Battery Maintenance
2240 SRM 1
HYDROMETER
NOTE:
Some batteries have sealed cells. The specific
gravity of the electrolyte cannot be checked on these
batteries.
Use a hydrometer to measure the specific gravity of
the battery. Make sure there is enough electrolyte in
the battery cells. See Adding Water to Battery in this
section. A high level or low level of the electrolyte can
change the specific gravity measurement. When the
electrolyte level is low, the percent of sulfuric acid in
the electrolyte increases. When the electrolyte level
decreases by 3 mm (1/8 in.), the specific gravity mea-
surement can increase by 3 to 5 points.
When you use the hydrometer, make sure there is
enough liquid in the barrel to let the float move freely.
The float must not touch the side, top, or bottom of
the barrel. See Figure 14 for the correct method of
reading the hydrometer.
1.
HIGH FLOAT MEANS HIGH SPECIFIC GRAVITY.
2.
LOW FLOAT MEANS LOW SPECIFIC GRAVITY.
3.
CORRECT METHOD OF READING
HYDROMETER: EYE LEVEL EVEN WITH
SURFACE OF ELECTROLYTE.
Figure 14. Reading Hydrometer
BATTERY TEMPERATURE
The temperature of the electrolyte will change the
reading of the specific gravity. When the tempera-
ture increases approximately 6 C (10
F)
, the specific
gravity will decrease by 0.003 point. See Figure 15
for making specific gravity corrections. If the hy-
drometer you are using does not have a temperature
correction, you will have to use a thermometer. Spe-
cial battery thermometers are available that will in-
dicate the correction factor directly and add or sub-
tract the correct number of points. See Figure 16.
NEVER charge a battery at a rate that will raise the
electrolyte temperature above 49 C (120
F)
. NEVER
let a battery stay discharged for long periods. A tem-
perature above this amount will damage the battery.
The cells in the center of the battery are normally at
the highest temperature. If the battery temperature
is too hot, make sure the ventilation of the battery
is increased and make sure the charge or discharge
rate is not too high. A recommendation for a battery
in service is 8 hours of use (discharge), followed by 8
hours of charging, followed by 8 hours of cooling.
To charge the battery, a direct current must pass
through the cells in the opposite direction to the dis-
charging current. The ampere-hours must be equal
to the discharging ampere-hours plus the energy lost
as heat. This additional amount of charge will vary
according to the battery and the temperature, but the
average additional charge is 12 percent. When the
battery is nearly charged, the final charging must be
at a low rate. A charging rate that is too high will
cause heating in the battery and a high loss of wa-
ter from the electrolyte. The charging of the battery
must be done correctly, or the service life of the bat-
tery will be decreased.
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