Distilled water needs to be added periodically to the Lead Acid Batteries to compensate for the water lost during electrolysis. When a discharged battery is charged with the power from an external charger, the current causes an electrolysis of the water in the electrolyte. The water dissociates into hydrogen and oxygen. These gases are released through the vents. This process results in drop in the level of the electrolyte. To compensate for this, distilled water is added when the level of electrolyte falls below a particular level.
Distilled water alone should be used as it is pure water without any impurity. Tap water should never be used as the chemicals present in tap water can seriously damage the batteries. Care should be taken that water is not added in excess known as overwatering. Overwatering will dilute the electrolyte and affect battery performance.