Megger BITE2, BITE3 Application Guide
Battery
testing guide
▪ Why backup batteries are needed
▪ Battery types
▪ Failure modes
▪ Maintenance philosophies
▪ Practical battery testing
▪ Frequently asked questions
▪ Megger products overview
WWW.MEGGER.COM
Contents
Why backup batteries are needed ................ 4
Why test battery systems .......................................4
Why batteries fail ................................................... 4
Battery types .................................................. 5
Lead-acid overview ................................................5
Nickel-Cadmium Overview .....................................5
Battery construction and nomenclature ..................... 6
Configurations ....................................................... 6
Single post batteries .................................................... 6
Multiple post batteries ................................................ 6
Failure modes ................................................. 7
Lead-acid (flooded) failure modes ..........................7
Lead-acid (VRLA) failure modes ..............................7
Nickel-Cadmium failure modes ..............................8
Maintenance philosophies ............................ 9
How to maintain the battery ..................................... 9
Standards and common practices ........................... 9
IEEE 450 ...................................................................... 9
Inspections .............................................................. 9
Capacity test (discharge test) should be done ........... 9
IEEE 1188 .................................................................. 10
Inspections ............................................................ 10
Capacity test (capacity test) should be done ........... 10
Battery replacement criteria ................................... 10
IEEE 1106 .................................................................. 10
Inspections ............................................................ 10
Capacity test (discharge test) should be done ......... 10
Summary best way to test and evaluate your
battery ................................................................. 10
Test intervals ............................................................. 10
Practical battery testing .............................. 11
Capacity test ...........................................................11
Battery testing matrix – IEEE recommended
practices .............................................................. 11
Procedure for capacity test of vented lead acid
battery ................................................................. 12
Impedance test ....................................................... 13
Impedance theory ................................................13
Intercell connection resistance ................................... 14
Testing and electrical paths ........................................ 15
Voltage ..................................................................... 15
Specific gravity ......................................................... 15
Float current ............................................................. 16
Ripple current ........................................................... 16
Temperature .............................................................. 16
Data analysis ...........................................................17
Locating ground faults on DC systems without
sectionalizing .......................................................... 18
Overview ............................................................. 18
Current test methods ........................................... 18
A better test method ...........................................18
Frequently asked questions ........................ 19
Battery technology summary .................................. 19
Megger products overview ......................... 20
Impedance test equipment ...................................... 20
®
BITE
3 ................................................................20
®
BITE
2 and BITE®2P .............................................21
ProActiv battery database management software ...... 21
®
accessories ....................................................... 21
BITE
Capacity testing ...................................................... 23
TORKEL 820/840/860 ..........................................23
TORKEL accessories ................................................... 23
Ground fault tracing equipment .............................. 24
Battery Ground Fault Tracer (BGFT).......................24
Battery Ground-fault Locator (BGL) ...................... 24
Digital Low Resistance Ohmmeters (DLRO
Microhmmeters (MOM) ..........................................26
DLRO200 and DLRO600 ......................................26
DLRO 247000 series ............................................26
MJÖLNER 200 and MJÖLNER 600 ........................ 27
MOM200A and MOM600A .................................27
MOM690 ............................................................27
Multimeters ............................................................ 28
MMC850 Multi-conductor AC Digital
Clampmeter ........................................................28
Multimeters ......................................................... 28
Insulation Resistance Test Equipment ....................... 29
MIT400 series insulation resistance testers ............ 29
®
) and
BATTERY TESTING GUIDE
3
Why backup
batteries are
needed
Batteries are used to ensure that critical electrical equipment
is always on. There are so many places where batteries are
used – it is nearly impossible to list them all. Some of the
applications for batteries include:
▪ Electric generating stations and substations for protection
and control of switches and relays
▪ Telephone systems to support phone service, especially
emergency services
▪ Industrial applications for protection and control
▪ Back up of computers, especially financial data
and information
▪ “Less critical” business information systems
Without battery back-up hospitals would have to close their
doors until power is restored. But even so, there are patients
on life support systems that require absolute 100% electric
power. For those patients, as it was once said, “failure is not
an option.”
Just look around to see how much electricity we use and
then to see how important batteries have become in our everyday lives. The many blackouts of 2003 around the world
show how critical electrical systems have become to sustain
our basic needs. Batteries are used extensively and without
them many of the services that we take for granted would
fail and cause innumerable problems.
