For most car owners, the only thing you should ever put in a car battery is distilled water, and this applies exclusively to traditional flooded lead-acid batteries that have removable caps. These batteries can lose water over time through evaporation, especially in hot weather. The electrolyte level should be just above the internal lead plates. Using distilled water is critical because tap water contains minerals that can damage the battery and reduce its lifespan.
Modern Absorbent Glass Mat (AGM) and gel cell batteries are completely sealed and are considered maintenance-free; they are designed never to be opened. Attempting to add anything to these batteries will likely void the warranty and can cause serious damage.
It is a common misconception that you need to add new acid. The battery's acid doesn't evaporate; only the water does. Adding more acid will drastically alter the chemical balance, making the electrolyte too strong and accelerating the battery's failure. If the electrolyte level is consistently low, it often indicates an overcharging problem, which should be diagnosed by a professional.
| Battery Type | Can You Add Fluid? | What to Add | Key Consideration |
|---|
| Flooded Lead-Acid | Yes, if caps are removable | Distilled Water only | Check levels every few months; top up if plates are exposed. |
| AGM (Absorbent Glass Mat) | No | N/A | Sealed unit; adding fluid will damage it. |
| Gel Cell | No | N/A | Sealed unit; not designed for maintenance. |
| EFB (Enhanced Flooded Battery) | Sometimes | Distilled Water only | Some models have caps; follow manufacturer instructions. |
Always prioritize safety. Wear gloves and safety glasses when handling batteries, as the sulfuric acid inside is highly corrosive. If you're unsure about your battery type, consult your vehicle's owner's manual or take it to an auto parts store for a free check.
