Adding water to the electrolyte level of unsealed batteries.

The electrolyte in the battery is a mixture of sulphuric acid and water.  The amount of water in a battery can fall due to electrolysis or evaporation.  This may cause in a drop in the level of the electrolyte and consequently a drop in the battery output. 

Hence, it is necessary to periodically inspect the level of electrolyte in the battery. If the level of the electrolyte falls below the minimum level, it can be topped up by adding water.  Only distilled water should be added as ordinary water may contain a lot of impurities and ions which may contaminate the electrolyte.

The level of electrolyte in the battery tends to fall as the battery gets discharged and tends to rise as the battery gets charged.  Hence, water should be added to the electrolyte only when the battery is fully charged.  If the water is added to the battery when it is in the discharged condition, the level can increase beyond the limit when the battery is fully charged and may overflow

The acid used as the electrolyte is extremely corrosive and should be handled with extreme care.  Proper protective outfits should be worn while handling them.  Water can be added to a container of acid.  However,  acid can never be added to container of water as the heat generated can cause splashing.