How to Fill a 12-Volt Battery With Water

by Jack Brubaker

A 12-volt battery contains six individual electrochemical cells connected in series, with each cell producing approximately two volts. Each cell contains lead plates that serve as electrodes, which are flat pieces of metal that conduct electricity and allow chemical reactions to take place on their surfaces. The plates sit in a bath of aqueous sulfuric acid, which serves as the electrolyte--a compound that helps carry electric current between the electrodes. Over time, some of the water in the electrolyte undergoes electrolysis during the charging process and breaks down into gaseous hydrogen and oxygen. This causes the liquid levels inside the cells to slowly decrease. Any good automotive maintenance regimen should, therefore, include checking and, if necessary, replenishing the liquid levels in the battery at monthly intervals.

Disconnect the battery from any devices to which it may be attached. In the case of an automotive battery, loosen the negative (black) battery cable with a crescent wrench and position the cable's terminal so it cannot touch the battery's negative post. Always remove the negative cable before removing the positive cable. Position any metal tools away from the battery before attempting to remove the caps.

Pry the caps off the battery by grasping the caps and working them using a back-and-forth motion. Use a flat-head screwdriver or putty knife to help loosen the caps only if absolutely necessary and be certain no metal objects contact the battery's terminals.

Observe the liquid levels inside the cells. Each cell will contain a "fill ring" that indicates the ideal liquid level. Add distilled water to any cells with liquid levels below the fill ring. If you over-fill the cell, remove any excess with a 3-cc syringe and dispose of it by combining it with baking soda.

Replace the caps using firm, even, hand pressure across the entire cap. Be certain the caps are fully seated before reconnecting the battery. Reverse the disconnect sequence when reconnecting the battery--positive (red) cable first, then negative (black).

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