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 full charge cycle 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 also a major factor in the transfer
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
charge. Water is added at the end of the charge to reach the high level
line. Gassing during recharge 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. Distilled or deionized (DI) water is best for
batteries, but city tap water is generally acceptable. Automatic watering
systems can further reduce maintenance labor costs. Overfilling must be
avoided because the resultant overflow of acid
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.
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