The fluid inside a standard lead-acid car battery is a mixture of sulfuric acid and deionized water, formally called electrolyte. This chemical solution is the core of the battery's operation, facilitating the electrochemical reaction that stores and provides electrical energy to start your car and power its accessories. The electrolyte's specific gravity—a measure of its density—is a key indicator of the battery's state of charge. When the battery is charging, the sulfuric acid concentration increases; when discharging, it decreases.
It is critical to understand that this is not regular water. The deionized water is used because it lacks minerals that could cause damaging chemical reactions or build-up on the battery's internal plates. The acid concentration is typically around 30-50% sulfuric acid by volume when the battery is fully charged. This creates a highly corrosive and conductive environment.
Proper maintenance involves occasionally checking the fluid level in each cell and topping it off only with distilled or deionized water, never tap water and certainly not more acid. The water naturally evaporates over time, especially in hot climates, while the acid does not. If the fluid level drops too low, exposing the lead plates to air, it can cause permanent damage and significantly shorten the battery's lifespan.
Modern "maintenance-free" batteries are sealed, meaning you cannot and should not attempt to add water. Newer technologies like Absorbed Glass Mat (AGM) and Gel-Cell batteries also use a sulfuric acid electrolyte, but it is suspended in a fiberglass mat or gelled, respectively, preventing spills and making them more versatile for installation.
| Battery Type | Electrolyte State | Maintenance Required | Common Use Cases |
|---|
| Flooded Lead-Acid | Liquid mixture of sulfuric acid & water | Yes; periodic water top-off | Standard car batteries |
| Absorbed Glass Mat (AGM) | Acid is absorbed in fiberglass mats | Sealed; no maintenance | Performance cars, start-stop systems |
| Gel-Cell | Acid is suspended in a silica gel | Sealed; no maintenance | Deep cycle applications (e.g., RVs) |
| Lithium-Ion | Lithium salts in organic solvent | Sealed; completely maintenance-free | Electric vehicles, hybrids |