Why test battery systems
There are three main reasons to test battery systems:
battery. Nonetheless, a battery, to work the way it is supposed to work must be maintained properly. A good battery
maintenance program may prevent, or at least, reduce the
costs and damage to critical equipment due to an AC mains
outage.
Even thought there are many applications for batteries,
standby batteries are installed for only two reasons:
▪ To protect and support critical equipment during
an AC outage
▪ To protect revenue streams due to the loss of service
The following discussion about failure modes focuses on
the mechanisms and types of failure and how it is possible
to nd weak cells. Below is a section containing a more
detailed discussion about testing methods and their pros
and cons.
Why batteries fail
In order for us to understand why batteries fail, unfortunately a little bit of chemistry is needed. There are two main
battery chemistries used today – lead-acid and nickel-cadmium. Other chemistries are coming, like lithium, which is
prevalent in portable battery systems, but not stationary, yet.
Volta invented the primary (non-rechargeable) battery in
1800. Planté invented the lead-acid battery in 1859 and
in 1881 Faure rst pasted lead-acid plates. With renements over the decades, it has become a critically important
back-up power source. The renements include improved
alloys, grid designs, jar and cover materials and improved
jar-to-cover and post seals. Arguably, the most revolutionary
development was the valve-regulated development. Many
similar improvements in nickel-cadmium chemistry have
been developed over the years.
▪ To insure the supported equipment is adequately backed-
up
▪ To prevent unexpected failures by tracking the battery’s
health
▪ To forewarn/predict death
And, there are three basic questions that battery users ask:
▪ What is the capacity and the condition of the battery
now?
▪ When will it need to be replaced?
▪ What can be done to improve / not reduce its life?
Batteries are complex chemical mechanisms. They have
numerous components from grids, active material, posts,
jar and cover, etc. – any one of which can fail. As with all
manufacturing processes, no matter how well they are made,
there is still some amount of black art to batteries (and all
chemical processes).
A battery is two dissimilar metallic materials in an electrolyte. In fact, you can put a penny and a nickel in half
of a grapefruit and you now have a battery. Obviously, an
industrial battery is more sophisticated than a grapefruit
4 BATTERY TESTING GUIDE
Battery types
There are several main types of battery technologies with
subtypes:
▪ Lead-acid
▶ Flooded (wet): lead-calcium, lead-antimony
▶ Valve Regulated Lead-acid, VRLA (sealed): lead-calcium,
lead-antimony-selenium
▶ Absorbed Glass Matte (AGM)
▶ Gel
▶ Flat plate
▶ Tubular plate
▪ Nickel-cadmium
▶ Flooded
▶ Sealed
▶ Pocket plate
▶ Flat plate
Lead-acid overview
The basic lead-acid chemical reaction in a sulphuric acid
electrolyte, where the sulphate of the acid is part of the
reaction, is:
PbO
+ Pb + 2H2SO4 2PbSO4 + 2H2 + 1⁄2 O
2
The acid is depleted upon discharge and regenerated upon
recharge. Hydrogen and oxygen form during discharge and
oat charging (because oat charging is counteracting selfdischarge). In ooded batteries, they escape and water must
be periodically added. In valve-regulated, lead-acid (sealed)
batteries, the hydrogen and oxygen gases recombine to form
water. Additionally, in VRLA batteries, the acid is immobilized by an absorbed glass matte (AGM) or in a gel. The
matte is much like the bre-glass insulation used in houses.
It traps the hydrogen and oxygen formed during discharge
and allows them to migrate so that they react back to form
water. This is why VRLA never need water added compared
to ooded (wet, vented) lead-acid batteries.
A battery has alternating positive and negative plates separated by
micro-porous rubber in ooded lead-acid, absorbed glass matte
in VRLA, gelled acid in VRLA gel batteries or plastic sheeting in
NiCd. All of the like-polarity plates are welded together and to the
appropriate post. In the case of VRLA cells, some compression of
the plate-matte-plate sandwich is exerted to maintain good contact
between them. There is also a self-resealing, pressure relief valve
(PRV) to vent gases when over-pressurization occurs.
