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BATTERY TESTING GUIDE
TABLE OF CONTENTS
Nickel-Cadmium Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Lead-acid (VRLA) Failure Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Single Post Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
A Better Test Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Insulation Resistance Test Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
BATTERY TESTING GUIDE
1
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
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.
WHY BATTERIES ARE NEEDED
Batteries are used to ensure that critical electrical equipment
is always on. There are so many places that 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
Even thought there are many applications for batteries,
they are installed for only two reasons:
Industrial applications for protection and control
To protect and support critical equipment during
an ac outage
Back up of computers, especially financial data
and information
To protect revenue streams due to the loss of service
■
“Less critical” business information systems
The following discussion about failure modes focuses on the
mechanisms and types of failure and why impedance works
so well at finding weak cells. Below is a section containing a
more detailed discussion about testing methods and their
pros and cons.
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.”
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.
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.
Volta invented the primary (non-rechargeable) battery in
1800. Planté invented the lead-acid battery in 1859 and in
1881 Faure first pasted lead-acid plates. With refinements
over the decades, it has become a critically important back-up
power source. The refinements 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.
WHY TEST BATTERY SYSTEMS
There are three main reasons to test battery systems:
■
To insure the supported equipment is adequately
backed-up
■
To prevent unexpected failures
■
To forewarn/predict death
And, there are three basic questions that battery users ask:
BATTERY TYPES
There are several main types of battery technologies with
subtypes:
■
What are the capacity and the condition of the
battery now?
■
When will it need to be replaced?
■
Lead-acid
■
Flooded (wet): lead-calcium, lead-antimony
■
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).
Valve Regulated Lead-acid, VRLA (sealed): lead-calcium,
lead-antimony-selenium
Absorbed Glass Matte (AGM)
■
Gel
■
Flat plate
■
Tubular plate
■
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ATTERY TESTING GUIDE
 ■
Nickel-cadmium
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.
■
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:
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.
PbO
2
+ Pb + 2H
2
SO
4
2PbSO
4
+ 2H
2
+
1
⁄
2
O
2
The acid is depleted upon discharge and regenerated upon
recharge. Hydrogen and oxygen form during discharge and
float charging (because float charging is counteracting self-
discharge). In flooded 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 fibre-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
flooded (wet, vented) lead-acid batteries.
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 sufficiently large
enough to safely carry the electrons, then overheating may
occur (i
2
R heating) and damage the battery or in the worst
cases, damage installed electronics due to smoke or fire.
A battery has alternating positive and negative plates
separated by micro-porous rubber in flooded 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.
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.
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:
FAILURE MODES
Lead-acid (flooded) Failure Modes
■
Positive grid corrosion
■
Sediment (shedding) build-up
2 NiOOH + Cd +2 H
2
O Ni(OH)
2
+ Cd(OH)
2
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.
■
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 flooded lead-acid batteries,
the predominant failure modes are listed above. Some of
them manifest themselves with use such as sediment build-
BATTERY TESTING GUIDE
3
  Intercell Connector 1
Intercell Connector 2
Neg post 1
Pos post 1
Plate#15 (neg
)
Cell #2
Cell #1
Plate #1 (neg)
)
Pos
Ð
top lea
Ñ
Neg
top lea
Ñ
Pos post 2
Neg post 2
Intercell connector 4
Intercell connector 3
Figure 1: Battery Construction Diagram
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.
capacity decreases as depicted in the graph in Figure 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 builds-up to
the point of shorting across the bottom of the plates
rendering the battery useless. The float voltage will drop and
the amount of the voltage drop depends upon how hard the
short is. Shedding, in reasonable amounts, is normal.
The expected failure mode of flooded lead-acid batteries is
positive grid corrosion. 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 specification
for clearance at the bottom of the jar to allow for plate
growth in accordance with its rated lifetime, for example, 20
years.
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.
At the designed end-of-life, the plates will have grown
sufficiently 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
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
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ATTERY TESTING GUIDE
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