Is it necessary to use distilled water in an auto battery?
yes always use distilled water if you can,but if you dont have to add very much and its an emergency then use plain tap water,and socalled matinance free batterys usualy have caps that can be removed and water added,if your regulator goes bad and wont shut of the electric flow to the battery it will over charge and generate heat that evaporates the water out of the acid solution and the battery will eventualy start internal arcking and ruien a cell making the battery junk
