4h 10°C to 45°C

1C 1h+ 10°C to 45°C

Typical
charge time

Maximum permissible
charge temperatures

(32°F to 113°F)

(50°F to 113°F)

(50°F to 113°F)

Charge termination method

Fixed timer. Subject to overcharge. Remove battery
when charged.

NDV set to 10mV/cell, uses voltage plateau, absolute
temperature and time-out-timer. (At 0.3C, dT/dt fails
to raise the temperature sufficiently to terminate the
charge.)

NDV responds to higher settings; uses dT/dt, voltage
plateau absolute temperature and time-out-t ime r

Figure 4-2: Characteristics of various charger types.

These values also apply to NiMH and NiCd cells.

Charging the Lead Acid Battery

The charge algorithm for lead acid batteries differs from nickel-based chemistry in that voltage
limiting rather than current limiting is used. Charge time of a sealed lead acid (SLA) is 12 to
16 hours. With higher charge currents and multi-stage charge methods, charge time can be
reduced to 10 hours or less. SLAs cannot be fully charged as quickly as nickel-based systems.

A multi-stage charger applies constant-current charge, topping charge and float charge (see
Figure 4-3). During the constant current charge, the battery charges to 70 percent in about
five hours; the remaining 30 percent is completed by the slow topping charge. The topping
charge lasts another five hours and is essential for the well-being of the battery. This can be
compared to a little rest after a good meal before resuming work. If the battery is not
completely saturated, the SLA will eventually lose its ability to accept a full charge and the
performance of the battery is reduced. The third stage is the float charge, which compensates
for the self-discharge after the battery has been fully charged.

Figure 4-3: Charge stages of a lead acid battery.

A multi-stage charger applies constant-current charge, topping charge and float charge.

Correctly setting the cell-voltage limit is critical. A typical voltage limit is from 2.30V to 2.45V.
If a slow charge is acceptable, or the room temperature may exceed 30°C (86°F), the
recommended voltage limit is 2.35V/cell. If a faster charge is required, and the room
temperature will remain below 30°C, 2.40 to 2.45V/cell may be used. Figure 4-4 compares
the advantages and disadvantages of the different voltage settings.

Advantage

Disadvantage

Maximum service life; battery
remains cool during charge;
ambient charge temperature may
exceed 30°C (86°F).

Slow charge time; capacity
readings may be low and
inconsistent. If no periodic
topping charge is applied, under­charge conditions (sulfation) may

2.30V to 2.35V/cell 2.40V to 2.45V/cell

Faster charge times; higher and
more consistent capacity
readings; less subject to damage
due to under-charge condition.

Battery life may be reduced due
to elevated battery temperature
while charging. A hot battery may
fail to reach the cell voltage limit,

causing harmful over charge.
occur, which can lead to
unrecoverable capacity loss.

Figure 4-4: Effects of charge voltage on a plastic SLA battery.

Large VRLA and the cylindrical Hawker cell may have different requirements.

The charge voltage limit indicated in Figure 4-4 is a momentary voltage peak and the battery
cannot dwell on that level. This voltage crest is only used when applying a full charge cycle to
a battery that has been discharged. Once fully charged and at operational readiness, a float

charge is applied, which is held constant at a lower voltage level. The recommended float
charge voltage of most low-pressure lead acid batteries is between 2.25 to 2.30V/cell. A good
compromise is 2.27V.

The optimal float charge voltage shifts with temperature. A higher temperature demands
slightly lower voltages and a lower temperature demands higher voltages. Chargers that are
exposed to large temperature fluctuations are equipped with temperature sensors to optimize
the float voltage.

Regardless of how well the float voltage may be compensated, there is always a compromise.
The author of a paper in a battery seminar explained that charging a sealed lead acid battery
using the traditional float charge techniques is like 'dancing on the head of a pin'. The battery
wants to be fully charged to avoid sulfation on the negative plate, but does not want to be
over-saturated which causes grid corrosion on the positive plate. In addition to grid corrosion,
too high a float charge contributes to loss of electrolyte.

Differences in the aging of the cells create another challenge in finding the optimum float
charge voltage. With the development of air pockets within the cells over time, some batteries
exhibit hydrogen evolution from overcharging. Others undergo oxygen recombination in an
almost starved state. Since the cells are connected in series, controlling the individual cell
voltages during charge is virtually impossible. If the applied cell voltage is too high or too low
for a given cell, the weaker cell deteriorates further and its condition becomes more
pronounced with time. Companies have developed cell-balancing devices that correct so me
of these problems but these devices can only be applied if access to individual cells is
possible.

A ripple voltage imposed on the charge voltage also causes problems for lead acid batteries,
especially the larger VRLA. The peak of the ripple voltage constitutes an overcharge, causing
hydrogen evolution; the valleys induce a brief discharge causing a starved state. Electrolyte
depletion may be the result.

Much has been said about pulse charging lead acid batteries. Although there are obvious
benefits of reduced cell corrosion, manufacturers and service technicians are not in
agreement regarding the benefit of such a charge method. Some advantages are apparent if
pulse charging is applied correctly, but the results are non-conclusive.

Whereas the voltage settings in Figure 4-4 apply to low-pressure lead acid batteries with a
pressure relief valve setting of about 34 kPa (5 psi), the cylindrical SLA by Hawker requires
higher voltage settings. These voltage limits should be set according to the manufacturer’s
specifications. Failing to apply the recommended voltage threshold for these batteries causes
a gradual decrease in capacity due to sulfation. Typically, the Hawker cell has a pressure
relief setting of 345 kPa (50 psi). This allows some recombination of the gases during charge.

An SLA must be stored in a charged state. A topping charge should be applied every six
months to avoid the voltage from dropping below 2.10V/cell. The topping charge requirements
may differ with cell manufacturers. Always follow the time intervals recommended by the
manufacturer.

By measuring the open cell voltage while in storage, an approximate charge-level indication
can be obtained. A voltage of 2.11V, if measured at room temperature, reveals that the cell
has a charge of 50 percent and higher. If the voltage is at or above this threshold, the battery
is in good condition and only needs a full charge cycle prior to use. If the voltage drops bel ow

2.10V, several discharge/charge cycles may be required to bring the battery to full
performance. When measuring the terminal voltage of any cell, the storage temperature
should be observed. A cool battery raises the voltage slightly and a warm one lowers it.

Plastic SLA batteries arriving from vendors with less than 2.10V per cell are rejected by some
buyers who inspect the battery during quality control. Low voltage suggests that the battery
may have a soft short, a defect that cannot be corrected with cycling. Although cycling may
increase the capacity of these batteries, the extra cycles compromise the service life of the
battery. Furthermore, the time and equipment required to make the battery fully functional
adds to operational costs.

The Hawker cell can be stored at voltages as low as 1.81V. However, when reactivating the
cells, a higher than normal charge voltage may be required to convert the large sulfite crystals
back to good active material.

Caution: When charging a lead acid battery with over-voltage, current limiting must be

applied once the battery starts to draw full current. Always set the current limit to the lowest
practical setting and observe the battery voltage and temperature during the procedure. If the
battery does not accept a normal charge after 24 hours under elevated voltage, a return to
normal condition is unlikely.

The price of the Hawker cell is slightly higher than that of the plastic equivalent, but lower than
the NiCd. Also known as the ‘Cyclone’, this cell is wound similar to a cylindrical NiCd. This
construction improves the cell’s stability and provides higher discharge currents when
compared to the flat plate SLA. Because of its relatively low self-discharge, Hawker cells are
well suited for defibrillators that are used on standby mode.

Lead acid batteries are preferred for UPS systems. During prolonged float charg e, a periodic
topping charge, also known as an ‘equalizing charge’, is recommended to fully charg e the
plates and prevent sulfation. An equalizing charge raises the battery voltage for several hours
to a voltage level above that specified by the manufacturer. Loss of electrolyte through
elevated temperature may occur if the equalizing charge is not administered co rrectly.
Because no liquid can be added to the SLA and VRLA systems, a reduction of the electrolyte
will cause irreversible damage. Manufacturers and service personnel are often divided on the
benefit of the equalizing charge.

Some exercise, or brief periodic discharge, is believed to prolong battery life of lead acid
systems. If applied once a month as part of an exercising program, the depth of discharge
should only be about 10 percent of its total capacity. A full discharge as part of regular
maintenance is not recommended because each deep discharge cycle robs service life from
the battery.

More experiments are needed to verify the benefit of exercising lead acid batteries. Again,
manufacturers and service technicians express different views on how preventive
maintenance should be carried out. Some experts prefer a topping charge while others
recommend scheduled discharges. No scientific data is available on the benefit of frequent
shallow discharges as opposed to fewer deep discharges or discharge pulses.

Disconnecting the float charge while the VRLA is on standby is another method of prolonging
battery life. From time-to-time, a topping charge is applied to replenish the energy lost through
self-discharge. This is said to lower cell corrosion and prolong battery life. In essence, the
battery is kept as if it was in storage. This only works for applications that do not draw a load
current during standby. In many applications, the battery acts as an energy buffer and needs
to be under continuous charge.

Important: In case of rupture, leaking electrolyte or any other cause of exposure to the

electrolyte, flush with water immediately. If eye exposure occurs, flush with water for
15 minutes and consult a physician immediately.

Charging the Lithium Ion Battery

The Li-ion charger is a voltage-limiting device similar to the lead acid battery char ger. The
difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of
trickle or float charge when full charge is reached.

While the lead acid battery offers some flexibility in terms of voltage cut-off, manufacturers of
Li-ion cells are very strict on setting the correct voltage. When the Li- ion was first introduced,
the graphite system demanded a charge voltage limit of 4.10V/cell. Although higher voltages
deliver increased energy densities, cell oxidation severely limited the service life in the early
graphite cells that were charged above the 4.10V/cell threshold. This effect has been solved
with chemical additives. Most commercial Li-ion cells can now be charged to 4.20V. The
tolerance on all Li-ion batteries is a tight +/-0.05V/cell.

Industrial and military Li-ion batteries designed for maximum cycle life use an end-of-charge
voltage threshold of about 3.90V/cell. These batteries are rated lower on the watt-hour-per­kilogram scale, but longevity takes precedence over high energy density and small size.

The charge time of all Li-ion batteries, when charged at a 1C initial current, is about 3 hours.
The battery remains cool during charge. Full charge is attained after the voltage has reached
the upper voltage threshold and the current has dropped and leveled off at about 3 percent of
the nominal charge current.

Increasing the charge current on a Li-ion charger does not shorten the charge ti me by much.
Although the voltage peak is reached quicker with higher current, the topping charge will take
longer. Figure 4-5 shows the voltage and current signature of a charger as the Li-ion cell
passes through stage one and two.

Some chargers claim to fast-charge a Li-ion battery in one hour or less. Such a charger
eliminates stage 2 and goes directly to ‘ready’ once the voltage threshold is reached at the
end of stage 1. The charge level at this point is about 70 percent. The topping charge typically
takes twice as long as the initial charge.

No trickle charge is applied because the Li-ion is unable to absorb overcharge. Trickl e charge
could cause plating of metallic lithium, a condition that renders the cell unstable. Instead, a
brief topping charge is applied to compensate for the small amount of self-discharge the
battery and its protective circuit consume.

Depending on the charger and the self-discharge of the battery, a topping charge may be
implemented once every 500 hours or 20 days. Typically, the charge kicks in when the open
terminal voltage drops to 4.05V/cell and turns off when it reaches 4.20V/cell agai n.

Figure 4-5: Charge stages of a Li-ion battery.

Increasing the charge current on a Li-ion charger does not shorten the charge time by much. Although the voltage
peak is reached quicker with higher current, the topping charge will take longer.

What if a battery is inadvertently overcharged? Li-ion batteries are designed to operate safely
within their normal operating voltage but become increasingly unstable if charged to higher
voltages. On a charge voltage above 4.30V, the cell causes lithium metal plating on the
anode. In addition, the cathode material becomes an oxidizing agent, loses stability and
releases oxygen. Overcharging causes the cell to heat up.

Much attention has been placed on the safety of the Li-ion battery. Commercial Li-ion battery
packs contain a protection circuit that prevents the cell voltage from going too high while
charging. The typical safety threshold is set to 4.30V/cell. In addition, temperature
sensing disconnects the charge if the internal temperature approaches 90°C (194°F). Most
cells feature a mechanical pressure switch that permanently interrupts the current path if a
safe pressure threshold is exceeded. Internal voltage control circuits cut off the battery at low
and high voltage points.

Exceptions are made on some spinel (manganese) packs containing one or two small cells.
On overcharge, this chemistry produces minimal lithium plating on the anode because most
metallic lithium has been removed from the cathode during normal charging. The cathode
material remains stable and does not generate oxygen unless the cell gets extremely hot.

Important: In case of rupture, leaking electrolyte or any other cause of exposure to the

electrolyte, flush with water immediately. If eye exposure occurs, flush with water for
15 minutes and consult a physician immediately.

Charging the Lithium Polymer Battery

The charge process of a Li-Polymer is similar to that of the Li-ion. Li-Polymer uses dry
electrolyte and takes 3 to 5 hours to charge. Li-ion polymer with gelled electrolyte, on the
other hand, is almost identical to that of Li-ion. In fact, the same charge algorithm can be
applied. With most chargers, the user does not need to know whether the battery being
charged is Li-ion or Li-ion polymer.

Almost all commercial batteries sold under the so-called ‘Polymer’ category are a variety of
the Li-ion polymer using some sort of gelled electrolyte. A low-cost dry polymer battery
operating at ambient temperatures is still some years away.

Charging at High and Low Temperatures

Rechargeable batteries can be used under a reasonably wide temperature range. This,
however, does not automatically mean that the batteries can also be charged at these
temperature conditions. While the use of batteries under hot or cold conditions cannot always
be avoided, recharging time is controlled by the user. Efforts should be made to charge the
batteries only at room temperatures.

In general, older battery technologies such as the NiCd are more tolerant to charging at low
and high temperatures than the more advanced systems. Figure 4-6 indicates the permissible
slow and fast charge temperatures of the NiCd, NiMH, SLA and Li-ion.

Nickel Cadmium
Nickel-Metal Hydride
Lead Acid
Lithium Ion

Figure 4-6: Permissible temperature limits for various batteries.

Older battery technologies are more tolerant to charging at extreme temperatures than newer, more advanced
systems.

NiCd batteries can be fast-charged in an hour or so, however, such a fast charge can only be
applied within temperatures of 5°C and 45°C (41°F and 113°F). More moderate temperatures
of 10°C to 30°C (50°F to 86°F) produce better results. When charging a NiCd bel ow 5°C
(41°F), the ability to recombine oxygen and hydrogen is greatly reduced and pressure build
up occurs as a result. In some cases, the cells vent, releasing oxygen and hydrogen. Not only
do the escaping gases deplete the electrolyte, hydrogen is highly flammable!

Chargers featuring NDV to terminate full-charge provide some level of protection when fast­charging at low temperatures. Because of the battery’s poor charge acceptance at low
temperatures, the charge energy is turned into oxygen and to a lesser amount hydrogen. This
reaction causes cell voltage drop, terminating the charge through NDV detection. When this
occurs, the battery may not be fully charged, but venting is avoided or minimized.