2
Nickel-Cadmium Overview
Nickel-Cadmium chemistry is similar in some respects
to lead-acid in that there are two dissimilar metals in an
electrolyte. The basic reaction in a potassium hydroxide
(alkaline) electrolyte is:
2 NiOOH + Cd +2 H
However, in NiCd batteries the potassium hydroxide (KOH) does
not enter the reaction like sulphuric acid does in lead-acid batteries.
The construction is similar to lead-acid in that there are alternating positive and negative plates submerged in an electrolyte. Rarely
seen, but available, are sealed NiCd batteries.
O Ni(OH)2 + Cd(OH)2
2
BATTERY TESTING GUIDE
5
Battery construction and
nomenclature
Now that we know everything there is to know about battery
chemistry, except for Tafel curves, ion diffusion, Randles
equivalent cells, etc., let’s move on to battery construction. A
battery must have several components to work properly: a jar
to hold everything and a cover, electrolyte (sulphuric acid or
potassium hydroxide solution), negative and positive plates,
top connections welding all like-polarity plates together and
then posts that are also connected to the top connections of
the like-polarity plates.
All batteries have one more negative plate than positive plate.
That is because the positive plate is the working plate and if
there isn’t a negative plate on the outside of the last positive
plate, the whole outer side of last positive plate will not have
anything with which to react and create electricity. Hence,
there is always an odd number of plates in a battery, e.g., a
100A33 battery is comprised of 33 plates with 16 positive
plates and 17 negative plates. In this example, each positive
plate is rated at 100 Ah. Multiply 16 by 100 and the capacity
at the 8-hour rate is found, namely, 1600 Ah. Europe uses a
little different calculation than the US standards.
In batteries that have higher capacities, there are frequently
four or six posts. This is to avoid overheating of the current-carrying components of the battery during high current
draws or lengthy discharges. A lead-acid battery is a series of
plates connected to top lead connected to posts. If the top
lead, posts and intercell connectors are not sufciently large
enough to safely carry the electrons, then overheating may oc-
2
cur (i
R heating) and damage the battery or in the worst cases,
damage installed electronics due to smoke or re.
To prevent plates from touching each other and shorting
the battery, there is a separator between each of the plates.
Figure 1 is a diagram of a four-post battery from the top
looking through the cover. It does not show the separators.
Configurations
Batteries come in various congurations themselves. Add to
that the many ways that they can be arranged, the number
of possible congurations is endless. Of course, voltage
plays the biggest part in a battery conguration. Batteries
have multiple posts for higher current draws. The more current needed from a battery, the bigger the connections must
be. That includes posts, intercell connectors and buss bars
and cables.
Single post batteries
Smaller battery systems are usually the simplest battery
systems and are the easiest to maintain. They usually have
single post batteries connected with solid intercell connectors. Frequently, they are quite accessible but because they
are small and can be installed in a cubby hole occasionally,
they may be quite inaccessible for testing and maintenance.
Multiple post batteries
Batteries with multiple posts per polarity start to become
interesting quickly. They are usually larger and frequently are
more critical.
Figure 1 Battery construction diagram
6 BATTERY TESTING GUIDE
Failure modes
Lead-acid (flooded) failure
modes
▪ Positive grid corrosion
▪ Sediment (shedding) build-up
▪ Top lead corrosion
▪ Plate sulphation
▪ Hard shorts (paste lumps)
Each battery type has many failure modes, some of which
are more prevalent than others. In ooded lead-acid batteries, the predominant failure modes are listed above. Some
of them manifest themselves with use such as sediment
build-up due to excessive cycling. Others occur naturally
such as positive grid growth (oxidation). It is just a matter
of time before the battery fails. Maintenance and environmental conditions can increase or decrease the risks of
premature battery failure.
Positive grid corrosion is the expected failure mode of
ooded lead-acid batteries. The grids are lead alloys (lead-
calcium, lead-antimony, lead-antimony-selenium) that convert to lead oxide over time. Since the lead oxide is a bigger
crystal than lead metal alloy, the plate grows. The growth
rate has been well characterized and is taken into account
when designing batteries. In many battery data sheets, there
is a specication for clearance at the bottom of the jar to
allow for plate growth in accordance with its rated lifetime,
for example, 20 years.
