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Using Tap Water For Batteries |
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Written by Staff of the Quadrant Marine Institute
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 Q. Battery labels recommend distilled water only. What is the risk of using plain tap water?
A. Water must be routinely added to standard non-sealed, lead-acid batteries as part of ongoing maintenance procedures. The most distinguishing feature of this kind of battery is removable vented caps on the top. Sealed gel cell and AGM batteries do not have these.
When under charge, some of the electrolyte (acid) in the battery cells is changed into hydrogen and oxygen. This will gas off, causing the electrolyte level to fall; so it must be topped up as needed with water. Fill only to the little “ridge” inside the cell hole; don’t overfill or extremely corrosive acid could escape.
Distilled water is always recommended because regular tap water usually contains chemicals such as chlorine and various minerals/salts. These can interfere with the chemical reactions going on in the electrolyte and/or form deposits on the plates of the battery cells, shortening the working life.
Note, this water needs to be labelled “distilled,” not “purified,” “sterilized” or otherwise, as the water may still contain chemicals. Distilled water is generally available in grocery stores.
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