Slow Charge (0.1) Fast Charge (0.5-1C)
0°C to 45°C (32°F to 113°F) 5°C to 45°C (41°F to 113°F)
0°C to 45°C (32°F to 113°F) 10C° to 45°C (50°F to 113°F)
0°C to 45°C (32°F to 113°F) 5C° to 45°C (41°F to 113°F)
0°C to 45°C (32°F to 113°F) 5C° to 45°C (41°F to 113°F)

To compensate for the slower reaction at temperatures below 5°C, a low charge rate of 0.1C
must be applied. Special charge methods are available for charging at cold temperatures.
Industrial batteries that need to be fast-charged at low temperatures include a thermal blanket
that heats the battery to an acceptable temperature. Among commercial batteries, the NiCd is
the only battery that can accept charge at extremely low temperatures.

Charging at high temperatures reduces the oxygen generation. This reduces the NDV effect
and accurate full-charge detection using this method becomes difficult. To avoid overcharge,
charge termination by temperature measurement becomes more practical.

The charge acceptance of a NiCd at higher temperatures is drastically reduced. A battery that
provides a capacity of 100 percent if charged at moderate room temperature can only accept
70 percent if charged at 45°C (113°F), and 45 percent if charged at 60°C (140°F) (se e
Figure 4-7). Similar conditions apply to the NiMH battery. This demonstrates the typically poor
summer performance of vehicular mounted chargers using nickel-based batteries.

Another reason for poor battery performance, especially if charged at high ambient
temperatures, is premature charge cutoff. This is common with chargers that use absolute
temperature to terminate the fast charge. These chargers read the SoC on battery
temperature alone and are fooled when the room temperature is high. The battery may not be
fully charged, but a timely charge cut-off protects the battery from damage due to excess heat.

The NiMH is less forgiving than the NiCd if charged under high and low temperatures. The
NiMH cannot be fast charged below 10°C (45°F), neither can it be slow charged below 0°C
(32°F). Some industrial chargers adjust the charge rate to prevailing temperatures. Price
sensitivity on consumer chargers does not permit elaborate temperature control features.

Figure 4-7: Effects of temperature on NiCd charge acceptance.

Charge acceptance is much reduced at higher temperatures. NiMH cells follow a similar pattern.

The lead acid battery is reasonably forgiving when it comes to temperature extremes, as in
the case of car batteries. Part of this tolerance is credited to the sluggishness of the lead acid
battery. A full charge under ten hours is difficult, if not impossible. The recommended charge
rate at low temperature is 0.3C.

Figure 4-8 indicates the optimal peak voltage at various temperatures when recharging and
float charging an SLA battery. Implementing temperature compensation on the charger to
adjust to temperature extremes prolongs the battery life by up to 15 percent. This is especially
true when operating at higher temperatures.

An SLA battery should never be allowed to freeze. If this were to occur, the battery would be
permanently damaged and would only provide a few cycles when it returned to normal
temperature.

Voltage limit on recharge
Continuous float voltage

Figure 4-8: Recommended voltage limits on recharge and float charge of SL As.

These voltage limits should be applied when operating at temperature extremes.

0°C (32°F) 25°C (77°F) 40°C (104°F)

2.55V/cell 2.45V/cell 2.35V/cell

2.35V/cell or lower 2.30V/cell or lower 2.25V/cell or lower

To improve charge acceptance of SLA batteries in colder temperatures, and avoid thermal
runaway in warmer temperatures, the voltage limit of a charger should be compensated by
approximately 3mV per cell per degree Celsius. The voltage adjustment has a negative
coefficient, meaning that the voltage threshold drops as the temperature increases. For
example, if the voltage limit is set to 2.40V/cell at 20°C, the setting should be lowered to

2.37V/cell at 30°C and raised to 2.43V/cell at 10°C. This represents a 30mV correction per
cell per 10 degrees Celsius.

The Li-ion batteries offer good cold and hot temperature charging performa nce. Some cells
allow charging at 1C from 0°C to 45°C (32°F to 113°F). Most Li-ion cells prefer a lower charge
current when the temperature gets down to 5°C (41°F) or colder. Charging below freezing
must be avoided because plating of lithium metal could occur.

Ultra-fast Chargers

Some charger manufacturers claim amazingly short charge times of 30 minutes or less. With
well-balanced cells and operating at moderate room temperatures, NiCd batteries designed
for fast charging can indeed be charged in a very short time. This is done by simply dumping
in a high charge current during the first 70 percent of the charge cycle. Some NiCd batteries
can take as much a 10C, or ten times the rated current. Precise SoC detection and
temperature monitoring are essential.

The high charge current must be reduced to lower levels in the second phase of the charge
cycle because the efficiency to absorb charge is progressively reduced as the battery moves
to a higher SoC. If the charge current remains too high in the later part of the charge cycle,
the excess energy turns into heat and pressure. Eventually venting occurs, releasing
hydrogen gas. Not only do the escaping gases deplete the electrolyte, they are also highly
flammable!

Several manufacturers offer chargers that claim to fully charge NiCd batteries in half the time
of conventional chargers. Based on pulse charge technology, these chargers intersperse one
or several brief discharge pulses between each charge pulse. This promotes the
recombination of oxygen and hydrogen gases, resulting in reduced pressure buildup and a
lower cell temperature. Ultra-fast-chargers based on this principle can charg e a nickel-based
battery in a shorter time than regular chargers, but only to about a 90 percent SoC. A trickle
charge is needed to top the charge to 100 percent.

Pulse chargers are known to reduce the crystalline formation (memory) of nickel-based
batteries. By using these chargers, some improvement in battery performance can be realized,
especially if the battery is affected by memory. The pulse charge method does not replace a
periodic full discharge. For more severe crystalline formation on nickel-based batteries, a full
discharge or recondition cycle is recommended to restore the battery.

Ultra-fast charging can only be applied to healthy batteries and those designed for fast
charging. Some cells are simply not built to carry high current and the conductive path heats

up. The battery contacts also take a beating if the current handling of the spring-loaded
plunger contacts is underrated. Pressing against a flat metal surface, these contacts may
work well at first, and then wear out prematurely. Often, a fine and almost invisible crater
appears on the tip of the contact, which causes a high resistive path or forms an isolator. The
heat generated by a bad contact can melt the plastic.

Another problem with ultra-fast charging is servicing aged batteries that commonly have high
internal resistance. Poor conductivity turns into heat, which further deteriorates the cells.
Battery packs with mismatched cells pose another challenge. The weak cells holding less
capacity are charged before those with higher capacity and start to heat up. This process
makes them vulnerable to further damage.

Many of today’s fast chargers are designed for the ideal battery. Charging less than perfect
specimens can create such a heat buildup that the plastic housing starts to distort. Provisions
must be made to accept special needs batteries, albeit at lower charging speeds.
Temperature sensing is a prerequisite.

The ideal ultra-fast charger first checks the battery type, measures its SoH and then applies a
tolerable charge current. Ultra-high capacity batteries and those that have aged are identified,
and the charge time is prolonged because of higher internal resistance. Such a charger would
provide due respect to those batteries that still perform satisfactorily but are no longer ‘spring
chickens’.

The charger must prevent excessive temperature build-up. Sluggish heat detection, especially
when charging takes place at a very rapid pace, makes it easy to overcharge a battery before
the charge is terminated. This is especially true for chargers that control fast charge using
temperature sensing alone. If the temperature rise is measured right on the skin of the cell,
reasonably accurate SoC detection is possible. If done on the outside surface of the battery
pack, further delays occur. Any prolonged exposure to a temperature of 45°C (113°F) harms
the battery.

New charger concepts are being studied which regulate the cha rge current according to the
battery's charge acceptance. On the initial charge of an empty battery when the charge
acceptance is high and little gas is generated, a very high charge current can be applied.
Towards the end of a charge, the current is tapered down.

Charge IC Chips

Newer battery systems demand more complex chargers than batteries with olde r chemistries.
With today’s charge IC chips, designing a charger has been simplified. These ch ips apply
proven charge algorithms and are capable of servicing all major battery chemistries. As the
price of these chips decreases, design engineers make more use of this product. With the
charge IC chip, an engineer can focus entirely on the portable equipment rather than devoting
time to developing a charging circuit.

The charge IC chips have some limitations, however. The charge algorithm is fixed and does
not allow fine-tuning. If a trickle charge is needed to raise a Li-ion that has dropped below

2.5V/cell to its normal operating voltage, the charge IC may not be able to perform this
function. Similarly, if an ultra-fast charge is needed for nickel-based batteries, the charge IC
applies a fixed charge current and does not take into account the SoH of the battery.
Furthermore, a temperature compensated charge would be difficult to administer if the IC
chips do not provide this feature.

Using a small micro controller is an alternative to selecting an off-the-shelf charge IC. The
hardware cost is about the same. When opting for the micro controller, custom firmware will
be needed. Some extra features can be added with little extra cost. They are fast charging
based on the SoH of the battery. Ambient temperatures can also be taken into account.

Whether an IC chip or micro controller is used, peripheral components are required consisting
of solid-state switches and a power supply.

Chapter 5: Discharge Methods

The purpose of a battery is to store energy and release it at the appropriate time in a
controlled manner. Being capable of storing a large amount of energy is one thing; the ability
to satisfy the load demands is another. The third criterion is being able to deliver all available
energy without leaving precious energy behind when the equipment cuts off.

In this chapter, we examine how different discharge methods can affect the deliverance of
power. Further, we look at the load requirements of various portable devices and evaluate the
performance of each battery chemistry in terms of discharge.

C-rate

The charge and discharge current of a battery is measured in C-rate. Most portable batteries,
with the exception of the lead acid, are rated at 1C. A discharge of 1C draws a current equal
to the rated capacity. For example, a battery rated at 1000mAh provides 1000mA for one hour
if discharged at 1C rate. The same battery discharged at 0.5C provides 500mA for two hours.
At 2C, the same battery delivers 2000mA for 30 minutes. 1C is often referred to as a one-hour
discharge; a 0.5C would be a two-hour, and a 0.1C a 10 hour discharge.

The capacity of a battery is commonly measured with a battery analyzer. If the analyzer’s
capacity readout is displayed in percentage of the nominal rating, 100 percent is shown if
1000mA can be drawn for one hour from a battery that is rated at 1000mAh. If the battery only
lasts for 30 minutes before cut-off, 50 percent is indicated. A new battery sometimes provides
more than 100 percent capacity. In such a case, the battery is conservatively rated and can
endure a longer discharge time than specified by the manufacturer.

When discharging a battery with a battery analyzer that allows setting different discharge C­rates, a higher capacity reading is observed if the battery is discharged at a lower C-rate and
vice versa. By discharging the 1000mAh battery at 2C, or 2000mA, the analyzer is scaled to
derive the full capacity in 30 minutes. Theoretically, the capacity reading should be the same
as a slower discharge, since the identical amount of energy is dispensed, only over a shorter
time. Due to energy loss that occurs inside the battery and a drop in voltage that causes the
battery to reach the low-end voltage cut-off sooner, the capacity reading is lower and may be
97 percent. Discharging the same battery at 0.5C, or 500mA over two hours would increase
the capacity reading to about 103 percent.

The discrepancy in capacity readings with different C-rates largely depends on the internal
resistance of the battery. On a new battery with a good load current characteristic or low
internal resistance, the difference in the readings is only a few percentage points. On a
battery exhibiting high internal resistance, the difference in capacity readings could swing
plus/minus 10 percent or more.

One battery that does not perform well at a 1C discharge rate is the SLA. To obtain a practical
capacity reading, manufacturers commonly rate these batteries at 0.05C or 20 hour discharge.
Even at this slow discharge rate, it is often difficult to attain 100 percent capacity. By
discharging the SLA at a more practical 5h discharge (0.2C), the capacity readings are
correspondingly lower. To compensate for the different readings at various discharge currents,
manufacturers offer a capacity offset.

Applying the capacity offset does not improve battery performance; it merely adjusts the
capacity calculation if discharged at a higher or lower C-rate than specified. The battery
manufacturer determines the amount of capacity offset recommended for a given battery type.

Li-ion/polymer batteries are electronically protected against high discharge currents.
Depending on battery type, the discharge current is limited somewhere between 1C and 2C.

This protection makes the Li-ion unsuitable for biomedical equipment, power tools and high­wattage transceivers. These applications are commonly reserved for the NiCd battery.

Depth of Discharge

The typical end-of-discharge voltage for nickel-based batteries is 1V/cell. At that voltage level,
about 99 percent of the energy is spent and the voltage starts to drop rapidly if the discharge
continues. Discharging beyond the cut-off voltage must be avoided, especially under
heavy load.

Since the cells in a battery pack cannot be perfectly matched, a negative voltage potential
(cell reversal) across a weaker cell occurs if the discharge is allowed to continue beyond the
cut-off point. The larger the number of cells connected in series, the greater the likelihood of
this occurring.

A NiCd battery can tolerate a limited amount of cell reversal, which is typically about 0.2V.
During that time, the polarity of the positive electrode is reversed. Such a condition can only
be sustained for a brief moment because hydrogen evolution occurs on the positive electrod e.
This leads to pressure build-up and cell venting.

If the cell is pushed further into voltage reversal, the polarity of both electrodes is being
reversed, resulting in an electrical short. Such a fault cannot be corrected and the pack will
need to be replaced.

On battery analyzers that apply a secondary discharge (recondition), the current is controlled
to assure that the maximum allowable current, while in sub-discharge range, does not exceed
a safe limit. Should a cell reversal develop, the current would be low enough as not to cause
damage. A cell breakdown through recondition is possible on a weak or aged pack.

If the battery is discharged at a rate higher than 1C, the more common end-of-discharge point
of a nickel-based battery is 0.9V/cell. This is done to compensate for the voltage drop induced
by the internal resistance of the cell, the wiring, protection devices and contacts of the pack. A
lower cut-off point also delivers better battery performance at cold temperatures.

The recommended end-of-discharge voltage for the SLA is 1.75V/cell. Unlike the prefe rred
flat discharge curve of the NiCd, the SLA has a gradual voltage drop with a rapid drop
towards the end of discharge (see Figure 5-1). Although this steady decrease in voltage is a
disadvantage, it has a benefit because the voltage level can be utilized to display the state-of­charge (SoC) of a battery. However, the voltage readings fluctuate with load and the SoC
readings are inaccurate.

Figure 5-1: Discharge characteristics of NiCd, NiMH and SLA batteries.

While voltage readings to measure the SoC are not practical on nickel-based batteries, the SLA enables some level
of indication as to the SoC.

°C (77°F) with respect to the depth of discharge is:

• 150 – 200 cycles with 100 percent depth of discharge (full discharge)

• 400 – 500 cycles with 50 percent depth of discharge (partial discharge)

• 1000 and more cycles with 30 percent depth of discharge (shallow discharge)

The SLA should not be discharged beyond 1.75V per cell, nor can it be stored in a discharged
state. The cells of a discharged SLA sulfate, a condition that renders the battery useless if left
in that state for a few days.

