GMC 1976 ZEO 6083 Maintenance Manual page 388

6Y- 8 ENGINE ELECTRICAL
2. If able to obtain a specific gravity of 1 .200 or
more @ 80°F. (26.7°C .) in all cells, remove the vent
caps and connect a 300 amp. load for 15 seconds.
a. If smoke occurs in one or more cells, re-
place the battery.
b. If smoke does not occur proceed to step 3.
3. Place a thermometer in one cell and apply a
specified load from specifications . Read the voltage
at 15 seconds with load connected, then remove load
and read electrolyte temperature. Compare tempera-
ture and voltage readings with voltage-temperature
chart in "Battery Specifications" .
a. If reading is less than voltage on chart,
replace battery.
b. If reading is same as or greater than voltage
on chart, fully charge, clean and return battery to
service.
Specific Gravity Readings (Figure 13)
A hydrometer can be used to measure the specific
gravity of the electrolyte in each cell .
The hydrometer measures the percentage of sulphu-
ric acid in the battery electrolyte in terms of specific
gravity. As a battery drops from a charged to a dis-
charged condition, the acid leaves the solution and
enters the plates, causing a decrease in specific
gravity of electrolyte . An indication of the concen-
tration of the electrolyte is obtained with a hydrome-
ter .
When using a hydrometer, observe the following
points :
TAKE READING
AT EYE LEVEL
Figure 13-Checking Specific Gravity
1 . Hydrometer must be clean, inside and out, to
insure an accurate reading.
2. Hydrometer readings must never be taken im-
mediately after water has been added. The water
must be thoroughly mixed with the electrolyte by
charging for at least 15 minutes at a rate high enough
to cause vigorous gassing .
3 . If hydrometer has built-in thermometer, draw
liquid into it several times to insure correct tempera-
ture before taking reading.
4. Hold hydrometer vertically and draw in just
enough liquid from battery cell so that float is free
floating . Hold hydrometer at eye level so that float
is vertical and free of outer tube, then take reading
at surface of liquid . Disregard the curvature where
the liquid rises against float stem due to surface ten-
sion .
5. Avoid dropping battery fluid on vehicle or
clothing as it is extremely corrosive. Any fluid that
drops should be washed off immediately with baking
soda solution .
The specific gravity of the electrolyte varies not
only with the percentage of acid in the liquid but also
with temperature. As temperature increases, the
electrolyte expands so that the specific gravity is re-
duced. As temperature drops, the electrolyte con-
tracts so that the specific gravity increases. Unless
these variations in specific gravity are taken into
account, the specific gravity obtained by the hy-
drometer may not give a true indication of the con-
centration of acid in the electrolyte .
A fully charged Battery will have a specific
gravity reading of approximately 1 .270 at an elec-
trolyte temperature of 80°F . (26.7° C.) If the elec
trolyte temperature is above or below 80°F. (26.7°C .),
additions or subtractions must be made in order to
obtain a hydrometer reading corrected to the 80°F .
(26.7° C.) standard . For every 10° above 80°F .
(26.7°C .), add four specific gravity points (.004) to
the hydrometer reading. Example: A hydrometer
reading of 1 .260 at 110°F. (43 .3°C .) would be 1 .272
corrected to 80 °C. (26.7°C .), indicating a fully
charged battery. For every 10° below 80°F. (26.7°C .),
subtract four points (.004) from the reading. Exam-
ple: A hydrometer reading of 1 .272 at 0°F. (-17 .8°C .)
would be 1 .240 corrected to 80°F . (26.7°C), indicat-
ing a partially charged battery .
Specific Gravity Cell Comparison Test
To perform this test measure the specific gravity
of each cell, regardless of state of charge, and inter-
pret the results as follows :