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
80F, 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 = Ha x 100
1.210 minus 1.150, or 60 points, and the number
Ht
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
Ha = 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.
STATE OF CHARGE
After a battery is discharged completely from
GASSING
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