The Li-ion typically discharges to 3.0V/cell. The spinel and coke versions can be discharged
to 2.5V/cell. The lower end-of-discharge voltage gains a few extra percentage points. Since
the equipment manufacturers cannot specify which battery type may be used, most
equipment is designed for a three-volt cut-off.

Caution should be exercised not to discharge a lithium-based battery too low. Discharging a
lithium-based battery below 2.5V may cut off the battery’s protection circuit. Not all chargers
accommodate a recharge on batteries that have gone to sleep because of low voltage.

Some Li-ion batteries feature an ultra-low voltage cut-off that permanently disconnects the
pack if a cell dips below 1.5V. This precaution prohibits recharge if a battery has dwelled in an
illegal voltage state. A very deep discharge may cause the formation of copper shunt, which
can lead to a partial or total electrical short. The same occurs if the cell is driven into negative
polarity and is kept in that state for a while. A fully discharged battery should be charged at

0.1C. Charging a battery with a copper shunt at the 1C rate would cause excessive heat.
Such a battery should be removed from service.

Discharging a battery too deeply is one problem; equipment that cuts off before the energy is
consumed is another. Some portable devices are not properly tuned to harvest the optimal

energy stored in a battery. Valuable energy may be left behind if the voltage cut-off-point is
set too high.

Digital devices are especially demanding on a battery. Momentary pulsed loads cause a brief
voltage drop, which may push the voltage into the cut-off region. Batteries with high internal
resistance are particularly vulnerable to premature cut-off. If such a battery is removed from
the equipment and discharged to the appropriate cut-off point with a battery analyzer on DC
load, a high level of residual capacity can still be obtained.

Most rechargeable batteries prefer a partial rather than a full discharge. Repeated full
discharge robs the battery of its capacity. The battery chemistry which is most affected by
repeat deep discharge is lead acid. Additives to the deep-cycle version of the lead acid
battery compensate for some of the cycling strain.

Similar to the lead acid battery, the Li-ion battery prefers shallow over repetitive deep
discharge cycles. Up to 1000 cycles can be achieved if the battery is only partially discharged.
Besides cycling, the performance of the Li-ion is also affected by aging. Capacity loss through
aging is independent of use. However, in daily use, there is a combination of both.

The NiCd battery is least affected by repeated full discharge cycles. Several thousand
charge/discharge cycles can be obtained with this battery system. This is the reason why the
NiCd performs well on power tools and two-way radios that are in constant use. The NiMH is
more delicate with respect to repeated deep cycling.

Pulse Discharge

Battery chemistries react differently to specific loading requirements. Discharge loads range
from a low and steady current used in a flashlight, to intermittent high current bursts in a
power tool, to sharp current pulses required for digital communications equipment, to a
prolonged high current load for an electric vehicle traveling at highway speed. Because
batteries are chemical devices that must convert higher-level active materials into an alternate
state during discharge, the speed of such transaction determines the load characteristics of a
battery. Also referred to as concentration polarization, the nickel and lithium-based batteries
are superior to lead-based batteries in reaction speed. This reflects in good lo ad
characteristics.

The lead acid battery performs best at a slow 20-hour discharge. A pulse discharge also
works well because the rest periods between the pulses help to disperse the depleted acid
concentrations back into the electrode plate. In terms of capacity, these two discharge
methods provide the highest efficiency for this battery chemistry.

A discharge at the rated capacity of 1C yields the poorest efficiency for the lead acid battery.
The lower level of conversion, or increased polarization, manifests itself in a momentary
higher internal resistance due to the depletion of active material in the reaction.

Different discharge methods, notably pulse discharging, also affect the longevity of some
battery chemistries. While NiCd and Li-ion are robust and show minimal deterioration when
pulse discharged, the NiMH exhibits a reduced cycle life when powering a digital load.

In a recent study, the longevity of NiMH was observed by discharging these batteries with
analog and digital loads. In both tests, the battery discharged to 1.04V/cell. The analog
discharge current was 500mA; the digital mode simulated the load requirem ents of the Global
System for Mobile Communications (GSM) protocol and applied 1.65-ampere peak current for
12 ms every 100 ms. The current in between the peaks was 270mA. (Note that the GSM
pulse for voice is about 550 ms every 4.5 ms).

With the analog discharge, the NiMH wore out gradually, providing an above average service
life. At 700 cycles, the battery still provided 80 percent capacity. By contrast, the cells faded
more rapidly with a digital discharge. The 80 percent capacity threshold was reached after
only 300 cycles. This phenomenon indicates that the kinetic characteristics for the NiMH
deteriorate more rapidly with a digital rather than an analog load.

Discharging at High and Low Temperature

Batteries function best at room temperature. Operating batteries at an elevated temperature
dramatically shortens their life. Although a lead acid battery may deliver the highest capacity
at temperatures above 30°C (86°F), prolonged use under such conditions decreases the life
of the battery.

Similarly, a Li-ion performs better at high temperatures. Elevated temperatures temporarily
counteracts the battery’s internal resistance, which is a result of a ging. The energy gain is
short-lived because elevated temperature promotes aging by further increasing the internal
resistance.

There is one exception to running a battery at high temperature — it is the lithium polymer
with dry solid polymer electrolyte, the true ‘plastic battery’. While the commercial Li-ion
polymer uses some moist electrolyte to enhance conductivity, the dry solid polymer version
depends on heat to enable ion flow. This requires that the battery core be kept at an operation
temperature of 60°C to 100°C.

The dry solid polymer battery has found a niche market as backup power in warm climates.
The battery is kept at the operating temperature with built-in heating elements. During normal
operation, the core is kept warm with power derived from the utility grid. Only on a power
outage would the battery need to provide power to maintain its own heat. To minimize heat
loss, the battery is insulated.

The Li-ion polymer as standby battery is said to outperform VRLA batteries in terms of size
and longevity, especially in shelters in which the temperature cannot be controlled. The high
price of the Li-ion polymer battery remains an obstacle.

The NiMH chemistry degrades rapidly if cycled at higher ambient temperatures. Optimum
battery life and cycle count are achieved at 20°C (68°F). Repeated charging and discharging
at higher temperatures will cause irreversible capacity loss. For example, if operated at 30°C
(86°F), the cycle life is reduced by 20 percent. At 40°C (104°F), the loss jumps to a whopping
40 percent. If charged and discharged at 45°C (113°F), the cycle life is only half of what can
be expected if used at moderate room temperature. The NiCd is also affected by high
temperature operation, but to a lesser degree.

At low temperatures, the performance of all battery chemistries drops drastically. While -20°C
(-4°F) is threshold at which the NiMH, SLA and Li-ion battery stop functioning, the NiCd can
go down to -40°C (-40°F). At that frigid temperature, the NiCd is limited to a discharge rate of

0.2C (5 hour rate). There are new types of Li-ion batteries that are said to operate down to ­40°C.

It is important to remember that although a battery may be capable of operating at cold
temperatures, this does not automatically mean it can also be charged under those conditions.
The charge acceptance for most batteries at very low temperatures is extremely confined.
Most batteries need to be brought up to temperatures above the freezing point for charging.
The NiCd can be recharged at below freezing provided the charge rate is reduced to 0.1C.

Part Two

You and the Battery

Chapter 6: The Secrets of Battery Runtime

Is the runtime of a portable device directly related to the size of the battery and the energy it
can hold? In most cases, the answer is yes. But with digital equipment, the length of time a
battery can operate is not necessarily linear to the amount of energy stored in the battery.

In this chapter we examine why the specified runtime of a portable device cannot always be
achieved, especially after the battery has aged. We address the four renegades that are
affecting the performance of the battery. They are: declining capacity, increasing internal
resistance, elevated self-discharge, and premature voltage cut-off on discharge.

Declining Capacity

The amount of charge a battery can hold gradually decreases due to usage, aging and, with
some chemistries, lack of maintenance. Specified to deliver about 100 percent capacity when
new, the battery eventually requires replacement when the capacity drops to the 70 or
60 percent level. The warranty threshold is typically 80 percent.

The energy storage of a battery can be divided into three imaginary sections consisting of
available energy, the empty zone that can be refilled and the rock content that has become
unusable. Figure 6-1 illustrates these three sections of a battery.

In nickel-based batteries, the rock content may be in the form of crystalline formation, also
known as memory. Deep cycling can often restore the capacity to full service. Also known as
‘exercise’, a typical cycle consists of one or several discharges to 1V/cell with subsequent
discharges.

Figure 6-1: Battery charge capacity.

Three imaginary sections of a battery consisting of available
energy, empty zone and rock content.

With usage and age, the rock content grows. Without regular
maintenance, the user may end up carrying rocks instead of
batteries.

The loss of charge acceptance of the Li-ion/polymer batteries is due to cell oxidation, which
occurs naturally during use and as part of aging. Li-ion batteries cannot be restored with
cycling or any other external means. The capacity loss is permanent because the metals used
in the cells are designated to run for a specific time only and are being consumed during their
service life.

Performance degradation of the lead acid battery is often caused by sulfation, a thin layer that

A

forms on the negative cell plates, which inhibits current flow. In addition, there is grid
corrosion that sets in on the positive plate. With sealed lead acid batteries, the issue of water
permeation, or loss of electrolyte, also comes into play. Sulfation can be reversed to a certain
point with cycling and/or topping charge but corrosion and permeation are permanent. Adding
water to a sealed lead acid battery may help to restore operation but the long-term results are
unpredictable.

Increasing Internal Resistance

To a large extent, the internal resistance, also known as impedance, determines the
performance and runtime of a battery. If measured with an AC signal, the internal resistance
of a battery is also referred to as impedance. High internal resistance curtails the flow of
energy from the battery to the equipment.

A battery with simulated low and high internal resistance is illustrated below. While a battery
with low internal resistance can deliver high current on demand, a battery with high resistance
collapses with heavy current. Although the battery may hold sufficient capacity, the voltage
drops to the cut-off line and the ‘low battery’ indicator is triggered. The equipment stops
functioning and the remaining energy is undelivered.

Figure 6-2: Effects of impedance on
battery load.

battery with low impedance provides
unrestricted current flow and delivers all
available energy. A battery with high impedance
cannot deliver high-energy bursts due to a
restricted path, and equipment may cut off
prematurely.

NiCd has the lowest internal resistance of all commercial battery systems, even after
delivering 1000 cycles. In comparison, NiMH starts with a slightly higher resistance and the
readings increase rapidly after 300 to 400 cycles.

Maintaining a battery at low internal resistance is important, especially with digital devices that
require high surge current. Lack of maintenance on nickel-based batteries can increase the
internal resistance. Readings of more than twice the normal resistance have been observed
on neglected NiCd batteries. After applying a recondition cycle with the Cadex 7000 Series
battery analyzer, the readings on the batteries returned to normal. Reconditioning clears the
cell plates of unwanted crystalline formations, which restores proper current flow.

Li-ion offers internal resistance characteristics that a re between th ose of NiMH and NiCd.
Usage does not contribute much to the increase in resistance, but aging does. The typical life
span of a Li-ion battery is two to three years, whether it is used or not. Cool storage and
keeping the battery in a partially charged state when not in use retard the aging process.

The internal resistance of the Li-ion batteries cannot be improved with cycling. The cell
oxidation, which causes high resistance, is non-reversible. The ultimate cause of failure is
high internal resistance. Energy may still be present in the battery, but it can no longer be
delivered due to poor conductivity.

With effort and patience, lead acid batteries can sometimes be improved by cycling or
applying a topping and/or equalizing charge. This reduces the current-inhibiting sulfation layer
but does not reverse grid corrosion.

Figure 6-3 compares the voltage signature and correspondin g runtime of a battery with low,
medium and high internal resistance when connected to a digital load. Similar to a soft ball
that easily deforms when squeezed, the voltage of a battery with high internal resistance
modulates the supply voltage and leaves the imprint of the load. The current pulses push the
voltage towards the end-of-discharge line, resulting in a premature cut-off.

When measuring the battery with a voltmeter after the equipment has cut off and the load is
removed, the terminal voltage commonly recovers and the voltage reading appears normal.
This is especially true of nickel-based batteries. Measuring the open terminal voltage is an
unreliable method to establish the state-of-charge (SoC) of the battery.

A battery with high impedance may perform well if loaded with a low DC current such as a
flashlight, portable CD player or wall clock. With such a gentle load, virtually all of the stored
energy can be retrieved and the deficiency of high impedance is masked.

Figure 6-3: Discharge curve.

This chart compares the runtime of batteries with similar capacities under low, medium and high impedance when
connected to a pulsed load.

The internal resistance of a battery can be measured with dedicated impedance meters.
Several methods are available, of which the most common are applying DC loads and AC
signals. The AC method may be done with different frequencies. Depending on the level of
capacity loss, each technique provides slightly different readings. On a good battery, the
measurements are reasonably close; on a weak battery, the readings between the methods
may disperse more drastically.

Modern battery analyzers offer internal resistance measurements as a battery quick-test.
Such tests can identify batteries that would fail due to high internal resistance, even though
the capacity may still be acceptable. Internal battery resistance measurements are available
in the Cadex 7000 Series battery analyzers. (See Chapter 9: Internal Battery Resistance.)

Elevated Self-Discharge

A

All batteries exhibit a certain amount of self-discharge; the highest is visible on nickel-based
batteries. As a rule, a nickel-based battery discharges 10 to 15 percent of its capacity in the
first 24 hours after charge, followed by 10 to 15 percent every month thereafter.

The self-discharge on the Li-ion battery is lower compared to the nickel-based systems. The
Li-ion self-discharges about five percent in the first 24 hours and one to two percent thereafter.
Adding the protection circuit increases the self-discharge to ten percent per month.

One of the best batteries in terms of self-discharge is the lead acid system; it only self­discharges five percent per month. It should be noted, however, that the lead acid family has
also the lowest energy density among current battery systems. This makes the system
unsuitable for most hand-held applications.

At higher temperatures, the self-discharge on all battery chemistries increases. Typically, the
rate doubles with every 10°C (18°F). Large energy losses occur through self-discharge if a
battery is left in a hot vehicle. On some older batteries, stored energy may get lost during the
course of the day through self-discharge rather than actual use.

The self-discharge of a battery increases with age and usage. For example, a NiMH battery is
good for 300 to 400 cycles, whereas a NiCd adequately performs over 1000 cycles before
high self-discharge affects the performance of the battery. Once a battery exhibits high self­discharge, no remedy is available to reverse the effect. Factors that accelerate self-discharge
on nickel-based batteries are damaged separators (induced by excess crystalline formation,
allowing the packs to cook while charging), and high cycle count, which promotes swelling in
the cell.

Figure 6-4: Effects of high load impedance.

battery may gradually self-discharge as a result of high
temperature, high cycle count and age. In older batteries,
stored energy may be lost during the course of the day
through self-discharge rather than actual use.

At present, no simple quick-test is available to measure the self-discharge of a battery. A
battery analyzer can be used by first reading the initial capacity after full charge, then
measuring the capacity again after a rest period of 12 hours. The Cadex 7000 Series
performs this task automatically. In the future, quick test methods may be available that are
able to measure the self-discharge of a battery within a few seconds.

