What Is Battery Acid?

Battery acid caution sign on a plastic container.
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Battery acid could refer to any acid used in a chemical cell or battery, but usually, this term describes the acid used in a lead-acid battery, such as those found in motor vehicles. 

Car or automotive battery acid is 30-50% sulfuric acid (H2SO4) in water. Usually, the acid has a mole fraction of 29%-32% sulfuric acid, a density of 1.25–1.28 kg/L and concentration of 4.2–5 mol/L. Battery acid has a pH of approximately 0.8.​

Construction and Chemical Reaction

A lead-acid battery consists of two lead plates separated by a liquid or gel containing sulfuric acid in water. The battery is rechargeable, with charging and discharging chemical reactions. When the battery is being used (discharged), electrons move from the negatively-charged lead plate to the positively-charged plate.

The negative plate reaction is:

Pb(s) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2 e-

The positive plate reaction is:

PbO2(s) + HSO4- + 3H+(aq) + 2 e- → PbSO4(s) + 2 H2O(l)

Which may be combined to write the overall chemical reaction:

Pb(s) + PbO2(s) + 2 H2SO4(aq) → 2 PbSO4(s) + 2 H2O(l)

Charging and Discharging

When the battery is fully charged, the negative plate is lead, the electrolyte is concentrated sulfuric acid, and the positive plate is lead dioxide. If the battery is overcharged, electrolysis of water produces hydrogen gas and oxygen gas, which are lost.

Some types of batteries allow water to be added to make up for the loss.

When the battery is discharged, the reverse reaction forms lead sulfate on both plates. If the battery is fully discharged, the result is two identical lead sulfate plates, separated by water. At this point, the battery is considered completely dead and can't recover or charged again.