Specific Gravity Readings; Specific Gravity Cell Comparison Test - Mercury 30 Service Manual

Specific Gravity Readings

Use a hydrometer (a) to measure specific gravity of
electrolyte in each cell.
Hydrometer measures percentage of sulfuric acid in
battery electrolyte in terms of specific gravity. As a
battery drops from a charged to a discharged condi-
tion, acid leaves the solution and chemically com-
bines with the plates, causing a decrease in specific
gravity of electrolyte. An indication of concentration
of electrolyte is obtained with a hydrometer.
When using a hydrometer, observe the following
points:
1. Hydrometer must be clean (inside and out) to in-
sure an accurate reading.
2. Never take hydrometer readings immediately af-
ter water has been added. Water must be thor-
oughly mixed with 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 liq-
uid in several times to ensure correct tempera-
ture before taking reading.
2B-2 - ELECTRICAL
a
22532
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 read-
ing at surface of liquid. Disregard curvature
where liquid rises against float stem due to capil-
lary action.
5. Avoid dropping electrolyte on boat or clothing, as
it is extremely corrosive. Wash off immediately
with baking soda solution.
Specific gravity of electrolyte varies not only with per-
centage of acid in liquid but also with temperature.As
temperature drops, electrolyte contracts, so that spe-
cific gravity increases. Unless these variations in
specific gravity are taken into account, specific grav-
ity obtained by hydrometer may not give a true indica-
tion of concentration of acid in electrolyte.
A fully charged battery will have a specific gravity
reading of approximately 1.270 at an electrolyte tem-
perature of 80 F (27 C). If electrolyte temperature
is above or below 80 F, additions or subtractions
must be made in order to obtain a hydrometer read-
ing corrected to 80 F standard. For every 10 F (3.3
C) above 80 F, add 4 specific gravity points (.004) to
hydrometer reading. Example: A hydrometer reading
of 1.260 at 110 F (43 C) would be 1.272 corrected
to 80 F, indicating a fully charged battery.
For every 10 below 80 F, subtract 4 points (.004)
from the reading. Example: A hydrometer reading of
1.272 at 0 F (-18 C) would be 1.240 corrected to 80
F, indicating a partially charged battery.
Specific Gravity Cell Comparison
Test
This test may be used when an instrumental tester is
not available. To perform this test, measure specific
gravity of each cell, regardless of state of charge, and
interpret results as follows: If specific gravity read-
ings show a difference between highest and lowest
cell of .050 (50 points) or more, battery is defective
and should be replaced.
90-826148R2 MARCH 1997