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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
relationship holds for cells that are truly open-
gravity readings, taken within 72 hours of the ter-
circuited (that is, there is no current flowing
mination of an equalizing charge or a water addi-
through the cell). The battery should have a well-
tion, will not be correct. These specific-gravity
mixed electrolyte and been off charge for more than
readings are inaccurate because the added water
16 hours. A voltage below that expected by equation
has not been properly mixed with the existing elec-
14-l indicates there may be a problem.
trolyte solution and stratification occurs.
(1) Differences in specific gravity. Lead-
Open-circuit voltage =
(eq. 14-1)
antimony or lead-calcium battery electrolytes do not
Specific gravity + 0.84
always have the same nominal specific gravities,
e. Connection resistance. A connection resistance
even if the plate alloy is the same. Maintenance
check is very important but is often neglected, even
personnel should not install a replacement cell
though it can be conducted with the battery in ser-
which requires a specific-gravity electrolyte differ-
vice. The instruments normally used are the same
ent from the existing cells. In similar cells with
as those used to measure a power circuit-breaker's
different specific gravities, the higher specific grav-
contact resistance. A moderate to high current is
ity cells will have higher float voltage requirements,
passed through the connection under test and the
provide increased local action, and consume more
voltage drop is measured and converted at the
water. Some application considerations may also
meter output to microhms. These measurements
cause a manufacturer to vary the nominal 1.200
are difficult to perform, especially when the cells
specific gravity for stationary cells. High or low am-
have multiple posts per cell and multiple intercell
bient temperatures influence specific-gravity re-
connectors per post. In these cases, multiple mea-
quirements. A higher specific gravity electrolyte is
surements per cell must be made or there will be
provided when ambient temperatures are extremely
significant errors in the measurement. The test,
low. This increases cell performance and serves to
performed at the initial installation, should be re-
lower the freezing point of the electrolyte. Similarly,
peated periodically and the results compared. High
with high ambient temperatures, normally above 90
connection resistance, if not detected, can cause se-
degrees F (32 degrees C), a lower specific-gravity
vere damage, especially in a stationary cell required
electrolyte is provided to reduce losses and main-
to discharge at a high current rate for a period of
tain expected life.
time. High connection resistance can actually melt
(2) Comparisons. The measured specific grav-
battery posts.
ity should be corrected to the reference temperature
14-7. Battery maintenance specifics.
and compared to previous data. Readings should be
uniform, with a minimum different between the
The battery installations must be kept clean. Mea-
high and low readings. Where specific gravities vary
surements which determine the need for water or
considerably over the battery, they are termed
electrolyte additions are required. Pilot cell com-
"ragged" and corrective action is required, as cov-
parisons are necessary. Battery charging and water
ered in ANSI/IEEE 450 and ANSE/IEEE 1106.
quality help in providing the optimum operation
(3) Method of measurement. With all cells con-
and life for battery installations.
nected in series, the specific gravity reading of one
a. Cleaning. Keep batteries; connections, and
cell, known as a pilot cell, indicates the state of
battery racks clean at all times.
discharge or charge of the whole battery. When a
(1) Battery cells. Jars or covers should be wiped
reading is being taken, the nozzle of the hydrometer
with a clean lint-free cloth or wiper moistened with
syringe is inserted into the cell, and just enough
clean water. The cloth should be moistened with a
electrolyte is drawn from the cell to float the hy-
suitable neutralizing agent to clean any electrolyte
drometer freely without touching at the top or bot-
spilled on the cover or jar. Remove moisture with a
tom. Read the specific gravity on the float, making
clean dry cloth or wiper, once cleaning is finished.
sure to obtain this reading at the bottom of the
Never use solvents, detergents, oils, waxes, pol-
meniscus (the bottom of the liquid-surface curva-
ishes, or ammonia to clean the jars, as this may
ture). For correct readings be sure to hold the hy-
cause permanent damage to the jar. For lead-acid
drometer in a true vertical position to avoid the
batteries; use tin acid-neutralizing agent consisting
float's touching the cylinder walls. After testing, the
of a soda solution. The soda solution should consist
electrolyte should ways be returned to the cell. Cell
of one pound (454 grams) of bicarbonate of soda to
readings can be inaccurate if taken sooner than 72
one gallon (3.785 liters) of water. Other neutralizing
hours after equalizing or water corrections.
agents may damage the jar. The solution must not
get into the cells.
d. Voltage readings. The open-circuit voltage of a
(2) Battery connections. Terminal posts and
lead-acid cell is a direct function of specific gravity
connections should be wiped with a clean lint-free
and can be approximated by equation 14-1. This
14-7
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