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What Constitutes Battery Failures?
Edward P. Rafter, P. E.
President
Tier IV Consulting Group
Lee’s Summit, Missouri

INTRODUCTION
The lead-acid battery is the primary source in use today in support of today’s mission critical systems and
will probably remain so for the foreseeable future. Uninterruptible Power Supplies, telecommunications
and switchgear equipment rely on this battery technology as the heart of their back-up power systems.
Given the high reliability expectations we place on the stationary battery, what are the causes for the
reports of failures often described?

All lead acid batteries have a limited useful life. The normal failure mode that dictates the end of life of a
well-maintained Vented Lead Acid (VLA), commonly known as flooded, battery is positive grid
corrosion. As the grid corrodes, the effective cross section of the conduction path narrows, and the
internal cell resistance starts to increase. At the same time, the grid structure starts to swell and deform to
the point where the paste or active material loses contact with the grid structure. This problem also leads
to increased internal cell resistance as the contact resistance between paste and grid increases. If the
resistance increases are ignored, meaning that the battery is not taken out of service at the appropriate
time, the positive grids will eventually lose their mechanical strength and start to break apart.

The Valve Regulated Lead Acid (VRLA), commonly known as sealed, battery only has about a seven to
ten year life span, and these cells do not live long enough to die of normal positive grid corrosion. The
most common cause for their early demise has been a drying out or loss of water in the electrolyte. There
are investigations being conducted that indicate that secondary reactions from internal recombination of
hydrogen and oxygen gases may adversely affect the polarization voltage of the negative plates and/or
accelerating positive grid corrosion. Both problems lead to a loss of capacity.

The predominant reasons that most batteries fail prematurely are related to one or more of the following:
1. Excessive cycling
2. Improper charging
3. Lack of temperature control
4. Installation
5. Manufacturing problems
6. Operational issues

Excessive Cycling
Every time a battery cycles (a discharge followed by a recharge), the electrochemical generator has to go
to work, which involves converting acid and paste. As the paste on the positive grid changes from PbO2 to
PbSO4, there is a large increase in volume, which puts pressure on the paste. The more the paste is
expanded and then later contracted, the more the wear and tear on it. This means that deeper discharges
are more harmful to the battery. Also, cycling a battery causes accelerated corrosion of the grid structure,
which leads to shorter life. This is especially true for lead calcium batteries, which happens to be the most
popular technology in use today.

The lead calcium battery’s cycling capability depends on the depth of discharge. For example, it is only
capable of 50 deep cycles (the removal of more than 80% of energy), but can deliver 300 cycles for a
25% depth of discharge cycle. A UPS battery which normally only delivers about 25% of its stored
energy during its 15 minute rated reserve time can deliver 300 such cycles.

Improper Charging
Battery manufacturers specify a voltage range for their various cells that must be adhered to. If the
voltage on a given cell is allowed to go either higher or lower than the recommended value, it will have a
detrimental effect on the life of the battery. It should also be noted that the specified voltage range is very
temperature dependent. The right voltage for a battery at 77°F would be too high if the battery was
operated in an ambient temperature of 90°F. It is important for a user to understand the interaction
between voltage and temperature.

Low float voltage (Undercharging) – Undercharging causes sulfate crystals to form on the plate
surfaces, since there is not enough current flowing to keep the battery fully charged. Sulfate crystals that
harden over a long period of time will not go back in solution when proper voltage is applied and,
therefore, result in a permanent loss of capacity. Extended undercharging will also cause a loss of active
material from the negative plates.

High float voltage (Overcharging) – Overcharging causes excessive gassing of hydrogen and oxygen.
This leads to loss of water in flooded cells and dryout in VRLA cells. High float voltage also causes
higher float current, which in turn causes accelerated corrosion and shedding of active material from
positive plates. The recombination of gases to form water in VRLA cells generates heat, and heat causes
higher float currents. Therefore, excessive gassing in VRLA cells can lead to thermal runaway.

Lack of Temperature Control
Batteries are very temperature sensitive, and efforts should be made to maintain the operating temperature
near 77°F. The proper temperature will optimize battery life and is especially critical for VRLA cells. The
recombination of gases within a VRLA cell can only take place at a certain rate. If this rate is exceeded,
gas pressure will build up beyond the safety valve level, and gases/water will be vented out and
permanently lost. At 77°F, the highest float voltage at which a cell can still recombine all the gases driven
off the plates is approximately 2.32 volts. If the cell temperature increased to 90°F while holding the
voltage constant, the cell would dry out and possibly go into thermal runaway. Thermal runaway leads to
a melting down of the jar and, under worst-case scenario, will lead to an explosion and fire.