Premature Voltage Cut-off

Some portable equipment does not fully utilize the low-end voltage spectrum of a battery. The
equipment cuts off before the designated end-of-discharge voltage is reached and some
precious battery power remains unused.

A high cut-off voltage problem is more widespread than is commonly assumed. For example,
a certain brand of mobile phone that is powered with a single-cell Li-ion battery cuts off at

3.3V. The Li-ion can be designed to be used to 3V and lower. With a discharge to 3.3V, only
about 70 percent of the expected 100 percent capacity is utilized. Another mobile phone using
NiMH and NiCd batteries cuts off at 5.7V. The four-cell nickel-based batteries are designed to
discharge to 5V.

Figure 6-5: Illustration of equipment with
high cut-off voltage.

Some portable devices do not utilize all available
battery power and leave precious energy behind.

When discharging these batteries to their respective end-of-discharge threshold with a battery
analyzer after the equipment has cut off, up to 60 percent residual capacity readings can be
retrieved. High residual capacity is prevalent with batteries that have elevated internal
resistance and are operated at warm ambient temperatures. Digital devices that load the
battery with current bursts are more receptive to premature voltage cut-off than analog
equipment.

A ’high cut-off voltage’ is mostly equipment related. In some cases the problem of premature
cut-off is induced by a battery with low voltage. A low table voltage is often caused by a
battery pack that contains a cell with an electrical short. Memory also causes a decrease in
voltage; however, this is only present in nickel-based systems. In addition, elevated
temperature lowers the voltage level on all battery systems. Voltage reduction due to high
temperatures is temporary and normalizes once the battery cools down.

Chapter 7: The ‘Smart’ Battery

Aspeaker at a battery seminar remarked that, “The battery is a wild animal and artificial
intelligence domesticates it.” An ordinary or ‘dumb’ battery has the inherit problem of not
being able to display the amount of reserve energy it holds. Neither weight, color, nor size
provides any indication of the battery’s state-of-charge (SoC) and state-of-health (SoH). The
user is at the mercy of the battery when pulling a freshly charged battery from the charger.

Help is at hand. An increasing number of today’s rechargeable batteries are made ‘smart’.
Equipped with a microchip, these batteries are able to communicate with the charger and
user alike to provide statistical information. Typical applications for ‘smart’ batteries are
notebook computers and video cameras. Increasingly, these batteries are also used in
advanced biomedical devices and defense applications.

There are several types of ‘smart’ batteries, each offering different complexities, performance
and cost. The most basic ‘smart’ battery may only contain a chip to identify its chemistry and
tell the charger which charge algorithm to apply. Other batteries claim to be smart simply
because they provide protection from overcharging, under-discharging and short-circuiting. In
the eyes of the Smart Battery System (SBS) forum, these batteries cannot be called ‘smart’.

What then makes a battery ‘smart’? Definitions still vary among organizations and
manufacturers. The SBS forum states that a ‘smart’ battery must be able to provide SoC
indications. Benchmarq was the first company to commercialize the concept of the battery
fuel gauge technology. Early IC chips date back to 1990. Several manufacturers followed suit
and produced ‘smart’ chips for batteries.

During the early nineties, numerous ‘smart’ battery architectures with a SoC read-out have
emerged. They range from the single wire system, the two-wire system and the system
management bus (SMBus). Most two-wire systems are based on the SMBus protocol. This
book will address the single wire system and the SMBus.

The Single Wire Bus

The single wire system is the simpler of the two and does all the data communications
through one wire. A battery equipped with the single wire system uses only three wires, the
positive and negative battery terminals and the data terminal. For safety reasons, most
battery manufacturers run a separate wire for temperature sensing. Figure 7-1 shows the
layout of a single wire system.

The modern single wire system stores battery-specific data and tracks battery parameters,
including temperature, voltage, current and remaining charge. Because of simplicity and
relatively low hardware cost, the single wire enjoys a broad market acceptance for high-end
mobile phones, two-way radios and camcorders.

Most single wire systems do not have a common form factor; neither do they lend themselves
to standardized SoH measurements. This produces problems for a universal charger concept.
The Benchmarq single wire solution, for example, cannot measure current directly; it must be
extracted from a change in capacity over time.

In addition, the single wire bus allows battery SoH measurement only when the host is
‘married’ to a designated battery pack. Such a fixed host-battery relationship is feasible with
notebook computers, mobile phones or video cameras, provided the appropriate OEM battery
is used. Any discrepancy in the battery type from the original will make the system unreliable
or will provide false readings.

Figure 7-1: Single wire system of a
‘smart’ battery.

Only one wire is needed for data communications.
Rather than supplying the clock signal from the
outside, the battery includes an embedded clock
generator. For safety reasons, most battery
manufacturers run a separate wire for temperature
sensing.

The SMBus

The SMBus is the most complete of all systems. It represents a large effort from the portable
electronic industry to standardize to one communications protocol and one set o f data. The
SMBus is a two-wire interface system through which simple power-related chips can
communicate with the rest of the system. One wire handles the data; the second is the clock.
It uses I²C as its backbone. Defined by Philips, the I²C is a synchronous multi-drop bi­directional communications system, which operates at a speed of up to 100 kilohertz (kHz).

The Duracell/Intel SBS, in use today, was standardized in 1993. In previous years, computer
manufacturers had developed their own proprietary ‘smart’ batteries. With the new SBS
specification, a broader interface standard was made possible. This reduces the hurdles of
interfering with patents and intellectual properties.

In spite of an agreed standard, many large computer manufacturers, such as IBM, Compaq
and Toshiba, have retained their proprietary batteries. The reason for going their own way is
partly due to safety, performance and form factor. Manufacturers claim that they cannot
guarantee safe and enduring performance if a non-brand battery is used. To make the
equipment as compact as possible, the manufacturers explain that the common form factor
battery does not optimally fit their available space. Perhaps the leading motive for using their
proprietary batteries is pricing. In the absence of competition, these batteries can be sold for a
premium price.

The early SMBus batteries had problems of poor accuracy. Electronic circuits did not provide
the necessary resolution; neither was real time reporting of current, voltage and temperature
adequate. On some batteries, the specified accuracy could only be achieved if the battery
was new, operated at room temperature and was discharged at a steady rate of 1C.
Operating in adverse temperatures or discharging at uneven loads reduced the accuracy
dramatically. Most loads for portable equipment are uneven and fluctuate with power demand.
There are power surges on a laptop at start up and refresh, high inrush currents on
biomedical equipment during certain procedures and sharp pulse bursts on digital
communications devices on transmit.

In the absence of a reliable reporting system on the older generation of ‘smart’ batteries,
capacity estimation was inaccurate. This resulted in powering down the equipment before the
battery was fully depleted, leaving precious energy behind. Most batteries introduced in the
late 1990s have resolved some or all of these deficiencies. Further improvements will be
necessary.

Design— The design philosophy behind the SMBus battery is to remove the charge control
from the charger and assign it to the battery. With a true SMBus system, the battery becomes
the master and the charger serves as a slave that must follow the dictates of the battery. This
is done out of concerns over charger quality, compatibility with new and old battery
chemistries, administration of the correct amount of charge currents and accurate full-charge
detection. Simplifying the charging for the user is an issue that is important when considering
that some battery packs share the same footprint but contain radically different chemistries.

The SMBus system allows new battery chemistries to be introduced without the charger
becoming obsolete. Because the battery controls the charger, the battery manages the
voltage and current levels, as well as cut-off thresholds. The user does not need to know
which battery chemistry is being used.

The analogy of charging a ‘smart’ and ‘dumb’ battery can be made with the eating habits of an
adult and a baby. Charging a ‘smart’ battery resembles the eating choices of a responsible
adult who knows best what food to select how much to take. The baby, in on the other hand,
has limited communications skills in expressing the type and amount of food desired. Putting
this analogy in parallel with charging batteries, the charger servicing ‘dumb’ batteries can only
observe the approximate SoC level and avoid overcharge conditions.

Architecture — An SMBus battery contains permanent and temporary data. The permanent
data is programmed into the battery at the time of manufacturing and include battery ID
number, battery type, serial number, manufacturer’s name and date of manufacture. The
temporary data is acquired during use and consists of cycle count, user pattern and
maintenance requirements. Some of the temporary data is being replaced and renewed
during the life of the battery.

The SMBus is divided into Level 1, 2 and 3. Level 1 has been eliminated because it does not
provide chemistry independent charging. Level 2 is designed for in-circuit charging. A laptop
that charges its battery within the unit is a typical example of Level 2. Another application of
Level 2 is a battery that contains the charging circuit within the pack. Level 3 is reserved for
full-featured external chargers.

Most external SMBus chargers are based on Level 3. Unfortunately, this level is complex and
the chargers are costly to manufacture. Some lower cost chargers have emerged that
accommodate SMBus batteries but are not fully SBS compliant. Manufacturers of SMBus
batteries do not readily endorse this shortcut. Safety is always a concern, but customers buy
these economy chargers because of the lower price.

Figure 7-2: Two-wire SMBus system.

The SMBus is based on a two-wire system using
a standardized communications protocol. This
system lends itself to standardized state-of­charge and state-of-health measurements.

Serious industrial battery users operating biomedical instruments, data collection devices and
survey equipment use Level 3 chargers with full-fledged charge protocol. No shortcuts are
applied. To assure compatibility, the charger and battery are matched and only approved
packs are used. The need to test and approve the marriage between specific battery and
charger types is unfortunate given that the ‘smart’ battery is intended to be universal.

Among the most popular SMBus batteries for portable computers are the 35 and 202 form­factors. Manufactured by Sony, Hitachi, GP Batteries, Moltech (formerly Energizer), Moli
Energy and many others, this battery works (should work) in all portable equipment designed
for this system.

Figure 7-3 illustrates the 35 and 202 series ‘smart' batteries. Although the ‘35’ has a smaller
footprint compared to the ‘202’, most chargers are designed to accommodate all sizes,
provided the common five-prong knife connector is used.

Figure 7-3: 35 and 202 series ‘smart’ batteries featuring SMBus.

Available in NiCd, NiMH and Li-ion chemistries, these batteries are used for mobile computing, biomedical

instruments and high-end survey equipment. The same form factor also accommodates NiCd and NiMH chemistries
but without SMBus (‘dumb’).

Negatives of the SMBus — Like any good invention, the SMBus battery has some serious
downsides that must be addressed. For starters, the ‘smart’ battery costs about 25 percent
more than the ‘dumb’ equivalent. In addition, the ‘smart’ battery was intended to simplify the
charger, but a full-fledged Level 3 charger costs substantially more than a regular dumb
model.

A more serious issue is maintenance requirements, better known as capa city re-learning. This
procedure is needed on a regular basis to calibrate the battery. The Engineering Manager of
Moli Energy, a large Li-ion cell manufacturer commented, “With the Li-ion battery we have
eliminated the memory effect, but are we introducing digital memory with the SMBus battery?”

Why is calibration needed? The answer is in correcting the tracking errors that occur between
the battery and the digital sensing circuit during use. The most ideal battery use, as far as
fuel-gauge accuracy is concerned, is a full charge followed by a full discharge at a constant
1C rate. This ensures that the tracking error is less than one percent per cycle. However, a
battery may be discharged for only a few minutes at a time and commonly at a lower C-rate
than 1C. Worst of all, the load may be uneven and vary drastically. Eventually, the true
capacity of the battery no longer synchronizes with the fuel gauge and a full charge and
discharge are needed to ‘re-learn’ or calibrate the battery.

How often is calibration needed? The answer lies in the type of battery application. For
practical purposes, a calibration is recommended once every three months or after every
40 short cycles. Long storage also contributes to errors because the circuit cannot accurately
compensate for self-discharge. After extensive storage, a calibration cycle is recommended
prior to use.

Many applications apply a full discharge as part of regular use. If this occurs regularly, no
additional calibration is needed. If a full discharge has not occurred for a few months and the
user notices the fuel gauge losing accuracy, a deliberate full discharge on the equipment is
recommended. Some intelligent equipment advises the user when a calibrating discharge is
needed. This is done by measuring the tracking error and estimating the discrepancy between
the fuel gauge reading and that of the chemical battery.

What happens if the battery is not calibrated regularly? Can such a battery be used in
confidence? Most ‘smart’ battery chargers obey the dictates of the cells rather than the
electronic circuit. In this case, the battery will be fully charged regardless of the fuel gauge
setting. Such a battery is able to function normally, but the digital readout will be inaccurate. If
not corrected, the fuel gauge information simply becomes a nuisance.

The level of non-compliance is another problem with the SMBus. Unlike other tightly regulated
standards, such as the long play record introduced in the late 1950s, the audiocassette in the
1960s, the VCR in the 1970s, ISDN and GSM in the 1980s and the USB in the 1990s, some
variations are permitted in the SMBus protocol. These are: adding a check bid to halt the
service if the circuit crashes, counting the number of discharges to advise on calibration and
disallowing a charge if a certain fault condition has occurred. Unfortunately, these variations
cause problems with some existing chargers. As a result, a given SMBus battery should be
checked for compatibility with the designated charger before use to assure reliable service.
Ironically, the more features that are added to the SMBus charger and battery, the higher the
likelihood of incompatibilities.

‘Smart’ battery technology has not received the widespread acceptance that battery
manufacturers had hoped. Some engineers go so far as to suggest that the SMBus battery is
a ‘misguided principal’. Design engineers may not have fully understood the complexity of

charging batteries in the incubation period of the ‘smart’ battery. Manufacturers of SMBus

A

chargers are left to clean up the mess.

The forecast in consumer acceptance of the ‘smart’ battery has been too optimistic. In the
early 1990s when the SMBus battery was conceived, price many not have been as critical an
issue as it is now. Then, the design engineer would include many wonderful options. Today,
we look for scaled down products that are economically priced and perform the function
intended. When looking at the wireless communications market, adding high-level intelligence
to the battery is simply too expensive for most consumers. In the competitive mobile phone
market, for example, the features offered by the SMBus would be considered overkill.

SMBus battery technology is mainly used by higher-level industrial applications and battery
manufacturers are constantly searching for avenues to achieve a wider utilization of the
‘smart’ battery. According to a survey in Japan, about 30 percent of all mobile computing
devices are equipped with a ‘smart’ battery.

Improvements in the ‘smart’ battery system, such as better compatibilities, improved error­checking functions and higher accuracies will likely increase the appeal of the ‘smart’ battery.
Endorsement by large software manufacturers such as Microsoft will entice PC manufacturers
to make full use of these powerful features.

The State-of-Charge Indicator

Most SMBus batteries are equipped with a charge level indicator. When pressing a SoC
button on a battery that is fully charged, all signal lights illuminate. On a partially discharged
battery, half the lights illuminate, and on an empty battery, all lights remain dark. Figure 7-4
shows such a fuel gauge.

Figure 7-4: State-of-charge readout
of a ‘smart’ battery.

lthough the state-of-charge is displayed, the
state-of-health and its predicted runtime are
unknown.

