No, you should not use plain water in a car battery. You must use distilled water, and even then, only to top off certain types of flooded lead-acid batteries when the fluid level is low. Using tap water is harmful because minerals and chemicals like calcium and chlorine can corrode the battery's internal plates and degrade its performance, leading to premature failure. Most modern cars have maintenance-free batteries that are sealed and do not require any water addition.
The liquid in a lead-acid battery is a mixture of sulfuric acid and water, known as the electrolyte solution. Over time, the water portion can evaporate, especially in hot climates, exposing the lead plates. If the plates are exposed to air, they sulfate (form a crystalline coating), which drastically reduces the battery's ability to hold a charge. Topping off with distilled water restores the proper electrolyte level. However, this only applies to batteries with removable caps. Overfilling is also a problem, as it can cause the electrolyte to overflow, leading to corrosion on the battery terminals and surrounding components.
Here is a quick reference for battery maintenance:
| Battery Type | Can You Add Water? | Correct Fluid to Use | Key Consideration |
|---|
| Flooded Lead-Acid | Yes, if levels are low | Distilled Water Only | Check levels every 3-6 months; avoid overfilling. |
| Enhanced Flooded Battery (EFB) | Yes, but less frequently | Distilled Water Only | Often found in start-stop systems; check owner's manual. |
| Absorbent Glass Mat (AGM) | No | Sealed / Maintenance-Free | These are completely sealed and cannot be serviced. |
| Gel Cell | No | Sealed / Maintenance-Free | The electrolyte is in a gel form; no liquid to add. |
| Lithium-Ion | No | Sealed / Maintenance-Free | Used in electric vehicles and some high-performance cars. |
Always prioritize safety. Wear safety glasses and gloves when checking a battery, as the electrolyte is highly corrosive. If you're unsure about your battery type, consult your vehicle's owner's manual or a professional technician. The Battery Council International (BCI) provides standards that most manufacturers follow, and they consistently recommend against using anything other than distilled water for serviceable batteries.
