gravity for this battery, for any number of amp-

or 10 amperes. If the temperature of the electro-

hr delivered to a load. For example, if 70 amp-hr

lyte at the beginning of the charge is not exactly

are delivered by the battery at the 10-hour rate

80°F, correct the time duration of the discharge

or any other rate or collection of rates, the drop

for the actual temperature of the battery.

in specific gravity is 70 × 1.5, or 105 points.

A battery of 100 percent capacity discharges

at its 10-hour rate for 10 hours before reaching

Conversely, if the drop in specific gravity per

its low-voltage limit. If the battery or one of its

amp-hr and the total drop in specific gravity are

cells reaches the low-voltage limit before the

known, you can determine the amp-hr delivered

10-hour period has elapsed, discontinue the

by a battery. For example, if the specific gravity

discharge immediately and determine the per-

of the previously considered battery is 1.210 when

centage of capacity using the following equation:

the battery is fully charged and 1.150 when it is

partly discharged, the drop in specific gravity is

c = H*a *x 100

1.210 minus 1.150, or 60 points, and the number

H*t*

of amp-hr taken out of the battery is 60/1.5, or

40 amp-hr. Thus, the number of amp-hr expended

Where

in any battery discharge can be determined from

the following items:

C = percentage of amp-hr capacity available

1. The specific gravity when the battery is

H*a *= total hours of discharge

fully charged

Ht = total hours for 100 percent capacity

2. The specific gravity after the battery has

been discharged

For example, a 100 amp-hr, 6-volt battery

delivers an average current of 10 amperes for 10

3. The reduction in specific gravity per

hours. At the end of this period, the battery

amp-hr

voltage is 5.25 volts. On a later test the same

battery delivers an average current of 10 amperes

Voltage alone is not a reliable indication of

for only 7 hours. The discharge was stopped at

the state of charge of a battery, except when the

the end of this time because the voltage of the

voltage is near the low-voltage limit on discharge.

middle cell was found to be only 1.65 volts. The

During discharge, the voltage falls. The higher the

percentage of capacity of the battery is now

7/10 × 100, or 70 percent. Thus, the amp-hr

rate of discharge, the lower will be the terminal

voltage. Open-circuit voltage is of little value

c a p a c i t y of this battery is reduced to

0.7 × 100 = 70 amp-hr.

because the variation between full charge and

complete discharge is so small--only about 0.1

Record the date for each test discharge on the

volt per cell. However, abnormally low voltage

storage battery record sheet.

does indicate injurious sulfation or some other

serious deterioration of the plates.

After a battery is discharged completely from

full charge at the 10-hour rate, the specific gravity

has dropped about 150 points to about 1.060. You

When a battery is being charged, a portion of

can determine the number of points the specific

the energy is dissipated in the electrolysis of the

gravity drops per amp-hr for each type of battery.

water in the electrolyte. Thus, hydrogen is released

For each amp-hr taken out of a battery, a definite

at the negative plates and oxygen at the positive

amount of acid is removed from the electrolyte

plates. These gases bubble up through the electro-

and combined with plates.

lyte and collect in the air space at the top of the

For example, if a battery is discharged from

cell. If violent gassing occurs when the battery is

full charge to the low-voltage limit at the 10-hour

first placed on charge, the charging rate is too

rate, and if 100 amp-hr are obtained with a

high. If the rate is not too high, steady gassing,

specific gravity drop of 150 points, there is a drop

which develops as the charging proceeds, indicates

of 150/100, or 1.5 points per amp-hr delivered.

that the battery is nearing a fully charged

If you know the reduction in specific gravity per

condition.

amp-hr, you can predict the drop in specific

10-16

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