While SoC information displayed on a battery or computer screen is helpful, the fuel gauge
resets to 100 percent each time the battery is recharged, regardless of the battery’s SoH. A
serious miscount occurs if an aged battery shows 100 percent after a full-charge, when in fact
the charge acceptance has dropped to 50 percent or less. The question remains:
“100 percent of what?” A user unfamiliar with this battery has little information about the
runtime of the pack.

The Tri-State Fuel Gauge

The SoC information alone is incomplete without knowing the battery’s SoH. To fully evaluate
the present state of a battery, three levels of information are needed. They are: SoC, SoH and
the empty portion of the battery that can be replenished with a charge. (The empty portion is
derived by deducting the SoC from the SoH.)

How can the three levels of a battery be measured and made visible to the user? While the
SoC is relatively simple to produce, as discussed above, measuring the SoH is more complex.
Here is how it works:

At time of manufacture, each SMBus battery is given its specified SoH status, which is
100 percent by default. This information is permanently programmed into the pack and does
not change. With each charge, the battery resets to the full-charge status. During discharge,

the energy units (coulombs) are counted and compared against the 100 percent setting. A
perfect battery would indicate 100 percent on a calibrated fuel gauge. As the battery ages and
the charge acceptance drops, the SoH begins to indicate lower readings. The discrepancy
between the factory set 100 percent and the actual delivered coulombs is used to calculate
the SoH.

Knowing the SoC and SoH, a simple linear display can be made. The SoC is indicated with
green LED’s; the empty part remains dark; and the unusable part is shown with red LED’s.
Figure 7-5 shows such a tri-state fuel gauge. As an alternative, the colored bar display may
be replaced with a numeric display indicating SoH and SoC.

Figure 7-5: Tri-state fuel gauge.

The Battery Health Gauge reads the ‘learned’ battery information available on the SMBus and displays it on a multi­colored LED bar. This illustration shows a partially discharged battery of 50% SoC with a 20% empty portion and an
unusable portion of 30%.

The most practical setting to place the tri-state-fuel gauge is on a charger. Only one display
would be needed for a multi-bay charging unit. To view the readings of a battery, the user
would simply press a button. The SoC and SoH information would be displayed within five
seconds after inserting the battery into the charger bay. During charge, the gauge would
reveal the charge level of each battery. This information would be handy when a functional
battery is needed in a hurry. Cadex offers a series of SMBus chargers that feature the tri-state
fuel gauge as an option.

The Target Capacity Selector

For users that simply need a go/no go answer and do not want to bother about other battery
information, chargers are available that feature a target capacity selector. Adjustable to 60, 70
or 80 percent, the target capacity selector acts as a performance check and flags batteries
that do not meet set requirements.

If a battery falls below target, the charger triggers the condition light. The user is prompted to
press the condition button to cycle the battery. Condition consists of charge/discharge/ch arge
and performs calibration and conditioning functions. If the battery does not recover after the
conditioning service, the fail light illuminates, indicating that the battery should be replaced. A
green ready light at the completion of the program assures that the battery meets the required
performance level.

An SMBus charger with the above described features acts as charger, conditioner and quality
control system. Figure 7-6 illustrates a two-bay Cadex charger featuring the target capacity
selector and discharge circuit. This unit is based on Level 3 and services both SMBus and
‘dumb’ batteries.

Some SMBus chargers can be fully automated to apply a conditioning cycle whenever the
battery falls below the target setting. An override button cancels the discharge if a fast-charge

is needed instead. Such a system maximizes the life of fleet batteries and assures that
deadwood is identified and removed.

Figure 7-6:The Cadex SM2+ charger

This Level 3 charger serves as charger,
conditioner and quality control system. It reads
the battery’s true state-of-health and flags those
that fall below the set target capacity. Each bay
operates independently and charges NiCd, NiMH
and Li-ion chemistries in approximately
three hours. ‘Dumb’ batteries can also be
charged.

By allowing the user to set the desired battery performance level, the question is raised as to
what level to select. The answer is governed by the applications, reliability standards and cost
policies.

It should be noted that the batteries are always charged to 100 percent, regardless of the
target setting. The target capacity simply refers to the amount of charge the battery has
delivered on the last discharge.

A practical target capacity setting for most applications is 80 percent. Decreasing the
threshold to 70 percent will lower the performance standard but pass more batteries. A direct
cost saving will result. The 60 percent level may suit those users who run a low budget
operation, have ready access to replacement batteries and can live with shorter, less
predictable runtimes.

Battery SoH readings are only available with Level 3 SMBus chargers servicing valid SMBus
batteries. ‘Dumb’ batteries cannot provide SoH readings, even if they share the identical
footprint of the ‘smart’ battery and can use the same charger.

Fuel Gauges for Large Batteries

Most ‘smart’ battery applications today are limited to portable electronic equipment, and the
electronic circuit is built right into the battery pack. In applications where larger ‘smart’
batteries are needed, such as electric wheelchairs, scooters, robots and forklifts, the
electronic circuit may be placed in a box external to the battery.

The main benefit of adding intelligence to the battery is to enable the measurement of SoH
and reserve energy. Most measuring devices used are based on voltage, which is known to
be highly inaccurate.

Cadex is extending the ‘smart’ battery technology to wheeled applications. Called the Cadex
Fuelcheck™, the device is based on the tri-state fuel gauge described earlier in this section.
The system consists of a controller, current measuring device and a display unit. The Cadex
Fuelcheck™ device can be added to new equipment when it is manufactured and can be

installed in existing equipment as a retrofit. The installation is permanent and each wheeled
appliance requires one system.

A tri-state linear bar graph consisting of colored LED’s would be the preferred display for a
personal user, such as a wheelchair operator. By replacing the light display with a digital
readout, additional information can be shown. For example, a display could indicate the
remaining runtime based on the average power consumption logged. Corrections would be
applied if the load factor changes during the course of the day. Figure 7-7 illustrates a digital
fuel gauge using the LCD panel.

To initialize the Cadex Fuelcheck™ system, a one-time setup procedure is required. A PC will
prompt the technician to enter information such as battery chemistry, desired end-of­discharge voltage, Ah rating of the battery, and designated service flags. The system is then
calibrated by applying a full charge, followed by a full discharge. The initial discharge
assesses the battery, or learns the performance. The SoH readings will be known after the
first full discharge.

7-7: Cadex Fuelcheck™ digital
fuel gauge.

The display indicates SoC, SoH, remaining
power in hours, the voltage and current
draw.

The simplest way to discharge the battery as part of calibration is to run it down through
normal use in the equipment. The most accurate readings are obtained by using a load bank.
Such a device may not be always available.

With a data logging option built into the system, the PC allows downloading service data that
have been collected during use. Such information would benefit large equipment fleets and
rental places that need to check the performance of the batteries on demand.

Similar to other ‘smart’ battery systems, calibration will be needed once every 6 to 12 months.
More frequent calibrations may be required if the equipment is used for short durations
between each recharge. The most practical way to calibrate the system is by occasionally
allowing the battery to run down through continued use. Information relating to the date of the
last calibration, or an early call for calibration if certain conditions occur, can be calculated
and displayed.

Knowing the SoH of a battery at any time and scheduling timely service or replacement is a
major benefit for industrial battery users. Such a system would be especially helpful for
organizations in which different individuals use the equipment and no one is given
maintenance responsibilities. Equipment rental places fall into this category.

Chapter 8: Choosing the Right Battery

What causes a battery to wear down — is it mechanical or chemical? The answer is ‘both’. A
battery is a perishable product that starts deteriorating from the time it leaves the factory.
Similar to a spring under tension, a battery seeks to revert to its lowest denominator. The rate
of aging is subject to depth of discharge, environmental conditions, charge methods and
maintenance procedures (or lack thereof). Each battery chemistry behaves differently in terms
of aging and wear through normal use.

What’s the best battery for mobile phones?

When buying a replacement battery, the buyer often has the choice of different battery
chemistries. Li-ion and Li-ion polymer batteries are used on newer phones, wh ereas the
NiMH and NiCd are found in older models. If the buyer has a choice, the sales person may
advise a customer to go for the highest capacity rating and to stay away from the NiCd
because of the memory effect. The customer may settle for the slim-line NiMH because it
offers relatively high capacity in a small package and is reasonably priced.

Seemingly a wise choice, an analysis in this chapter reveals that other chemistries may have
served better. The NiMH offers good value for the price but falls short in expected cycle life.
Although excellent when new, the performance trails off quickly after about 300 cycles due to
decreased capacity and rising internal resistance. In comparison, the Li-ion can be used for
about 500 cycles. The best cycle count is achieved with NiCd. Properly maintained, the NiCd
delivers over 1000 cycles and the internal resistance remains low. However, the NiCd offers
about 30 percent less capacity compared to the NiMH. In addition, the NiCd is being removed
from the mobile phone market because of environmental concerns.

Switching to environmentally friendlier batteries is fitting, especially in the mobile phone
market where the NiMH performs reasonably well and can be economical. The battery
disposal issue is difficult to control, particularly in the hands of a diverse user group.

The NiMH and NiCd are considered high maintenance batteries, which require regular
discharge cycles to prevent what is referred to as ‘memory’. Although the NiMH was originally
advertised as memory-free, both NiCd and NiMH are affected by the phenomenon. The
capacity loss is caused by crystalline formation that is generated by the positive nickel plate, a
metal shared by both systems.

Nickel-based batteries, especially NiCd’s, should be fully discharged once per month. If such
maintenance is omitted for four months or more, the capacity drops by as much as one third.
A full restoration becomes more difficult the longer service is withheld.

It is not recommended to discharge a battery before each charge because this wears down
the battery unnecessarily and shortens the life. Neither is it advisable to leave a battery in the

charger for a long period of time. When not in use, the battery should be put on a shelf and
charged before use. Always store the battery in a cool place.

Is the Li-ion a better choice? Yes, for many applications. The Li-ion is a low maintenance
battery which offers high energy, is lightweight and does not require periodic full discharge.
No trickle charge is applied once the battery reaches full charge. The Li-ion battery can stay
in most chargers until used. The charging process of a Li-ion is, in many ways, simpler and
cleaner than that of nickel-based systems, but requires tighter tolerances. Repeated insertion
into the charger or cradle does not affect the battery by inducing overcharge.

On the negative side, the Li-ion gradually loses charge acceptance as part of aging, even if
not used. For this reason, Li-ion batteries should not be stored for long periods of time but be
rotated like perishable food. The buyer should be aware of the manufacturing date when
purchasing a replacement battery.

The Li-ion is most economical for those who use a mobile phone daily. Up to
1000 charge/discharge cycles can be expected if used within the expected service life of
about two to three years. Because of the aging effect, the Li-ion does not provide an
economical solution for the occasional user. If the Li-ion is the only battery choice and the
equipment is seldom used, the battery should be removed from the equipment and stored in a
cool place, preferably only partially charged.

So far, little is known about the life expectancy of the Li-ion polymer. Because of the
similarities with the Li-ion, the long-term performance of both systems is expected to be
similar. Much effort is being made to prolong the service life of lithium-based systems. New
chemical additives have been effective in retarding the aging process.

What’s the best battery for two-way radios?

The two-way radio market uses mostly NiCd batteries. In the last few years, environmental
agencies have been attempting to discourage the use of NiCd, especially in Europe. NiMH
have been tried and tested in two-way radios for a number of years but the results are mixed.
Shorter cycle life compared to NiCd is the major drawback.

The reasons for the relatively short life of NiMH are multi-fold. NiMH is less robust than NiCd
and has a cycle life expectancy that is half or one third that of the standard NiCd. In addition,
NiMH prefers a moderate discharge current of 0.5C or less. A two-way radio, on the other
hand, draws a discharge current of about 1.5A when transmitting at 4W of power. High
discharge loads shorten the life of the NiMH battery considerably.

NiCd has the advantage of maintaining a low and steady internal resistance throughout most
of its service life. Although low when new, NiMH increases the resistance with advanced cycle
count. A battery with high internal resistance causes the voltage to drop when a load is
applied. Even though energy may still be present, the battery cannot deliver the high current
flow required during transmit mode. This results in a drop in voltage, which triggers the ‘low
battery’ condition and the radio cuts off. This happens mostly during transmission.

The Li-ion has been tested for use with two-way radios but has not been able to provide the
ultimate answer. Higher replacement costs, restrictions posed by the safety circuit and aging
pose limitations on this battery system.

What’s the best battery for laptops?

Batteries for laptops have a unique challenge because they must be small and lightweight. In
fact, the laptop battery should be invisible to the user and deliver enough power to last for a
five-hour flight from Toronto to Vancouver. In reality, a typical laptop battery provides only
about 90 minutes of service.

Computer manufacturers are hesitant to add a larger battery because of increased size and
weight. A recent survey indicated that, given the option of larger size and more weight to
obtain longer runtimes, most users would settle for what is being offered today. For better or
worse, we have learned to accept the short runtime of a laptop.

During the last few years, batteries have improved in terms of energy density. Any benefit in
better battery performance, however, is being eaten up by the higher power requirements of
the laptops. It is predicted that the even more power-hungry PC’s of the future will counteract
any improvements in battery technology, as marginal as they might be. The net effect will
result in the same runtimes but faster and more powerful computers.

The length of time the battery can be used will get shorter as the battery ages. A battery
residing in a laptop ages more quickly than when used in other applications. After a warm-up,
the official operating temperature inside a laptop computer is 45°C (113°F). Such a high
ambient temperature drastically lowers the battery’s life expectation. At a temperature of 45°C,
for example, the life expectancy of a NiMH battery is less than 50 percent as compared to
running it at the ideal operating temperature of 20°C (68°F).

The Li-ion does not fare much better. At this high ambient temperature, the wear-down effect
of the battery is primarily governed by temperature as opposed to cycle count. The situation is
worsened by the fact that the battery resides in a high SoC most of the time. The combination
of heat and high SoC promotes cell oxidation, a condition that cannot be reversed once
afflicted.

A fully charged Li-ion battery that is stored at 45°C suffers a capacity loss from 100 percent to
about 70 percent in as little as six months. If this condition persists, the capacity degrades
further to 50 percent in twelve months. In reality, the battery in a laptop is exposed to elevated
temperatures just during use and the battery is in a full charge state only part of the time. But
leaving the laptop in a parked car under the hot sun will aggravate the situation.

Some Japanese computer manufacturers have introduced a number of sub-notebooks in
which the battery is mounted externally, forming part of the hinge. This design improves

battery life because the battery is kept at room temperature. Some models carry several size
batteries to accommodate different user patterns.

What then is the best battery for a laptop? The choices are limited. The NiCd has virtually
disappeared from the mobile computer scene and the NiMH is loosing steam, paving the way
for the Li-ion. Eventually, very slim geometry will also demand thin batteries, and this is
possible with the prismatic Li-ion polymer.

Besides providing reliable performance for general portable use, the Li-ion battery also offers
superior service for laptop users who must continually switch from fixed power to battery use,
as is the case for many sales people. Many biomedical and industrial applications follow this
pattern also. Here is the reason why such use can be hard on some batteries:

On a nickel-based charging system, unless smart, the charger ap plies a full charge each time
the portable device is connected to fixed power. In many cases, the battery is already fully
charged and the cells go almost immediately into overcharge. The battery heats up, only to be
detected by a sluggish thermal charge control, which finally terminates the fast charge.
Permanent capacity loss caused by overcharge and elevated temperature is the result.