At the designed end-of-life, the plates will have grown suf-
ciently to pop the tops off of the batteries. But excessive
cycling, temperature and over-charging can also increase the
speed of positive grid corrosion. Impedance will increase
over time corresponding to the increase in electrical resistance of the grids to carry the current. Impedance will also
increase as capacity decreases as depicted in the graph in
gure 2.
Sediment build-up (shedding) is a function of the amount
of cycling a battery endures. This is more often seen in UPS
batteries but can be seen elsewhere. Shedding is the sloughing off of active material from the plates, converting to
white lead sulphate. Sediment build-up is the second reason
battery manufacturers have space at the bottom of the jars
to allow for a certain amount of sediment before it buildsup to the point of shorting across the bottom of the plates
rendering the battery useless. The oat voltage will drop and
the amount of the voltage drop depends upon how hard the
short is. Shedding, in reasonable amounts, is normal.
Some battery designs have wrapped plates such that the
sediment is held against the plate and is not allowed to drop
to the bottom. Therefore, sediment does not build-up in
wrapped plate designs. The most common application of
wrapped plates is UPS batteries.
Corrosion of the top lead, which is the connection between
the plates and the posts is hard to detect even with a visual
inspection since it occurs near the top of the battery and is
hidden by the cover. The battery will surely fail due to the
high current draw when the AC mains drop off. The heat
build-up when discharging will most likely melt then crack
open and then the entire string drops off-line, resulting in a
catastrophic failure.
Plate sulphation is an electrical path problem. A thorough
visual inspection can sometimes nd traces of plate sulphation. Sulphation is the process of converting active plate
material to inactive white lead sulphate. Sulphation is due
to low charger voltage settings or incomplete recharge after
an outage. Sulphates form when the voltage is not set high
enough. Sulphation will lead to higher impedance and a
lower capacity.
Lead-acid (VRLA) failure modes
▪ Dry-out (Loss-of-Compression)
▪ Plate Sulphation (see above)
▪ Soft and Hard Shorts
▪ Post leakage
▪ Thermal run-away
▪ Positive grid corrosion (see above)
Dry-out is a phenomenon that occurs due to excessive heat
(lack of proper ventilation), over charging, which can cause
elevated internal temperatures, high ambient (room) temperatures, etc. At elevated internal temperatures, the sealed
cells will vent through the PRV. When sufcient electrolyte
is vented, the glass matte no longer is in contact with the
plates, thus increasing the internal impedance and reducing
battery capacity. In some cases, the PRV can be removed
and distilled water added (but only in worst case scenarios
and by an authorized service company since removing the
PRV may void the warranty). This failure mode is easily
detected by impedance and is one of the more common
failure modes of VRLA batteries.
Soft (a.k.a. dendritic shorts) and Hard shorts occur for a
number of reasons. Hard sorts are typically caused by paste
lumps pushing through the matte and shorting out to the
adjacent (opposite polarity) plate. Soft shorts, on the other
hand, are caused by deep discharges. When the specic
gravity of the acid gets too low, the lead will dissolve into
it. Since the liquid (and the dissolved lead) are immobilized
by the glass matte, when the battery is recharged, the lead
comes out of solution forming threads of thin lead metal,
known as dendrites inside the matte. In some cases, the lead
dendrites short through the matte to the other plate. The
oat voltage may drop slightly but impedance can nd this
failure mode easily but is a decrease in impedance, not the
typical increase as in dry-out. See gure 2, Abnormal Cell.
Thermal run-away occurs when a battery’s internal components melt-down in a self-sustaining reaction. Normally, this
phenomenon can be predicted by as much as four months
or in as little as two weeks. The impedance will increase in
BATTERY TESTING GUIDE
7
advance of thermal run-away as does oat current. Thermal
run-away is relatively easy to avoid, simply by using temperature-compensated chargers and properly ventilating the
battery room/cabinet. Temperature-compensated chargers
reduce the charge current as the temperature increases.
Remember that heating is a function of the square of the
current. Even though thermal run-away may be avoided by
temperature-compensation chargers, the underlying cause is
still present.
Nickel-Cadmium failure modes
NiCd batteries seem to be more robust than lead-acid. They are
more expensive to purchase but the cost of ownership is similar
to lead-acid, especially if maintenance costs are used in the cost
equation. Also, the risks of catastrophic failure are considerably
lower than for VRLAs.
