(2) Battery cells are filled with a solution of sulfuric acid and
water, known as electrolyte. This solution contains approximately onepart of
acid, by volume, threeparts water. The strength of the electrolyte is
measured in terms of specific gravity. Specific gravity is the ratio of weight
of a given volume of electrolyte to an equal volume of pure water. The
specific gravity of pure water is 1.000. Sulfuric acid has a specific gravity
of 1.830. A mixture of sulfuric acid and water will vary in strength from
1.000 to 1.830. Normally, electrolyte (water and acid mixture) will have a
strength of 1.280 when the battery is fully charged. As a storage battery
discharges, the sulfuric acid is depleted and the electrolyte is converted into
water. This action provides a guide in determining the state of discharge of
the lead acid cell. If the specific gravity reading is below 1.225, the
battery is in a low state of charge. Testing and recharging of a leadacid
battery will be covered later in the subcourse.
CAUTION
Sometimes, pure sulfuric acid will be issued. This acid must be mixed with
water to produce the desired specific gravity. When mixing electrolyte, always
pour the acid into the water. NEVER pour water into the acid.
d. Battery Classification. Storage batteries are classified according to
their rate of discharge and amperehour capacity. Most batteries are rated
according to a 20hour rate of discharge. If a fully charged battery is
completely discharged during a 20hour period, it is discharged at the 20hour
rate. At the end of the discharge time, the average voltage must be 1.75 volts
or higher. If a battery can deliver 20 amperes continuously for 20 hours, the
battery has a rating 20 X 20, or 400 amperehours. The batteries used in
military vehicles are the 6TN battery with a capacity of 45 amperehours. When
more capacity is required than can be supplied by one battery, additional
batteries can be connected in parallel to increase total capacity. If two 100
amp, 12 volt batteries were connected in parallel, they would have a combined
capacity of 200amps. The total voltage would remain the same (12 volts).
e. Maintenance of Storage Batteries.
(1) Visually inspect the battery to determine if maintenance or
repairs are required.
(a) Inspect the battery case for holes or cracks. If a hole or
large crack is evident, turn in the battery to intermediate support
maintenance.
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OD0611
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