Among the nickel-based batteries, NiMH is least capable of tolerating a recharge on top of a
charge. Adding elevated ambient temperatures to the charging irregularities, a NiMH battery
can be made inoperable in as little as six months. In severe cases, the NiMH is known to last
only 2 to 3 months.

For mixed battery and utility power use, the Li-ion system is a better choice. If a fully charged
Li-ion is placed on charge, no charge current is applied. The battery only receives a recharge
once the terminal voltage has dropped to a set threshold. Neither is there a concern if the
device is connected to fixed power for long periods of time. No overcharge can occur and
there is no memory to worry about.

NiMH is the preferred choice for a user who runs the laptop mostly on fixed power and
removes the battery when not needed. This way, the battery is only engaged if the device is
used in portable mode. The NiMH battery can thus be kept fresh while sitting on the shelf.
NiMH ages well if kept cool and only partially charged.

Selecting a Lasting Battery

As part of an ongoing research program to find the optimum battery system for selected
applications, Cadex has performed life cycle tests on NiCd, NiMH and Li-ion batteries. All
tests were carried out on the Cadex 7000 Series battery analyzers in the test labs of Cadex,
Vancouver, Canada. The batteries tested received an initial full-charge, and then underwent a
regime of continued discharge/charge cycles. The internal resistance was measured with
Cadex’s Ohmtest™ method, and the self-discharge was obtained from time-to-ti me by
reading the capacity loss incurred during a 48-hour rest period. The test program involved
53 commercial telecommunications batteries of different models and chemistries. One battery
of each chemistry displaying typical behavior was chosen for the charts below.

When conducting battery tests in a laboratory, it should be noted that the performance in a
protected environment is commonly superior to those in field use. Elements of stress and
inconsistency that are present in everyday use cannot always be simulated accurately in
the lab.

The NiCd Battery — In terms of life cycling, the standard NiCd is the most enduring battery.
In Figure 8-1 we examine the capacity, internal resistance and self-discharge of a 7.2V,
900mA NiCd battery with standard cells. Due to time constraints, the test was terminated after
2300 cycles. During this period, the capacity remains steady, the internal resistance stays flat
at 75mW and the self-discharge is stable. This battery receives a grade ‘A’ for almost perfect
performance.

The readings on an ultra-high capacity NiCd are less favorable but still better than other
chemistries in terms of endurance. Although up to 60 percent higher in energy density
compared to the standard NiCd version, Figure 8-2 shows the ultra-high NiCd gradually losing
capacity during the 2000 cycles delivered. At the same time, the internal resistance rises
slightly. A more serious degradation is the increase of self-discharge after 100 0 cycles. This
deficiency manifests itself in shorter runtimes because the battery consumes some energy
itself, even if not in use.

Figure 8-1: Characteristics of a standard cell NiCd battery.

This battery deserves an ‘A’ for almost perfect performance in terms of stable capacity, internal resistance and self­discharge over many cycles. This illustration shows results for a 7.2V, 900mA NiCd.

Figure 8-2: Characteristics of a NiCd battery with ultra-high capacity cells.

This battery is not as favorable as the standard NiCd but offers higher energy densities and performs better than
other chemistries in terms of endurance. This illustrations shows results for a 6V, 700mA NiCd.

The NiMH Battery — Figure 8-3 examines the NiMH, a battery that offers high energy
density at reasonably low cost. We observe good performance at first but past the 300-cycle
mark, the performance starts to drift downwards rapidly. One can detect a swift increase in
internal resistance and self-discharge after cycle count 700.

Figure 8-3: Characteristics of a NiMH battery.

This battery offers good performance at first but past the 300-cycle mark, the capacity, internal resistance and self­discharge start to deteriorate rapidly. This illustrations shows results for a 6V, 950mA NiMH.

The Li-ion Battery — The Li-ion battery offers advantages that neither the NiCd nor NiMH
can match. In Figure 8-4 we examine the capacity and internal resistance of a typical Li-ion. A
gentle capacity drop is observed over 1000 cycles and the internal resistance increases only
slightly. Because of low readings, self-discharge was omitted for this test.

The better than expected performance of this test battery may be due to the fact that the test
did not include aging. The lab test was completed in about 200 days. A busy user may charge
the battery once every 24 hours. With such a user pattern, 500 cycles would represent close
to two years of normal use and the effects of aging would become apparent.

Manufacturers of commercial Li-ion batteries specify a cycle count of 500. At that stage, the
battery capacity would drop from 100 to 80 percent. If operated at 40°C (104°F) rather than at
room temperature, the same battery would only deliver about 300 cycles.

Figure 8-4: Characteristics of a Li-ion battery.

The above-average performance of this battery may be due to the fact that the test did not include aging. This
illustration shows results for a 3.6V, 500mA Li-ion battery.

Chapter 9: Internal Battery Resistance

With the move from analog to digital devices, new demands are being placed on the battery.
Unlike analog equipment that draws a steady current, the digital mobile phone, for example,

loads the battery with short, high current bursts.

Increasingly, mobile communication devices are
moving from voice only to multimedia which
allows sending and receiving data, still pictures
and even video. Such transmissions add to the
bandwidth, which require several times the b
power compared to voice only.

One of the urgent requirements of a battery for
digital applications is low internal resistance.

Measured in milliohms (mΩ), the internal
resistance is the gatekeeper that, to a large extent, determines the runtime. The lower the
resistance, the less restriction the battery encounters in delivering the needed po wer b ursts. A
high mΩ reading can trigger an early ‘low battery’ indication on a seemingly good battery
because the available energy cannot be delivered in an appropriate manner.

Figure 9-1 examines the major global mobile phone systems and compares peak po wer and
peak current requirements. The systems are the AMP, GSM, TDMA and CDMA.

attery

Type
Used in
Peak Power
Peak current
In service since

Figure 9-1: Peak power requirements of popular global mobile phone systems.

Moving from voice to multi-media requires several times the battery power.

2

AMP GSM TDMA

Analog Digital Digital Digital
USA, Canada Globally USA, Canada USA, Canada

0.6W 1-2W 0.6-1W 0.2W

0.3A DC 1-2.5A 0.8-1.5A 0.7A
1985 1986 1992 1995

1

CDMA

1. Some TDMA handsets feature dual mode (analog 800mA DC load; digital 1500mA
pulsed load).

2. Current varies with battery voltage; a 3.6V battery requires higher current than a 7.2V
battery.

Why do seemingly good batteries fail on digital equipment?

Service technicians have been puzzled by the seemingly unpredictable battery behavior when
powering digital equipment. With the switch from analog to digital wireless communications
devices, particularly mobile phones, a battery that performs well on an analog device may
show irrational behavior when used on a digital device. Testing these batteries with a battery
analyzer produces normal capacity readings. Why then do some batteries fail prematurely on
digital devices but not on analog?

The overall energy requirement of a digital mobile phone is less than that of the analog
equivalent, however, the battery must be capable of delivering high current pulses that are
often several times that of the battery’s rating. Let’s look at the battery rating as expressed in
C-rates.

A 1C discharge of a battery rated at 500mAh is 500mA. In comparison, a 2C discharge of the
same battery is 1000mA. A GSM phone powered by a 500mA battery that draws 1.5A pulses
loads the battery with a whopping 3C discharge.

A 3C rate discharge is fine for a battery with very low internal resistance. However, aging
batteries, especially Li-ion and NiMH chemistries, pose a challenge because the mΩ readings
of these batteries increase with use.

Improved performance can be achieved by using a larger battery, also known as an extended
pack. Somewhat bulkier and heavier, an extended pack offers a typical rating of about
1000mAh or roughly double that of the slim-line. In terms of C-rate, the 3C discharge is
reduced to 1.5C when using a 1000mAh instead of a 500mAh battery.

As part of ongoing research to find the best battery system for wireless devices, Cadex has
performed life cycle tests on various battery systems. In Figure 9-2, Figure 9-3, and Figure 9­4, we examine NiCd, NiMH and Li-ion batteries, each of which generates a good capacity
reading when tested with a battery analyzer but produce stunning differences on a pulsed
discharge of 1C, 2C and 3C. These pulses simulate a GSM phone.

Figure 9-2: Talk-time of a NiCd battery under the GSM load schedule.

This battery has 113% capacity and 155mΩ internal resistance.

A closer look reveals vast discrepancies in the mΩ measurements of the test batteries. In fact,
these readings are typical of batteries that have been in use for a while. The NiCd shows
155mΩ, the NiMH 778mΩ and the Li-ion 320mΩ, although the capacities checked in at 113,
107 and 94 percent respectively when tested with the DC load of a battery analyzer. It should
be noted that the internal resistance was low when the batteries were new.

Figure 9-3: Talk-time of a NiMH battery under the GSM load schedule.

This battery has 107% capacity and 778mΩ internal resistance.

Figure 9-4: Talk-time of a Li-ion battery under the GSM load schedule.

This battery has 94% capacity and 320mΩ internal resistance.

From these charts we can see that the talk-time is in direct relationship with the battery’s
internal resistance. The NiCd performs best and produces a talk time of 140 minutes at 1C
and a long 120 minutes at 3C. In comparison, the NiMH is good for 140 minutes at 1C but
fails at 3C. The Li-ion provides 105 minutes at 1C and 50 minutes at 3C discharge.

How is the internal battery resistance measured?

A number of techniques are used to measure internal battery resistance. One co mmon
method is the DC load test, which applies a discharge current to the battery while measuring
the voltage drop. Voltage over current provides the internal resistance (see Figure 9-5).

Figure 9-5: DC load test.

The DC load test measures the battery’s internal resistance by reading the voltage drop. A large drop indicates high
resistance.

The AC method, also known as the conductivity test, measures the electrochemical
characteristics of a battery. This technique applies an alternating current to the battery
terminals. Depending on manufacturer and battery type, the frequency ranges from 10 to
1000Hz. The impedance level affects the phase shift between voltage and current, which
reveals the condition of the battery. The AC method works best on single cells. Figure 9-6
demonstrates a typical phase shift between voltage and current when testing a battery.

Figure 9-6: AC load test.

The AC method measures the phase shift between voltage and current. The battery’s reactance is used to calculate
the impedance.

Some AC resistance meters evaluate only the load factor and disregard the phase shift
information. This technique is similar to the DC method. The AC voltage that is superimposed
on the battery’s DC voltage acts as brief charge and discharge pulses. The amplitude of the
ripple is utilized to calculate the internal battery resistance.

Cadex uses the discreet DC method to measure internal battery resistance. Added to the
Cadex 7000 Series battery analyzers, a number of charge and discharge pulses are applied,

which are scaled to the mAh rating of the battery tested. Based on the voltage deflections, the
battery’s internal resistance is calculated. Known as Ohmtest™, the mΩ reading is obtained
in five seconds. Figure 9-7 shows the technique used.

Figure 9-7: Cadex Ohmtest™.

Cadex’s pulse method measures the voltage deflections by applying charge and discharge pulses. Higher deflections
indicate higher internal resistance.

Figure 9-8 compares the three methods of measuring the internal resistance of a battery and
observe the accuracy. In a good battery, the discrepancies between methods are minimal.
The test results deviate to a larger degree on packs with poor SoH.

Impedance measurement alone does not provide a definite conclusion as to the battery
performance. The mΩ readings may vary widely and are dependent on battery chemistry, cell
size (mAh rating), type of cell, number of cells connected in series, wiring and contact type.

Figure 9-8: Comparison of the AC, DC and Cadex Ohmtest™ methods.

State-of-health readings were obtained using the Cadex 7000 Series battery analyzer by applying a full
charge/discharge/charge cycle. The DC method on the 68% SoH battery exceeded 1000mΩ.

When using the impedance method, a battery with a known performance should be measure d
and its readings used as a reference. For best results, a reference reading should be on hand
for each battery type. Figure 9-9kl; provides a guideline for digital mobile phone batteries
based on impedance readings.

The milliohm readings are related to the battery voltage. Higher voltage batteries allow higher
internal resistance because less current is required to deliver the same po wer. The ratio
between voltage and milliohm is not totally linear. There are certain housekeeping
components that are always present whether the battery has one or several cells. These are
wiring, contacts and protection circuits.

Temperature also affects the internal resistance of a battery. The internal resistance of a
naked Li-ion cell, for example, measures 50mΩ at 25°C (77°F). If the temperature increases,
the internal resistance decreases. At 40°C (104°F), the internal resistance drops to about
43mΩ and at 60°C (140°F) to 40mΩ. While the battery performs better when exposed to heat,
prolonged exposure to elevated temperatures is harmful. Most batteries deliver a momentary
performance boost when heated.

Milli-Ohm Battery Voltage Ranking

75-150mOhm
150-250mOhm
250-350mOhm
350-500mOhm
Above 500mOhm

3.6V Excellent

3.6V Good

3.6V Marginal

3.6V Poor

3.6V Fail

Figure 9-9: Battery state-of-health based on internal resistance.

The milliohm readings relate to the battery voltage; higher voltage allows higher milliohm readings.

Cold temperatures have a drastic effect on all batteries. At 0°C (32°F), the internal resistance
of the same Li-ion cell drops to 70mΩ. The resistance increases to 80mΩ at -10°C (50°F) and
100mΩ at -20°C (-4°F).

The impedance readings work best with Li-ion batteries because the performance
degradation follows a linear pattern with cell oxidation. The performance of NiMH batteries
can also be measured with the impedance method but the readings are less dependable.
There are instances when a poorly performing NiMH battery can also exhibit a low mΩ
reading.

Testing a NiCd on resistance alone is unpredictable. A low resistance reading does not
automatically constitute a good battery. Elevated impedance readings are often caused by
memory, a phenomenon that is reversible. Internal resistance values have been reduced by a
factor of two and three after servicing the affected batteries with the recondition cycle of a
Cadex 7000 Series battery analyzer. Of cause, high internal resistance can hav e sources
other than memory alone.

What’s the difference between internal resistance and
impedance?

The terms ‘internal resistance’ and ‘impedance’ are often intermixed when addressing the
electrical conductivity of a battery. The differences are as follows: The internal resistance
views the conductor from a purely resistive value, or ohmic resistance. A comparison can b e
made with a heating element that produces warmth by the friction of electric current passing
through.

Most electrical loads are not purely resistive, rather, they have an element of reactance. If an
alternating current (AC) is sent through a coil, for example, an inductance (magnetic field) is
created, which opposes current flow. This AC impedance is always higher than the ohmic
resistance of the copper wire. The higher the frequency, the higher the inductive resistance
becomes. In comparison, sending a direct current (DC) through a coil constitutes an electrical
short because there is only a very small ohmic resistance.

Similarly, a capacitor does not allow the flow of DC, but passes AC. In fact, a capacitor is an
insulator for DC. The resistance that is present when sending an AC current flowing through a
capacitor is called capacitance. The higher the frequency, the lower the capacitive resistance.

A battery as a power source combines ohmic, inductive and capacitive resistance. Figure 9­10 represents these resistive values on a schematic diagram. Each battery type exhibits
slightly different resistive values.

