Specific Gravity Cell Comparison Test; Electrolyte Level; Charging A Discharged Battery - Mercury 40 Service Manual

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
specific 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.

Electrolyte Level

Check electrolyte level in battery regularly. A battery
in use in hot weather should be checked more fre-
quently because of more rapid loss of water. If elec-
trolyte level is found to be low, then distilled water
should be added to each cell until liquid level rises ap-
proximately 3/16" (4.8mm) over plate. DO NOT
OVERFILL, because this will cause loss of electro-
lyte and result in poor performance, short life and ex-
cessive corrosion.
CAUTION
During service, only distilled water should be
added to the battery, not electrolyte.
90-852572R1 JANUARY 1998
Charging A Discharged
Battery
The following basic rules apply to any battery charg-
ing situation:
1. Any battery may be charged at any rate, (in am-
peres) as long as spilling of electrolyte (from vio-
lent gassing) does not occur, and as long as elec-
trolyte temperature does not exceed 125 F (52
C). If spewing of electrolyte occurs, or if electro-
lyte temperature exceeds 125 F, charging rate
(in amperes) must be reduced or temporarily
halted to avoid damage to the battery.
2. Battery is fully charged when, over a 2-hour peri-
od at a low charging rate (in amperes), all cells
are gassing freely (not spewing liquid electro-
lyte), and no change in specific gravity occurs.
Full charge specific gravity is 1.260-1.275, cor-
rected for electrolyte temperature with electrolyte
level at 3/16" (4.8 mm) over plate. For most satis-
factory charging, lower charging rates in am-
peres are recommended.
3. If, after prolonged charging, specific gravity of at
least 1.230 on all cells cannot be reached, battery
is not in optimum condition and will not provide
optimum performance; however, it may continue
to provide additional service, if it has performed
satisfactorily in the past.
4. To check battery voltage while cranking engine
with electric starter motor, place red (+) lead of
tester on positive (+) battery terminal and black
(-) lead of tester on negative (-) battery terminal.
If the voltage drops below 9-1/2 volts while crank-
ing, the battery is weak and should be recharged
or replaced.
ELECTRICAL - 2B-5