A car battery delivers electric power that starts the car. Most auto batteries work by chemical action to make power. This can cause water to evaporate from the battery. When the water level drops too far, more water needs to be added. Battery makers recommend distilled water but many people use tap water to refill the battery.
Tap water is free and handy for refilling a battery. Distilled water does not come from the tap and costs a few cents per cup.
Tap water usually has dissolved minerals and salts compared to the distilled water that battery makers recommend.
The impurities in tap water interfere with the normal chemical action in the battery so that it does not operate as efficiently.
The battery often works harder and hotter with tap water than with distilled water. It does not last as long when the internal temperature is too hot.
A car battery that is maintenance free does not need to be refilled with water.
Always follow battery maker instructions for filling a battery. It may have corrosive acid and explosive gases. Wear protective face and hand gear.