Figure 9-10: Ohmic, inductive and capacitive resistance in batteries.

•

R = ohmic resistance

o

• Q = constant phase loop (type of capacitance)

c

• L = inductor

• Z = Warburg impedance (particle movement within the electrolyte)

w

• R = transfer resistance

t

Chapter 10: Getting the Most from your Batteries

A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service.
Although fully charged, the battery eventually regresses to a point where the available energy is less than half of its original
capacity, resulting in unexpected downtime.

Downtime almost always occurs at critical moments. This is especially true in the public safety
sector where portable equipment runs as part of a fleet operation and the battery is charged
in a pool setting, often with minimal care and attention. Under normal conditions, the battery
will hold enough power to last the day. During heavy activities and longer than expected
duties, a marginal battery cannot provide the extra power needed and the equipment fails.

Rechargeable batteries are known to cause more concern, grief and frustration than any other
part of a portable device. Given its relatively short life span, the battery is the most expensive
and least reliable component of a portable device.

In many ways, a rechargeable battery exhibits human-like characteristics: it needs good
nutrition, it prefers moderate room temperature and, in the case of the nickel-based system,
requires regular exercise to prevent the phenomenon called ‘memory’. Each battery seems to
develop a unique personality of its own.

Memory: myth or fact?

The word ‘memory’ was originally derived from ‘cyclic memory’, meaning that a NiCd battery
can remember how much discharge was required on previous discharges. Improvements in
battery technology have virtually eliminated this phenomenon. Tests performed at a Black &
Decker lab, for example, showed that the effects of cyclic memory on the modern NiCd were
so small that they could only be detected with sensitive instruments. After the same battery
was discharged for different lengths of time, the cyclic memory phenomenon could no longer
be noticed.

The problem with the nickel-based battery is not the cyclic memory but the effects of
crystalline formation. There are other factors involved that cause degeneration of a battery.
For clarity and simplicity, we use the word ‘memory’ to address capacity loss on nickel-based
batteries that are reversible.

The active cadmium material of a NiCd battery is present in finely divided crystals. In a good
cell, these crystals remain small, obtaining maximum surface area. When the memory
phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a
voltage depression, which leads to a loss of capacity. In advanced stages, the sharp edges of
the crystals may grow through the separator, causing high self-discharge or an electrical short.

Another form of memory that occurs on some NiCd cells is the formation of an inter-metallic
compound of nickel and cadmium, which ties up some of the needed cadmium and creates
extra resistance in the cell. Reconditioning by deep discharge helps to break up this
compound and reverses the capacity loss.

The memory phenomenon can be explained in layman’s terms as expressed by Duracell:
“The voltage drop occurs because only a portion of the active materials in the cells is
discharged and recharged during shallow or partial discharging. Th e active materials that
have not been cycled change in physical characteristics and increase in resistance.
Subsequent full discharge/charge cycling will restore the active materials to their original
state.”

When NiMH was first introduced there was much publicity about its memory-free status.
Today, it is known that this chemistry also suffers from memory but to a lesser extent than the
NiCd. The positive nickel plate, a metal that is shared by both chemistries, is responsible for
the crystalline formation.

New NiCd cell.

The anode is in fresh condition (capacity of 8.1Ah). Hexagonal cadmium
hydroxide crystals are about 1 micron in cross section, exposing large
surface area to the liquid electrolyte for maximum performance.

Cell with crystalline formation.

Crystals have grown to an enormous 50 to 100 microns in cross section,
concealing large portions of the active material from the electrolyte (capacity
of 6.5Ah). Jagged edges and sharp corners may pierce the separator, which
can lead to increased self-discharge or electrical short.

Restored cell.

After pulsed charge, the crystals are reduced to 3 to 5 microns, an almost
100% restoration (capacity of 8.0A). Exercise or recondition are needed if
the pulse charge alone is not effective.

Figure 10-1: Crystalline formation on NiCd cell.

Illustration courtesy of the US Army Electronics Command in Fort Monmouth, NJ, USA.

In addition to the crystal-forming activity on the positive plate, the NiCd also develops crystals
on the negative cadmium plate. Because both plates are affected by crystalline formation, the
NiCd requires more frequent discharge cycles than the NiMH. This is a non-scientific
explanation of why the NiCd is more prone to memory than the NiMH.

The stages of crystalline formation of a NiCd battery are illustrated in Figure 10-1. The
enlargements show the negative cadmium plate in normal crystal structure of a new cell,
crystalline formation after use (or abuse) and restoration.

Lithium and lead-based batteries are not affected by memory, but these chemistries have
their own peculiarities. Current inhibiting pacifier layers affect both batteries — plate oxidation
on the lithium and sulfation and corrosion on the lead acid systems. These degenerative
effects are non-correctible on the lithium-based system and only partially reversible on the
lead acid.

How to Restore and Prolong Nickel-based Batteries

The effects of crystalline formation are most pronounced if a nickel-based battery is left in the
charger for days, or if repeatedly recharged without a periodic full discharge. Since most
applications do not use up all energy before recharge, a periodic discharge to 1V/cell (known
as exercise) is essential to prevent the buildup of crystalline formation on the cell plates. This
maintenance is most critical for the NiCd battery.

All NiCd batteries in regular use and on standby mode (sitting in a charger for operational
readiness) should be exercised once per month. Between these monthly exercise cycles, no
further service is needed. The battery can be used with any desired user pattern without the
concern of memory.

The NiMH battery is affected by memory also, but to a lesser degree. No scientific research is
available that compares NiMH with NiCd in terms of memory degradation. Neither is
information on hand that suggests the optimal amount of maintenance required to obtain
maximum battery life. Applying a full discharge once every three months appears right.
Because of the NiMH battery’s shorter cycle life, over-exercising is not recommended.

A hand towel must be cleaned periodically. However, if it were washed after each use, its
fabric would wear out very quickly. In the same way, it is neither necessary nor advisable to
discharge a rechargeable battery before each charge — excessive cycling puts extra strain
on the battery.

Exercise and Recondition — Research has shown that if no exercise is applied to a NiCd
for three months or more, the crystals ingrain themselves, making them more difficult to break
up. In such a case, exercise is no longer effective in restoring a battery and reconditioning is
required. Recondition is a slow, deep discharge that removes the remaining battery energy by
draining the cells to a voltage threshold below 1V/cell.

Figure 10-2: Exercising and reconditioning batteries on a Cadex battery analyzer.

This illustration shows the battery voltage during a normal discharge to 1V, followed by the secondary discharge
(recondition). Recondition consists of a discharge to 1V/cell at a 1C load current, followed by a secondary discharge
to 0.4V at a much reduced current. NiCd batteries affected by memory often restore themselves to full service.

Tests performed by the US Army have shown that a NiCd cell needs to be discharged to at
least 0.6V to effectively break up the more resistant crystalline formation. During recondition,
the current must be kept low to prevent cell reversal. Figure 10-2 illustrates the battery
voltage during normal discharge to 1V/cell followed by the secondary discharge (recondition).

Figure 10-3 illustrates the effects of exercise and recondition. Four batteries afflicted with
various degrees of memory are serviced. The batteries are first fully charged, then discharged
to 1V/cell. The resulting capacities are plotted on a scale of 0 to 120 percent in the first
column. Additional discharge/charge cycles are applied and the battery capacities are plotted
in the subsequent columns. The solid black line represents exercise, (discharge to 1V/cell)
and the dotted line recondition (secondary discharge at reduced current to 0.4V/cell). On this
test, the exercise and recondition cycles are applied manually at the discretion of the research
technician.

Figure 10-3: Effects of exercise and recondition.

Battery A improved capacity on exercise alone; batteries B and C required recondition. A new battery with excellent
readings improved further with recondition.

Battery A responded well to exercise alone and no recondition was required. This result is
typical of a battery that has been in service for only a few months or has received periodic
exercise cycles. Batteries B and C, on the other hand, required recondition (dotted line) to
restore their performance. Without the recondition function, these two batteries would need to
be replaced.

After service, the restored batteries were returned to full use. When examined after six
months of field use, no noticeable degradation in the restored performance was visible. The
regained capacity was permanent with no evidence of falling back to the previous state.
Obviously, the batteries would need to be serviced on a regular basis to maintain the
performance.

Applying the recondition cycle on a new battery (top line on chart) resulted in a slight capacity
increase. This capacity gain is not fully understood, other than to assume that the battery
improved by additional formatting. Another explanation is the presence of early memory.
Since new batteries are stored with some charge, the self-discharge that occurs during
storage contributes to a certain amount of crystalline formation. Exercising and reconditioning
reverse this effect. This is why the manufacturers recommend storing rechargeable batteries
at about 40 percent charge.

The importance of exercising and reconditioning NiCd batteries is emphasized further by a
study carried out by GTE Government Systems in Virginia, USA, for the US Navy. To
determine the percentage of batteries needing replacement within the first year of use, one
group of batteries received charge only, another group was exercised and a third group
received recondition. The batteries studied were used for two-way radios on the aircraft
carriers USS Eisenhower with 1500 batteries and USS George Washington with 600 batteries,
and the destroyer USS Ponce with 500 batteries.

With charge only (charge-and-use), the annual percentage of battery failure on the USS
Eisenhower was 45 percent (see Figure 10-4). When applying exerci se, the failure rate was
reduced to 15 percent. By far the best results were achieved with recondition. The failure rate
dropped to 5 percent. Identical results were attained from the USS George Washington and
the USS Ponce.

Maintenance Method Annual Percentage of Batteries

Requiring Replacement

Charge only (charge-and-use) 45%
Exercise only (dischar ge to 1V/cell) 15%
Reconditioning (secondary deep discharge) 5%

Figure 10-4: Replacement rates of NiCd batteries.

The annual percentage of NiCd batteries requiring replacement when used without any maintenance decreases with
exercise and recondition. These statistics were drawn from batteries used by the US Navy on the USS Eisenhower,
USS George Washington and USS Ponce.

The GTE Government System report concluded that a battery analyzer featuring exercise and
recondition functions costing $2,500US would pay for itself in less than one month on battery
savings alone. The report did not address the benefits of increased system reliability, an issue
that is of equal if not greater importance, especially when the safety of human lives is at stake.

Another study involving NiCd batteries for defense applications was performed by the Dutch
Army. This involved battery packs that had been in service for 2 to 3 years during the Balkan
War. The Dutch Army was aware that the batteries were used under the worst possible
conditions. Rather than a good daily workout, the packs were used for patrol duties lasting
2 to 3 hours per day. The rest of the time the batteries remained in the chargers for
operational readiness.

After the war, the batteries were sent to the Dutch Military Headquarters and were tested with
Cadex 7000 Series battery analyzers. The test technician found that the capacity of some
packs had dropped to as low as 30 percent. With the recondition function, 90 percent of the
batteries restored themselves to full field use. The Dutch Army set the target capacity
threshold for field acceptability to 80 percent. This setting is the pass/fail acceptance level for
their batteries.

Based on the successful reconditioning results, the Dutch Army now assigns the battery
maintenance duty to individual battalions. The program calls for a service once every two
months. Under this regime, the Army reports reduced battery failure and prolonged service
life. The performance of each battery is known at any time and any under-performing battery
is removed before it causes a problem.

NiCd batteries remain the preferred chemistry for mobile communications, both in civil and
defense applications. The main reason for its continued use is dependable and enduring
service under difficult conditions. Other chemistries have been tested and found problematic
in long-term use.

During the later part of the 1990s, the US Army switched from mainly non-rechargeable to the
NiMH battery. The choice of chemistry was based on the benefit of higher energy densities as
compared to NiCd. The army soon discovered that the NiMH did not live up to the expected
cycle life. Their reasoning, however, is that the 100 cycles attained from a NiMH pack is still
more economical than using a non-rechargeable equivalent. The army’s focus is now on the
Li-ion Polymer, a system that is more predictable than NiMH and requires little or no
maintenance. The aging issue will likely cause some logistic concerns, especially if long-term
storage is required.

Simple Guidelines

Do not leave a nickel-based battery in a charger for more than a day after full charge is
reached.

• Apply a monthly full discharge cycle. Running the battery down in the equipment may
do this also.

• Do not discharge the battery before each recharge. This would put undue stress on
the battery.

• Avoid elevated temperature. A charger should only raise the battery temperature for a
short time at full charge, and then the battery should cool off.

• Use quality chargers to charge batteries.

The Effect of Zapping

To maximize battery performance, remote control (RC) racing enthusiasts have experimented
with all imaginable methods available. One technique that seems to work is zapping the cells
with a very high pulse current. Zapping is said to increase the cell voltage slightly, generating
more power.

Typically, the racecar motor draws 30A, delivered by a 7.2V battery. This calculates to over
200W of power. The battery must endure a race lasting about four minutes.

According to experts, zapping works best with NiCd cells. NiMH cells have been tried but they
have shown inconsistent results.

Companies specializing in zapping NiCd for RC racing use a very high quality Japanese NiCd
cell. The cells are normally sub-C in size and are handpicked at the factory for the application.
Specially labeled, the cells are delivered in a discharged state. When measuring the cell in
empty state-of-charge (SoC), the voltage typically reads between 1.11 to 1.12V. If the voltage
drops lower than 1.06V, the cell is considered suspect and zapping does not seem to
enhance the performance as well as on the others.

The zapping is done with a 47,000mF capacitor that is charged to 90V. Best results are
achieved if the battery is cycled twice after treatment, then is zapped again. After the battery
has been in service for a while, zapping no longer seems to improve the cell’s performance.
Neither does zapping regenerate a cell that has become weak.

The voltage increase on a properly zapped battery is between 20 and 40mV. This
improvement is measured under a load of 30A. According to experts, the voltage gain is
permanent but there is a small drop with usage and age.

There are no apparent side effects in zapping, however, the battery manufacturers remain
silent about this treatment. No scientific explanations are available why the method of zapping
improves battery performance. There is little information available regarding the longevity of
the cells after they have been zapped.

How to Restore and Prolong Sealed Lead Acid Batteries

The sealed version of the lead acid battery is designed with a low over-voltage potential to
prevent water depletion. Consequently, the SLA and VRLA systems never get fully charged
and some sulfation will develop over time.

Finding the ideal charge voltage limit for the sealed lead acid system is critical. Any voltage
level is a compromise. A high voltage limit produces good battery performance, but shortens
the service life due to grid corrosion on the positive plate. The corrosion is permanent and

cannot be reversed. A low voltage preserves the electrolyte and allows charging under a wide
temperature range, but is subject to sulfation on the negative plate. (In keeping with portability,
this book focuses on portable SLA batteries. Due to similarities between the SLA and VRLA
systems, references to the VRLA are made where applicable).

Once the SLA battery has lost capacity due to sulfation, regaining its performance is often
difficult and time consuming. The metabolism of the SLA battery is slow and cannot be
hurried.

A subtle indication on whether an SLA battery can be recovered is reflected in the behavior of
its discharge voltage. A fully charged SLA battery that starts its discharge with a high voltage
and tapers off gradually can be reactivated more successfully than one on which the voltage
drops rapidly when the load is applied.