The failure modes of NiCd are much more limited than leadacid. Some of the more important modes are:
▪ Gradual loss of capacity
▪ Carbonation
▪ Floating effects
▪ Cycling
▪ Iron poisoning of positive plates
Gradual loss of capacity occurs from the normal aging
process. It is irreversible but is not catastrophic, not unlike
grid growth in lead-acid.
Carbonation is gradual and is reversible. Carbonation is
caused by the absorption of carbon dioxide from the air
into the potassium hydroxide electrolyte which is why it is
a gradual process. Without proper maintenance, carbonation can cause the load to not be supported, which can be
catastrophic to supported equipment. It can be reversed by
exchanging the electrolyte.
Floating effects are the gradual loss of capacity due to long
periods on oat without being cycled. This can also cause
a catastrophic failure of the supported load. However,
through routine maintenance, this can be avoided. Floating
effects are reversible by deep-cycling the battery once or
twice.
NiCd batteries, with their thicker plates, are not well-suited
for cycling applications. Shorter duration batteries generally have thinner plates to discharge faster due to a higher
surface area. Thinner plates means more plates for a given
jar size and capacity, and more surface area. Thicker plates
(in the same jar size) have less surface area.
Iron poisoning is caused by corroding plates and is irreversible.
Figure 2 Changes in impedance as a result of battery capacity
8 BATTERY TESTING GUIDE
Maintenance
How to maintain the
philosophies
There are different philosophies and ambition levels for
maintaining and testing batteries. Some examples:
▪ Just replace batteries when they fail or die. Minimum or
no maintenance and testing.
Obviously, not testing batteries at all is the least costly with
considering only maintenance costs but the risks are great.
The consequences must be considered when evaluating
the cost-risk analysis since the risks are associated with
the equipment being supported. Batteries have a limited
lifetime and they can fail earlier than expected. Time
between outages is usually long and if outages are the
only occasions the battery shows its capability risk is high
that reduced or no back-up is available when needed.
Having batteries as back-up of important installations
without any idea of their current health spoils the whole
idea of a reliable system.
▪ Replace after a certain time. Minimum or no maintenance
and testing.
This might also be a risky approach. Batteries can
fail earlier than expected. Also it is waste of capital
if the batteries are replaced earlier than needed.
Properly maintained batteries can live longer than the
predetermined replacement time.
▪ A serious maintenance and testing program in order to
ensure the batteries are in good condition, prolong their
life and to find the optimal time for replacement .
A maintenance program including inspection, impedance
and capacity testing is the way to track the battery’s
state of health. Degradation and faults will be found
before they become serious and surprises can be avoided.
Maintenance costs are higher but this is what you have
to pay for to get the reliability you want for your back-up
system.
The best testing scheme is the balance between maintenance
costs and risks of losing the battery and the supported
equipment. For example, in some transmission substations,
there is upwards of $10 million per hour owing through
them. What is the cost of not maintaining battery systems
in those substations? A $3000 battery is fairly insignicant
compared to the millions of dollars in lost revenues. Each
company is different and must individually weigh the costrisk of battery maintenance.
battery
Standards and common
practices
There are a number of standards and company practices for
battery testing. Usually they comprise inspections (observa-
tions, actions and measurements done under normal oat
condition) and capacity tests. Most well-known are the
IEEE standards:
▪ IEEE 450 for flooded lead-acid
▪ IEEE 1188 for sealed lead-acid
▪ IEEE 1106 for nickel-cadmium
IEEE 450
IEEE 450, “IEEE Recommended Practice for Maintenance, Testing and Replacement of Vented Lead-acid Batteries for Stationary Applications” describes the frequency
and type of measurements that need to be taken to validate
the condition of the battery. The standard covers Inspections, Capacity test, Corrective actions, Battery replacement
criteria etc.
Below is a summarized description for the maintenance, for
the full instructions see the IEEE450 standards.
Inspections
▪ Monthly inspection include appearance and
measurements of string voltage, ripple voltage, ripple
current, charger output current and voltage, ambient
temperature, voltage and electrolyte temperature at pilot
cells, battery float charging current or specific gravity at
pilot cells, unintentional battery grounds etc.
▪ Quarterly inspections include same measurements as
monthly inspection and in addition, voltage of each cell,
specific gravity of 10% of the cells of the battery and float
charging current, temperature of a representative sample
of 10% or more of the battery cells.
