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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
rent vibration. Where signs indicate vibration, reex-
highest specific gravity, highest voltage, or combina-
amine the battery supporting/restraining system
tions of both. Pilot cells should be rotated periodi-
cally, usually on a monthly basis. One reason for
and eliminate this source of damaging activity.
this is to limit electrolyte loss. Whenever a cell's
(2) Hydration. Overdischarge of a lead-acid
specific gravity is read, some small amount of elec-
battery without immediate recharge can cause hy-
trolyte will remain in the hydrometer. For a fre-
dration. This can happen if the battery charger is
quently read pilot cell, this loss of electrolyte, al-
shut down or if a lead-acid battery is kept in storage
though very small, could ultimately affect the cell
for an extended period without recharging. The cell
over a long period of time.
must be replaced if irreversible damage is indicated,
h. Temperature readings. Electrolyte tempera-
for example, by a whitish "bathtub ring" visible ap-
tures should be read and be recorded any time
proximately halfway up a clear jar. The lead and
specific-gravity or voltage readings are taken. The
lead compounds in the cell dissolve in the water
specific gravity of the electrolyte varies with tem-
released during overdischarge and form lead hy-
perature. In order to compensate for this effect, the
drate, which is deposited on the separators. Thou-
temperature needs to be recorded at the same time
sands of short circuits between the positive and
that the hydrometer is read.
negative plates will occur when the battery is re-
(1) Differential temperature. Differential elec-
charged after hydration. Hydration can also occur
when a dry-charged battery is mistakenly filled
trolyte temperatures, greater than 5 degrees F (2.75
degrees C), between cells can be a problem. This
with water instead of the electrolyte solution.
problem normally occurs when one portion of a bat-
(3) Frozen electrolyte. Freezing of operating
tery is located near a localized heat source, such as
batteries is unlikely if care is taken when water is
a sunny window or when a battery rack with more
added. When water is added to a battery in freezing
than two steps or tiers is used. A battery tempera-
temperature, the battery must be charged to mix
ture differential will cause some cells to be over-
the water with the electrolyte, or the water will
charged and some cells to be undercharged.
remain on top and freeze. Nominal 1.200 specific-
(2) Ambient temperature. Ambient tempera-
gravity lead-acid electrolyte starts forming slush at
approximately minus 20 degrees F (minus 29 de-
ture of the battery area should be read and recorded
grees C). But, during discharge, a lead-acid cell's
periodically, even where the room or area is environ-
__
specific gravity decreases, and there is a resultant
mentally conditioned. Battery performance is based
increase in the temperature at which slush could
upon the cell electrolyte temperature, which can
form. Freezing would begin at 16 degrees F (minus
differ from the room ambient temperature. Opti-
8 degrees C), if the battery's specific gravity de-
mum battery performance is obtained when electro-
creased to 1.100. Irreparable damage occurs when
lyte temperature is maintained at 77 degrees F (25
ice crystals form within the battery, even though
degrees C).
damage may not be visible. In essence, the frozen
(3) Recording temperature. Some hydrometers
electrolyte will cause the active materials to expand
have a thermometer and table showing the tem-
and lose contact with the grid. The frozen electro-
perature correction that should be applied to the
lyte can also cause structural damage to the jar.
reading. If the hydrometer being used does not have
a thermometer, a battery thermometer should be
14-6. Measurements of battery condition.
placed into the cell and the electrolyte temperature
recorded.
Simultaneous measurements of specific gravity,
voltage, and temperature can identify the condition
(4) Temperature correction. Comparisons are
of the cell. Measurements and tests are often per-
made for readings corrected to 77 degrees F (25
formed on individual cells, referred to as pilot cells,
degrees C). The temperature correction for lead-acid
instead of on the entire battery.
batteries requires adding one point (.00l) to the
hydrometer reading for every 3 degrees F (1.67 de-
a. Pilot cells for voltage and specific gravity mea-
grees C) above 77 degrees F (25 degrees C) and
surements. One or more pilot cells may be chosen to
subtracting one point for every 3 degrees F (1.67
reduce the time necessary to perform inspections
degrees C) below 77 degrees F (25 degrees C).
and tests, while still affording some degree of confi-
c. Specific-gravity readings. Specific gravity is a
dence in the battery's condition. The selection is
arbitrary, but one cell per rack section should be
good indication of state-of-charge of lead-acid cells.
chosen, so that all levels are represented. Some-
Corrections for electrolyte temperature and level
times the pilot cells are selected after a quarterly
must be applied to adjust the specific-gravity read-
--
check of all of the cells' voltages and specific gravi-
ings to a standard reference temperature. Level cor-
ties have been made. Criteria for selection include
rections can vary for each cell type and should be
cells with the lowest specific gravity, lowest voltage,
obtained from the manufacturer. Note that specific-
14-6
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