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10-16
BATTERY
ELECTRICAL SYSTEMS
(292)
imate idea of specific gravity reading is re-
quired, it will not be necessary to make the
temperature correction. Hydrometers are avail-
able which have a built-in thermometer and
temperature correction scale similar to figure
10-4. This type of hydrometer simplifies the
operation of obtaining a true specific gravity
reading.
d. Using Hydrometer
When using a hydrometer, observe the fol-
lowing points:
1. Hydrometer must be clean, inside and
out, to insure an accurate reading.
2. Hydrometer readings must never be taken
immediately after water has been added. The
water must be thoroughly
mixed'
with the
elec-
trolyte by charging for at least 30 minutes be-
fore hydrometer values are reliable.
3.
If
hydrometer has built-in thermometer,
draw liquid into it several times to insure
cor-
rect temperature before taking a reading.
4. Hold hydrometer vertically and draw in
just enough liquid from battery cell so that float
is free floating, and with bulb fully released.
Hold hydrometer at eye level so that float is
vertical and free of outer tube, then take read-
ing at surface of
liquid.
Disregard the curva-
ture where the liquid rises against float stem
due to surface tension.
5.
Avoid dropping liquid on car or clothing
as it is extremely corrosive. Any liquid that
drops should be washed off immediately with
soda solution.
e. Meaning of Specific Gravity Reading
A battery in good condition should have
spe-
cific gravity of not less than 1.240 in seasons
when freezing of water may occur, or 1.215 in
seasons when freezing of water does not occur.
The battery must be recharged if specific grav-
ity is less than these values. The approximate
state of charge of a battery as indicated by
specific
gravity corrected to
80°
F. is as fol-
lows
:
1.265 to 1.290
Fully charged
1.235 to 1.260
Three-fourths
charged
1.205 to 1.230
One
-half charged
1.170 to 1.200
One-fourth charged
1.140 to 1.165
Barely operative
1.110 to 1.135
Completely discharged
The specific gravity of a charged battery
should not vary more than 25 points (.025) be-
tween cells. A greater variation could be due to
the low cell being shorted,
to
loss of electrolyte,
or to the fact that the battery is old. Such bat-
tery should be removed for a slow charge and
further tests.
10-20 HIGH DISCHARGE TEST OF
BATTERY
The hydrometer test (par. 10-19) reports
only the chemical condition of the battery. A
high discharge test with a voltmeter gives ad-
ditional information on the condition of bat-
tery and its ability to deliver current under
load.
If
the battery specific gravity is below
1.215 the high discharge test may be made.
If
specific gravity is below 1.215 the high dis-
charge test should not be made, since the bat-
tery will not maintain a very good voltage
reading under discharge if it is more than half
discharged.
CAUTION: Gas formed in battery during
charge is dangerously explosive. Sparks pro-
duced by test equipment may set off an explo-
sion if gas remains in top of battery cells. If
battery has been gassing recently, remove filler
plugs, blow the gas out of cells and away from
area, and install filler plugs before performing
high discharge tests.
Figure 10-5-High Discharge Test with Voltmeter, Ammeter, and
Rheos
tat
a. High Discharge Test With Voltmeter,
Ammeter, and Rheostat
A high discharge test of battery may be
made with equipment comprising a voltmeter
(5 volts minimum), ammeter of 300 or more
amperes capacity, and a carbon-pile rheostat
having a minimum capacity of 300 amperes
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