Lead-Acid Storage Batteries
Overdischarge may cause difficulties in recharging the cell by increasing the battery's
internal resistance. Also, overdischarging may cause lead to be precipitated in the
separator and cause a short in the cell or between cells.
Maintaining Electrolyte Levels
During normal operation, water is lost from a flooded lead-acid battery as a result of
evaporation and electrolysis into hydrogen and oxygen, which escape into the
atmosphere. One Faraday of overcharge will result in a loss of about 18 g of water.
Evaporation is a relatively small part of the loss except in very hot, dry climates. With a
fully charged battery, electrolysis consumes water at a rate of 0.336 cm per ampere-
hour overcharge. A 5000-Ah battery overcharged 10% can thus lose 16.8 cm , or
about 0.3%, of its water each cycle. It is important that the electrolyte be maintained at
the proper level in the battery. The electrolyte not only serves as the ionic conductor,
but is a major factor in the transfer of heat from the plates. If the electrolyte is below
the plate level, then an area of the plate is not electrochemically efficient; this causes a
concentration of heat in other parts of the battery. Periodic checking of water
consumption can also serve as a rough check on charging efficiency and may warn when
adjustment of the charger is required.
Since replacing water can be a major maintenance cost, water loss can be reduced by
controlling the amount of overcharge and by using hydrogen and oxygen recombining
devices in each cell where possible. Addition of water is best accomplished after
recharge and before an equalization charge. Water is added at the end of the charge to
reach the high level line. Gassing during overcharge will stir the water into the acid
uniformly. In freezing weather, water should not be added without mixing as it may
freeze before gassing occurs. Only distilled water should be added to batteries.
Although demineralized or tap water may be approved for some batteries, the low cost
of distilled water makes it the best choice. Automatic watering devices and reliability
testing can reduce maintenance labor costs further. Overfilling must be avoided because
the resultant overflow of acid electrolyte will cause tray corrosion, ground paths, and
loss of cell capacity. Although distilled water is no longer specified by most battery
manufacturers, good quality water, low in minerals and heavy metal ions such as iron,
will help prolong battery life.
Keeping the battery clean will minimize corrosion of cell post connectors and steel trays
and avoid expensive repairs. Batteries commonly pick up dry dirt which can be readily
blown off or brushed away. This dirt should be removed before moisture makes it a
conductor of stray currents. The top of the battery can become wet with electrolyte any
time a cell is overfilled. The acid in this electrolyte does not evaporate and should be
neutralized by washing the battery with a solution of baking soda and hot water,