Reasonably good results in regaining lost capacity are achieve d by applying a charge on top
of a charge. This is done by fully charging an SLA battery, then removing it for a 24 to 48 hour
rest period and applying a charge again. This is repeated several times, then the capacity of
the battery is checked with a full discharge. The SLA is able to accept some overcharge,
however, too long an overcharge could harm the battery due to corrosion and loss of
electrolyte.

The effect of sulfation of the plastic SLA can be reversed by applying an over-voltage charge
of up to 2.50V/cell for one to two hours. During that time, the battery must be kept cool and
careful observation is necessary. Extreme caution is required not to raise the cell pressure to
venting point. Most plastic SLA batteries vent at 34 kPa (5 psi). Cell venting causes the
membrane on some SLA to rupture permanently. Not only do the escaping gases deplete the
electrolyte, they are also highly flammable!

The VRLA uses a cell self-regulating venting system that opens and closes the cells based on
cell pressure. Changes in atmospheric pressure contribute to cell venting. Proper ventilation
of the battery room is essential to prevent the accumulation of hydrogen gas.

Cylindrical SLA — The cylindrical SLA (made by Hawker) resembles an enlarged D sized
cell. After long storage, the Hawker cell can be reactivated relatively easily. If affected by
sulfation, the cell voltage under charge may initially raise up to 5V, absorbing only a small
amount of current. Within about two hours, the small charging current converts the large
sulfate crystals back into active material. The internal cell resistance decreases and the
charge voltage eventually returns to normal. At a voltage between 2.10V and 2.40V, the cell is
able to accept a normal charge. To prevent damage, caution must be exercised to limit the
charge current.

The Hawker cells are known to regain full performance with the described voltage method,
leaving few adverse effects. This, however, does not give credence to store this cell at a very
low voltage. It is always best to follow the manufacturer’s recommended specifications.

Improving the capacity of an older SLA by cycling is mostly unsuccessful. Such a battery may
simply be worn out. Cycling would just wear down the battery further. Unlike nickel-based
batteries, the lead acid battery is not affected by memory.

SLA batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a
capacity of 100 percent is difficult to obtain. For practical reasons, most battery analyzers use
a 5-hour discharge when servicing SLA batteries. This typically produces 80 to 90 percent of
the rated capacity. SLA batteries are normally overrated and manufacturers are aware of this.

Caution: When charging an SLA with over-voltage, current limiting must be applied to protect

the battery. Always set the current limit to the lowest practical setting and observe the battery
voltage and temperature during charge. Prevent cell venting.

Important: In case of rupture, leaking electrolyte or any other cause of

exposure to the electrolyte, flush with water immediately. If eye
exposure occurs, flush with water for 15 minutes and consult a

physician immediately.

Simple Guidelines

• Always keep the SLA charged. Never store below 2.10V/cell.

• Avoid repeated deep discharges. Charge more often.

• If repeated deep discharges cannot be avoided, use a larger battery to ease the

strain.

• Prevent sulfation and grid corrosion by choosing the correct charge and float voltages.

How to Prolong Lithium-based Batteries

Today’s battery research is heavily focused on lithium chemistries, so much so that one could
assume that all future batteries will be lithium systems. Lithium-based batteries offer many
advantages over nickel and lead-based systems. Although maintenance free, no external
service is known that can restore the battery’s performance once degraded.

In many respects, Li-ion provides a superior service to other chemistries, but its performance
is limited to a defined lifespan. The Li-ion battery has a time clock that starts ticking as soon
as the battery leaves the factory. The electrolyte slowly ‘eats up’ the positive plate and the
electrolyte decays. This chemical change causes the internal resistance to increase. In time,
the cell resistance raises to a point where the battery can no longer deliver the energy,
although it may still be retained in the battery. Equipment requiring high current bursts is
affected most by the increase of internal resistance.

Battery wear-down on lithium-based batteries is caused by two activities: actual usage or
cycling, and aging. The wear-down effects by usage and aging apply to all batteries but this is
more pronounced on lithium-based systems.

The Li-ion batteries prefer a shallow discharge. Partial discharges produce less wear than a
full discharge and the capacity loss per cycle is reduced. A periodic full discharge is not
required because the lithium-based battery has no memory. A full cycle constitutes a
discharge to 3V/cell. When specifying the number of cycles a lithium-based battery can
endure, manufacturers commonly use an 80 percent depth of discharge. This method
resembles a reasonably accurate field simulation. It also achieves a higher cycle count than
doing full discharges.

In addition to cycling, the battery ages even if not used. The amount of permanent capacity
loss the battery suffers during storage is governed by the SoC and temperature. For best
results, keep the battery cool. In addition, store the battery at a 40 percent charge level.
Never fully charge or discharge the battery before storage. The 40 percent charge assures a
stable condition even if self-discharge robs some of the battery’s energy. Most battery
manufacturers store Li-ion batteries at 15°C (59°F) and at 40 percent charge.

Simple Guidelines

• Charge the Li-ion often, except before a long storage. Avoid repeated deep
discharges.

• Keep the Li-ion battery cool. Prevent storage in a hot car. Never freeze a battery.

• If your laptop is capable of running without a battery and fixed power is used most of

the time, remove the battery and store it in a cool place.

• Avoid purchasing spare Li-ion batteries for later use. Observe manufacturing date
when purchasing. Do not buy old stock, even if sold at clearance prices.

Battery Recovery Rate

The battery recovery rate by applying controlled discharge/charge cycles varies with
chemistry type, cycle count, maintenance practices and age of the battery. The best results
are achieved with NiCd. Typically 50 to 70 percent of discarded NiCd batteries can be
restored when using the exercise and recondition methods of a Cadex battery analyzer or
equivalent device.

Not all batteries respond equally well to exercise and recondition services. An older battery
may show low and inconsistent capacity readings with each cycle. Another will get worse
when additional cycles are applied. An analogy can be made to a very old man for whom
exercise is harmful. Such conditions indicate instabilities caused by aging, suggesting that
this pack should be replaced. In fact, some users of the Cadex analyzers use the recondition
cycle as the acid test. If the battery gets worse, there is strong evidence that this battery
would not perform well in the field. Applying the acid test exposes the weak packs, which can
no longer hide behind their stronger peers.

Some older NiCd batteries recover to near original capacity when serviced. Caution should be
applied when ‘rehiring’ these old-timers because they may exhibit high self-discharge. If in
doubt, a self-discharge test should be carried out.

The recovery rate of the NiMH is about 40 percent. This lower yield is, in part, due to the
NiMH’s reduced cycle count as compared to the NiCd. Some batteries may be afflicted by
heat damage that occurs during incorrect charging. This deficiency cannot be corrected.
Permanent loss of battery capacity is also caused by prolonged storage at elevated
temperatures.

The recovery rate for lead acid batteries is a low 15 percent. Unlike nickel-based batteries,
the restoration of the SLA is not based on reversing crystalline formation, but rather by
reactivating the chemical process. The reasons for low capacity readings are prolonged
storage at low terminal voltage, and poor charging methods. The battery also fails due to age
and high cycle count.

Lithium-based batteries have a defined age limit. Once the anticipated cycles have been
delivered, no method exists to improve the battery. The main reason for failure is high internal
resistance caused by oxidation. Operating the battery at elevated temperatures will
momentarily reduce this condition. When the temperature normalizes, the condition of high
internal resistance returns.

The speed of oxidation depends on the storage temperature and the battery’s charge state.

Keeping the battery in a cool place can prolong its life. The Li-ion
battery should be stored at 40 percent rather than full-charge state.

An increasing number of modern batteries fall prey to the cut-off
problem induced by a deep discharge. This is especially evident o
Li-ion batteries for mobile phones. If discharged below 2.5V/cell,
the internal protection circuit often opens. Many chargers cannot
apply a recharge and the battery appears to be dead.

Some battery analyzers fe it
and enable a recharge if discharged too low. If the cell voltage has fallen too low (1.5V/cell
and lower) and has remained in that state for a few days, a recharge should not be attem
because of safety concerns on the cell(s).

ature a boost, or wake-up function, to activate the protection circu

pted

n

It is often asked whether a restored battery will work as good as a new one. The breakdown
of the crystalline formation can be considered a full restoration. However, the crystalline
formation will re-occur with time if the battery is denied the required maintenance.

When the defective component of a machine is replaced, only the replaced part is new; the
rest of the machine remains in the same condition. If the separator of a nickel-based battery is
damaged by excess heat or is marred by uncontrolled crystalline formation, that part of the
battery will not improve.

Other methods, which claim to restore and prolong rechargeable batteries, have produced
disappointing results. One method is attaching a strong magnet on the side of the battery;
another is exposing the battery to ultrasound vibrations. No scientific evidence exists that
such methods will improve battery performance, or restore an ailing battery.

Chapter 11: Maintaining Fleet Batteries

Unlike individual battery users, who come to know their batteries like a good friend, fleet users
must share the batteries from a pool of unknown packs. While an individual user can detect
even a slight reduction in runtime, fleet operators have no way of knowing the behavior or
condition of the battery when pulling it from the charger. They are at the mercy of the battery.
It’s almost like playing roulette.

It is recommended that fleet battery users set up a battery maintenance program. Such a plan
exercises all batteries on a regular basis, reconditions those that fall below a set target
capacity and ‘weeds out’ the deadwood. Usually, batteries get serviced only when they no
longer hold a charge or when the equipment is sent in for repair. As a result, battery-operated
equipment becomes unreliable and battery-related failures often occur. The loss of adequate
battery power is as detrimental as any other malfunction in the system.

Implementing a battery maintenance plan requires an effort by management to schedule the
required service for the battery packs. This should become an integral component of an
organization’s overall equipment maintenance and repair activities. A properly managed
program improves battery performance, enhances reliability and cuts replacement costs.

The maintenance plan should include all rechargeable batteries in use. Large organizations
often employ a variety of batteries ranging from wireless communications, to mobile
computing, to emergency medical equipment, to video cameras, portable lighting and power
tools. The performance of these batteries is critical and there is little room for failure.

Whether the batteries are serviced in-house with their own battery analyzers or sent to an
independent firm specializing in that service, sufficient spare batteries are required to replace
those packs that have been temporarily removed. When the service is done on location and
the batteries can be reinstated within 24 hours, only five spares in a fleet of 100 batteries are
required. This calculation is based on servicing five batteries per day in a 20 workday month,
which equals100 batteries per month. If the batteries are sent away, five spares are needed
for each day the batteries are away. If 100 batteries are absent for one week, for example,
35 spare batteries are needed.

Manufacturers of portable equipment support battery maintenance programs. Not only does
such a plan reduce unexpected downtime, a well-performing battery fleet makes the
equipment work better. If the recurring problems relating to the battery can be eliminated, less
equipment is sent to the service centers. A well-managed battery maintenance program also
prolongs battery life, a benefit that looks good for the vendor.

The ‘Green Light’ Lies

When charging a battery, the ready light will eventually illuminate, indicating that the battery is
fully charged. The user assumes that the battery has reached its full potential and the battery
is taken in confidence.

In no way does the ‘green light’ guarantee sufficient battery capacity or assure good state-of­health (SoH). Similar to a toaster that pops up the bread when brown (or black), the cha rger
fills the battery with energy and ‘pops’ it to ready when full (or warm).

The rechargeable battery is a corrosive device that gradually loses its ability to hold a charge.
Many users in an organization are unaware that their fleet batteries barely last a day with no
reserve energy to spare. In fact, weak batteries can hide comfortably because little demand is
placed on them in a routine day. The situation changes when full performance is required
during an emergency. Total collapse of portable systems is common and such breakdowns
are frequently related to poor battery performance. Figure 11-1 shows five batteries in various
states of degradation.

Figure 11-1: Progressive loss of charge acceptance.

The rechargeable battery is a corrosive device that gradually loses its ability to hold charge as part of natural aging,
incorrect use and/or lack of maintenance. The unusable part of the battery that creeps in is referred to as ‘rock
content’.

Carrying larger packs or switching to higher energy-dense chemistries does n ot assure better
reliability if the weak batteries are not ‘weeded’ out at the appropriate time. Likewise, the
benefit of using ultra-advanced battery systems offers little advantage if packs are allowed to
remain in the fleet once their performance has dropped below an acceptable performance
level.

Figure 11-2 illustrates four batteries with different ratings and SoH
conditions. Batteries B, C and D show reduced performance because of
memory problems and other deficiencies. The worst pack is Battery D.
Because of its low charge acceptance, this battery might switch to r

eady
after only 14 minutes of charge (assumed time). Ironically, this battery
a likely candidate to be picked when a fresh battery is required in
hurry. Unfortunately, it will last only for a brief moment. Battery A, on t

a

he
other hand, has the highest capacity and takes the longest to charge.
Because the ready light is not yet lit, this battery is least likely picked.

is

Figure 11-2: Comparison of charge and discharge times.

This illustration shows typical charge and discharge times for batteries
Carrying larger batteries or switching to high energy-dense chemistries does not necessarily assure longer runtim
deadwood is allowed to remain in the battery fleet.

with different ratings and SoH conditions.

e if

The weak batteries are charged quicker and remain on ‘ready’ longer than the strong ones.
The bad batteries tend to gravitate to the top. They become a target for the unsuspecting us

er.
In an emergency situation that demands quick charge action, the batteries that show ready
may simply be those that are deadwood.

A weak battery can be compared to a fuel tank with an indentation. Refueling this tank is
quicker than a normal tank because it holds less fuel. Similar to the ‘green light’ on a char

ger,
the fuel gauge in the vehicle will show full when filled to the brim, but the distance traveled
before refueling will be short.

Battery Maintenance, a Function of Quality Control

The reliability of portable equipment relies almost entirely on the performance of the battery. A
dependable battery fleet can only be assured if batteries are maintained on a periodic basis.

Battery maintenance also needs proper documentation. One simple method is attaching a
color dot, each color indicating the month of service. A different color dot is applied when th
battery is re-serviced the following month. A numbering system indicating the month of
service also works well.

e

A better system is attaching a full battery label containing service date and capacity. Lik
pending service on a car,

the label shows the user when maintenance is due. For critical

e the

missions, the user will pick a battery with the highest capacity and the most recent service
date. The label ensures a properly serviced replacement pack.

Battery analyzers are available that print a label revealing the organization, group, service
date, expiry date (time to service the battery), battery capacity a

nd ID number. The label is
generated automatically when the battery is removed from the analyzer. Figure 11-3
illustrates such a label.

11-3: Sample battery label.

The battery label keeps track of the battery in the
same way a service sticker on a car reminds the
owner of pending service.

The battery labeling system is simple to
users would only pick a battery that is proper

manage. It is self-governing in the sense that the

ly labeled and has recently been serviced. The
system does not permit batteries to fall though the cracks and be forgotten. It is in the interest
of the user to ensure continued reliability by bringing in batteries with dated labels for servi

ce.

Battery Maintenance Made Simple

Several methods are available to maintain a fleet of batterie
is illustrated in Figure 11-4 to Figure 11-6. Only 30 m
technician to maintain the system. One or several battery analyzers are needed that are
capable of producing battery labels.

s. A simple, self-governing system

inutes per day should be required for a