Guide to VRLA Batteries

East Penn Manufacturing Co. Lyon Station, PA 19536-0147 Phone: [phone] Fax: [phone] Order Department Hotline: [phone]
E.P.M. Form No. 1927 04/15 © 2015 by EPM Printed in U.S.A.
All data subject to change without notice. No part of this document may be copied or
reproduced, electronically or mechanically, without written permission from the company.
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epending on the active material they hold.
POSITIVE — Designating, or pertaining to, a kind of electrical
potential; opposite of negative. A point or terminal on a battery
having higher relative electrical potential. The positive battery ter-
minal is the point to which electrons flow during discharge.
PRIMARY BATTERY — A battery that can store and deliver electri-
cal energy but cannot be recharged. A lead-acid battery is NOT a
p
rimary battery.
RESERVE CAPACITY RATING — The time in minutes that a new,
fully charged battery will deliver 25 amperes at 27°C (80°F) and
maintain a terminal voltage equal to, or higher than, 1.75 volts per
cell. This rating represents the time the battery will continue to
operate essential accessories if the alternator or generator of a
vehicle fails.
RESISTANCE — The opposition to the free flow of current in a
circuit or battery. It is commonly measured in Ohms.
SECONDARY BATTERY — A battery that can store and deliver
electrical energy and can be recharged by passing direct current
through it in a direction opposite to that of discharge. A lead-acid
battery is a secondary battery.
SEPARATOR — A porous divider between the positive and neg-
ative plates in a cell that allows the flow of ionic current to pass
through it, but not electronic current. Separators are made from
numerous materials such as: polyethylene, polyvinyl chloride,
rubber, glass fiber, cellulose, etc.
SHORT CIRCUIT — An unintended current-bypass in an electric
device or wiring. Outside the battery a short circuit is estab-
lished when a conductive path is established between the two
terminals of a battery. Inside a battery, a cell short circuit is the
result of contact between the positive and negative plates and
will cause a cell to discharge and render the battery useless.
SPECIFIC GRAVITY (Sp. Gr. or SG) — Specific Gravity is a
measure of the electrolyte concentration in a battery. This mea-
surement is based on the density of the electrolyte compared to
the density of water and is typically determined by the use of a
hydrometer (see Hydrometer). By definition, the specific gravity of
water is 1.00 and the specific gravity of the sulfuric acid elec-
trolyte in a typical fully charged battery is 1.265-1.285. Specific
gravity measurements are typically used to determine if the bat-
tery is fully charged or if the battery has a bad cell.
STATE OF CHARGE — The amount of deliverable low-rate elec-
trical energy stored in a battery at a given time expressed as a
percentage of the energy when fully charged and measured
u
nder the same discharge conditions. If the battery is fully
charged the “State of Charge” is said to be 100%.
STRATIFICATION — The unequal concentration of electrolyte due
to density gradients from the bottom to the top of a cell. This con-
dition is encountered most often in batteries recharged from a
deep discharge at constant voltage without a great deal of gassing.
Continued deep cycling of a ‘stratified’ battery will result in soften-
ing of the bottoms of the positive plates. Equalization charging is a
w
ay to avoid acid stratification.
SULFATION — The generation or conversion of the lead sulfate
discharge in the plates to a state that resists normal recharge.
Sulfation often develops when a battery is stored or cycled in a
partially discharged state at warm temperatures.
TERMINALS — The electrical structures on the battery to which
the external circuit is connected. Typically, batteries have either
top-terminals (posts) or side-terminals. Some batteries have both
types of terminals (dual-terminal).
VENTS — Mechanisms that allow gasses to escape from the bat-
tery while retaining the electrolyte within the case. Flame arresting
vents typically contain porous disks that reduce the probability of
an internal explosion as a result of an external spark. Vents come
in both permanently fixed and removable designs.
VOLT — The unit of measure for electrical potential or voltage.
VOLTMETER — An electronic device used to measure voltage,
normally in a digital format.
VOLTAGE DROP — The net difference in the electrical potential
(voltage) when measured across a resistance or impedance
(ohms). Its relationship to current is described in Ohm’s law.
VRLA — Valve Regulated Lead Acid battery. AGM and Gel are the
two types of VRLA batteries. These batteries have no “free” liquid
electrolyte and in the cell operate on the oxygen recombination
cycle, which is designed to minimize water loss. VRLA batteries
feature valves that have a one-way, pressure-relief design. These
low-pressure valves prohibit air entering the cell while permit-
ting gases to vent from the cell if necessary. The pressure
maintained in the battery, though only very slight, is required to
facilitate the oxygen recombination reaction, which converts the
oxygen generated at the positive plates back into water.