The electrolyte solution in nickel-cadmium batteries consists of purified
caustic potash (KOH potassium hydroxide) and other salts in distilled water.
Liquid electrolyte should be stored in a clean, glass, or porcelain container.
The electrolyte will readily absorb carbon dioxide from air to form potassium
carbonate. This process will temporarily lower battery capacity. Electrolyte
must therefore be stored in airtight containers. The specific gravity of
electrolyte does not change with state of charge but remains almost constant
on charge and discharge. The average specific gravity of a normal cell will
be about 1.190 at 72 EF;
; however, in servicing a battery, always refer to the
manufacturer's recommended specific gravity range. The specific gravity
range for ni-cad cells has limits. Below 1.200, the resistivity begins to
increase rapidly, making cells sluggish, and the lowest electrolyte density is
usually set at 1.170. High concentrations are damaging because of the
increased solubility of the iron electrodes, especially at higher temperatures.
Concentrations much higher than 1.170 also result in increased resistivity.
The proper density of the electrolyte is a compromise held within narrow
The freezing point of electrolyte with a specific gravity of 1.190 is about -10 °F,
at which the solution forms a slush but will not freeze solid. If the
battery will encounter temperatures colder than -10 EF, specific gravity is
usually raised to 1.230 for protection to -40 EF. Always consult the
manufacturer before attempting to change the specific gravity of electrolyte.
6.11 ELECTROLYTE LEVEL
6.11 ELECTROLYTE LEVEL
Cells lose water through natural evaporation and when gassing on equalizing
charge. Always keep the plates covered with electrolyte. Serious damage
will occur if the plate tops are exposed to air.
Check the electrolyte level in every cell during the visual inspection. The
level can be observed by looking closely at the cell. Cell cases are typically
translucent, and the electrolyte level can be seen through the cell case.
When electrolyte level is low, add distilled water to the proper height but do
not overfill. If cells are overfilled, the electrolyte may be forced out of vents
on charge. This condition can cause electrolysis between the cells,
corrosion of the cell containers, and grounds in the electrical circuit. The
maximum level of the electrolyte is
halfway between the tops of the plates and
the inside of the cell covers.
To retard natural evaporation, pure mineral oil should be floated on the
electrolyte in each cell. All cells should be checked annually for adequate
oil depth, (about 1/4 inch). See manufacturer's instructions for the
recommended oil type and depth.