You should only use distilled water or deionized water to top off a lead-acid car battery. Using tap water, bottled spring water, or any other type of water containing minerals and chemicals can significantly shorten your battery's lifespan and impair its performance. The impurities in non-distilled water, such as calcium, magnesium, and chlorine, cause sulfation (a buildup of lead sulfate crystals) on the battery plates and increase the rate of self-discharge.
A car battery's function relies on a precise chemical reaction between its lead plates and the sulfuric acid electrolyte. The water in the battery exists solely to maintain the proper acid-to-water ratio as water naturally evaporates over time. Adding anything other than pure water introduces contaminants that disrupt this delicate balance.
The following table compares the effects of different water types on a car battery:
| Water Type | Total Dissolved Solids (TDS) | Primary Contaminants | Effect on Battery | Recommendation |
|---|
| Distilled/Deionized | < 10 ppm (parts per million) | Virtually None | Maintains optimal chemical balance; no scale buildup | Highly Recommended |
| Reverse Osmosis (RO) | 10 - 50 ppm | Low levels of various ions | Generally acceptable in a pinch, but not ideal | Acceptable if distilled is unavailable |
| Filtered Tap Water | 50 - 200 ppm | Chlorine, Fluoride, Minerals | Accelerates plate corrosion and sulfation | Not Recommended |
| Standard Tap Water | 200 - 500+ ppm | Calcium, Magnesium, Chlorine | Rapid plate degradation and performance loss | Avoid |
| Bottled Spring Water | 100 - 400 ppm | Natural Minerals | Similar negative effects as tap water | Avoid |
When adding water, only do so when the battery is fully charged and the plates are exposed. Use a battery filler bottle to add just enough distilled water to cover the plates by about 1/4 to 1/2 inch. Overfilling can cause the acidic electrolyte to overflow, leading to corrosion on the battery terminals and surrounding components.
