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.