Low temperature – Battery capacity is diminished at low temperatures. For example, at 62°F, capacity is
approximately 90% versus 100% at 77°F. At low temperatures, a higher float voltage is required to
maintain full charge and, if the charger is not adjusted properly, cells may be undercharged, leading to the
problems described under low voltage.

High temperature – High temperature causes loss of life. For every 15°F rise in operating temperature,
the life is cut in half. High temperature causes increased float current, which means increased corrosion
and, therefore, the loss of life. High temperature also causes gassing, which means loss of water in
flooded cells and dryout and thermal runaway in VRLA cells.

Installation
A lot of battery problems stem from improper installations. A detailed discussion of these is beyond the
scope of this paper, but some of the more common ones are the following:

Loose intercell connections – These can lead to abrupt failures, including fires.

Damaged post seals – Improper cell handling or not supporting cables can damage post seals. This
allows acid to migrate up the post and corrode the post to intercell connection.

Not replacing shipping caps with vent caps – In flooded batteries, this creates internal gas pressures
that will force gases to escape past the post seals, causing post corrosion.

Manufacturing Problems
Manufacturing problems actually represent a small number of the total. Some of the more common
problems, which may not show up for years, are the following:

Faulty post seal design – A leaky post seal allows acid to migrate up to the post/intercell connection
area, causing a connection problem. Sometimes a new design appears to work well, but then suddenly
starts failing after six to eight years in the field.

Internal connection problems – Quality problems in the connection between grid tabs and the
interconnecting bus have been reported from time to time. In multicell jars like six or twelve volt
modules, the intercell connection between adjacent cells may fail as a result of a poor lead burn.
Paste – Problems in the paste formula or improper curing of the paste can have a major impact on the
capacity the battery can store. Some new batteries have been delivered at less than 50% capacity.

Operational Problems
Discharge without recharge – A fully discharged or near fully discharged cell will be damaged and
possibly ruined if not recharged within 24 to 48 hours. As a battery discharges, the electrolyte starts
changing from an acid solution to almost pure water when the battery is fully discharged. Lead
dissolves in water, and some of the plate material mixes with water to form lead hydrate. Lead
hydrate causes the plate surfaces to turn white and, because it is conductive, it forms a short circuit
between the plates, rendering the battery irreversibly damaged.

Over discharge – Over discharge causes abnormal expansion of the active material in the plates,
which leads to permanent damage and also recharges problems. This can happen in lightly loaded
UPS systems that experience an extended power outage.

Excessive discharging (same as excessive cycling) – Some users have local requirements that call
for testing their critical backup systems either weekly or monthly. If this testing includes cycling the
battery, it will severely limit the life of the batteries.

FAILURE ANALYSIS SUMMARY
Battery system failure modes can be broken down into the following two major categories:

1. Abrupt failure – This is a sudden loss of the battery system without any warning while the system is
trying to perform its intended mission. This is the worst-case scenario, as it will lead to very
expensive failures. In a data center application, even a momentary loss may result in millions of
dollars worth of damage. An abrupt failure is cause by an interruption in the conduction path. Typical
failures are:

• Faulty intercell connection – This could be an installation problem or a severely corroded
connection.

• Internal conductance path problems – Current has to flow through the post, to an internal bus,
to the grids, through the paste and electrolyte, to the opposite polarity plate, and then back out
through the other terminal post. Abrupt failures can result from a totally corroded grid that is
breaking apart and only able to pass a low current flow. It can also result from a terminal post that
has lost its copper insert. VRLA cells that have totally dried out can also be viewed as conduction
path failures, since they have no effective path between adjacent plates.

2. Low capacity failure – This is a failure to support the load for the required period of time. Low
capacity results from both mechanical conductance problems as well as electrochemical problems. As
a battery ages and its conduction path (grid, paste to grid connection) starts to deteriorate, the internal
resistance increases. When the battery is placed under load, the voltage drop across the internal
resistance will cause the overall battery voltage to reach the end voltage before its rated time.

The VRLA battery has the additional problem that, as it loses water due to dryout, it loses capacity. In
essence, it loses the energy storage required for a full capacity battery.

The capacity problem is a slowly developing problem that is easily detected and, most of the time,
does not cause expensive outages, since it is rare that full capacity is required during an outage.
Typically, an outage is a short momentary event, and an emergency generator is usually part of the
backup scheme.

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