The acid used in a standard car battery is sulfuric acid (H₂SO₄). It's not used alone but is diluted with distilled water to create an electrolyte solution with a typical concentration of around 35% acid and 65% water. This powerful acid is the key component that enables the battery's core function: initiating a chemical reaction to release electrons, which provides the massive burst of current needed to start your engine and powers the vehicle's electrical systems when the alternator isn't running.
Inside the battery case, the sulfuric acid electrolyte interacts with the lead plates (lead dioxide for the positive plates and sponge lead for the negative plates). This chemical reaction produces electrons (electricity) and converts the acid and plate materials into lead sulfate. When you charge the battery by driving, the alternator reverses this reaction, turning the lead sulfate back into lead, lead dioxide, and sulfuric acid. This charging cycle is what allows the battery to be reused.
Handling a car battery requires extreme caution due to the sulfuric acid. It is highly corrosive and can cause severe burns to skin and eyes. It also produces highly flammable hydrogen gas during charging, which is why avoiding sparks or flames near a battery is critical. Always wear protective gloves and safety glasses if you need to service a battery yourself. The specific gravity of the electrolyte, which is a measure of the acid-to-water ratio, is a primary indicator of a battery's state of charge.
| State of Charge | Average Specific Gravity | Average Voltage (12V Battery) |
|---|
| 100% | 1.265 - 1.275 | 12.6V - 12.7V |
| 75% | 1.225 - 1.235 | 12.4V |
| 50% | 1.190 - 1.200 | 12.2V |
| 25% | 1.155 - 1.165 | 12.0V |
| Discharged | 1.120 or below | 11.9V or below |
