When the battery is charged, the sulphate recombines with the water to form sulphuric acid. The specific gravity of the electrolyte thus varies from 1.1 to 1.3 depending on the level of charge.
Over a period of time, there is a chance of the water in the electrolyte evaporating. This may lead to a change in the level of the electrolyte along with a variation in the specific gravity. This may result in a deterioration in the battery performance. In such situations, it may be necessary to add water to the electrolyte.
Procedure
Only distilled and demineralized water should be used, as ordinary water contains minerals such as calcium, magnesium, etc which can cause deposits in the electrodes and affect the battery's life and performance. The vent caps of the batteries need to be removed. Once, the vent caps are removed, the plates will be visible. See if the plates are completely covered by the electrolyte. Add water if the level of the electrolyte is low and the plates are exposed. Take care that excess water is not added this will result in an "overflow". The acid can overflow and cause damage.
Safety Precautions
Follow the manufacturer's instructions before topping the electrolyte with water. Always wear protection goggles for your eyes. The acidic electrolyte is corrosive. Take care that you do not accidentally come in contact with it. The sulphurous gases which may escape the electrolyte are explosive. Do not smoke near the battery as a spark can trigger an explosion.