▪ Once a year a quarterly inspection should be extended
with, specific gravity of all cells of the battery, temperature
of each cell, cell-to-cell and terminal connection resistance
are performed on the entire string.
Capacity test (discharge test) should be done
▪ At the installation (acceptance test)
▪ Within the first two years of service
▪ Periodically. Intervals should not be greater than 25% of
the expected service life.
BATTERY TESTING GUIDE
9
▪ Annually when the battery shows signs of degradation or
has reached 85% of the expected service life. Degradation
is indicated when the battery capacity drops more than
10% from its capacity on the previous capacity test or is
below 90% of manufacturers rating. If the battery has
reached 85% of service life, delivers 100% or greater of
the manufacturer's rated capacity and has no signs of
degradation it can be tested at two-year Intervals until it
shows signs of degradations.
IEEE 1188
IEEE 1188, “IEEE Recommended Practice for Maintenance, Testing and Replacement of Valve-Regulated
Lead-Acid Batteries for Stationary Applications” denes the
recommended tests and frequency.
Below is a summarized description for the maintenance, for
the full instructions see the IEEE1188 standards.
Inspections
▪ Monthly inspection include battery terminal float voltage,
charger output current and voltage, ambient temperature,
visual inspection and DC float current per string.
▪ Quarterly same measurements as for monthly inspection
shall be done and additionally cell/unit impedance value,
temperature of negative terminal of each cell and voltage
of each cell. For applications with a discharge rate of one
hour or less, resistance of 10% of the intercell connections
shall be measured.
▪ Semi-Annual same measurements as for quarterly
inspection shall be done and additionally a check and
record of voltage of each cell/unit, cell/unit internal ohmic
values, temperature of the negative terminal of each cell/
unit of battery.
▪ Yearly and Initial above measurements should be taken
and in addition, cell-to-cell and terminal connection
resistance of entire battery and AC ripple current and/or
voltage imposed on the battery.
Capacity test (capacity test) should be done
▪ At the installation (acceptance test)
▪ Periodically. Intervals should not be greater than 25% of
the expected service life or two years, whichever is less.
▪ Where impedance values has changed significantly
between readings or physically changes has occurred
▪ Annually when the battery shows signs of degradation or
has reached 85% of the expected service life. Degradation
is indicated when the battery capacity drops more than
10% from its capacity on the previous capacity test or is
below 90% of manufacturers rating.
IEEE 1106
IEEE 1106, “IEEE Recommended Practice for Installation,
Maintenance, Testing and Replacement of Vented Nickel-Cadmium Batteries for Stationary Applications”.
Below is a summarized description for the maintenance, for
the full instructions see the IEEE1106 standards.
Inspections
▪ Inspection at least once per quarter include battery
terminal float voltage, appearance, charger output current
and voltage, pilot-cell electrolyte temperature.
▪ Semi-annually general inspection and measurement of
voltage of each cell shall be done.
Capacity test (discharge test) should be done
▪ Within the first two years of service
▪ At 5-year intervals until the battery shows signs of
excessive capacity loss.
▪ Annually at excessive capacity loss
Summary best way to test and
evaluate your battery
Test intervals
1. Make a capacity test when the battery is new as part of
the acceptance test.
2. Make an impedance test at the same time to establish
baseline values for the battery.
3. Repeat the above within 2 years for warranty purpose.
4. Make an impedance test every year on ooded cells and
quarterly on VRLA cells.
5. Make capacity tests at least for every 25% of expected
service life.
6. Make capacity test annually when the battery has
reached 85% of expected service life or if the capacity
has dropped more than 10% since the previous test or is
below 90% of the manufacturers rating.
7. Make a capacity test if the Impedance value has changed
signicantly.
8. Follow a given practice (preferably from the IEEE
standard) for all temperature, voltage, gravity measure-
ments etc. and ll in a report. This will be a great help
for trending and for fault tracing.
Battery replacement criteria
Both IEEE 450 and IEEE 1188 recommend replacing the
battery if its capacity is below 80% of manufacturer’s rating.
Maximum time for replacement is one year. Physical characteristics such as plate condition or abnormally high cell
temperatures are often determinants for complete battery or
individual cell replacements.
10 BATTERY TESTING GUIDE
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