Guide to VRLA Batteries

6
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Avoid ultra-deep discharges. The definition of ultra-deep
discharge may vary with application and battery type.
n
Don’t leave a battery at a low stage of charge for an
extended length of time. Charge a discharged battery as
soon as possible.
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Don’t cycle a battery at a low state of charge without
regularly recharging fully.
Use the highest initial charging current available (up to 30%
of the 20-hour capacity per hour) while staying within the
proper temperature-compensated voltage range.
What is a thermal runaway?
The appropriate charge voltage depends on the battery tem-
peratures. A warmer battery requires a reduced voltage. If the
voltage is not reduced, current accepted by the battery
increases. When the current increases, the internal heating
increases. This can rise to destructive levels if not taken into
consideration.
Thermal runaway can be prevented with:
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Temperature compensation monitoring at the battery —
not at the charger.
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Limiting charging currents to appropriate levels.
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Allowing for adequate air circulation around the batteries.
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Using timers or ampere-hour counters.
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Using smart chargers that recognize the signature of a
thermal runaway event which will shut the charger down.
Why do some VRLA batteries have a concave or
convex appearance?
CONCAVE APPEARANCE
The gas pressure in a VRLA battery can be lower than ambi-
ent pressure under certain circumstances. This partial
vacuum will pull the container walls and cover inward. This
may give the ends, sides of each cell, and tops of each cell a
noticeable concave (inwardly dished) appearance. This is
normal. The product is not defective. Its performance is not
compromised in any way. In some situations charging will
reduce or eliminate this appearance. Internal gas pressure
being lower than external pressure has several potential
causes that can operate alone or in combination. The
recombination process continues after charging ends
consuming most of the headspace oxygen — decreasing
internal pressure. An increase in external pressure will result
from a decrease in altitude. Cooling the battery reduces the
internal gas pressure by contraction and also by causing some
water vapor to return to liquid form. Severe discharging
shrinks the volume of the internal materials. This leaves more
volume for the gas to fill reducing the internal pressure.
CONVEX APPEARANCE
To prevent the permanent loss of gases so that recombination
has time to take place, each cell can hold an internal gas pres-
sure above external pressure before venting. Batteries with very
large cells will slightly bulge as this normal pressure builds.
This is especially noticeable at higher temperatures because
the polypropylene case is more pliable when warm. Therefore,
a certain amount of bulge is normal. If a battery bulges severe-
ly on charge, this is not normal. It is an indication of a blocked
valve or an overcharge situation. Such a battery should be
removed from service.
V. GLOSSARY TERMS
ACTIVE MATERIAL — The porous structure of lead compounds
that produce and store electrical energy within a lead-acid battery.
The active material in the positive plates is lead dioxide and that in
the negative is metallic sponge lead. When an electrical circuit is
created, these materials react with sulfuric acid during charging
and discharging according to the following chemical reaction:
PbO
2
+ Pb + 2H
2
SO
4
= 2PbSO
4
+ 2H
2
O.
AGM (Absorbed Glass Mat) — A type of non-woven separator
material comprised almost entirely of glass microfibers that
absorbs and retains the electrolyte leaving no free electrolyte in
the cell to spill. VRLA batteries made with this material are often
referred to as “AGM” batteries.
AMPERE (Amp, A) — The unit of measure of the electron flow
rate, or current, through a circuit.
AMPERE-HOUR (Amp-Hr, Ah) — A unit of measure for a battery’s
electrical storage capacity, obtained by multiplying the current in
amperes by the time in hours of discharge. (Example: A battery
that delivers 5 amperes for 20 hours delivers 5 amperes X 20
hours = 100 Amp-Hr of capacity.)
BOOST CHARGE — The process of ensuring that the cells and
plates within a battery are charged sufficiently for the battery to
perform its desired function. Boost charging is typically done for a
short duration at a high current.
CAPACITY — The capacity of a battery is specified as the number
of Amp-Hrs that the battery will deliver at a specific discharge rate
and temperature. The capacity of a battery is not a constant value
and is seen to decrease with increasing discharge rate. The capaci-
ty of a battery is affected by a number of factors such as: active
material weight, density of the active material, adhesion of the
active material to the grid, number, design and dimensions of
plates, plate spacing, design of separators, specific gravity and
quantity of available electrolyte, grid alloys, final limiting voltage,
discharge rate, temperature, internal and external resistance, age,
and life history of the battery.
CONTAINER AND COVER — The reservoir and lid containing the
battery parts and electrolyte made from impact and acid resistant
material such as polypropylene.
CELL — The basic electrochemical current-producing unit in a
battery, consisting of a set of positive plates, negative plates,
electrolyte, separators, and casing. In a lead-acid battery the cell
has an open-circuit voltage of approximately 2-volts. There are
six cells in a 12-volt lead-acid battery.
CIRCUIT — An electrical circuit is the path followed by a flow of