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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 de-mineralized 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.
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