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Why do lead-acid batteries use sulfuric acid as an electrolyte?

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A labeled diagram highlights sulfur's position on the periodic table, showing its atomic number 16, symbol S, atomic weight range, electron configuration, and classification as a solid, strongly acidic, orthorhombic nonmetal at room temperature. — sulfur Chemical properties of sulfur.

Lead-acid batteries use sulfuric acid as an electrolyte because it is an essential participant in the chemical reactions that generate electromotive force.12

During the battery's discharge process, the sulfuric acid acts as a reactant at both electrodes:
* At the positive electrode, lead dioxide combines with the electrolyte to produce lead sulfate and water.1
* At the negative electrode, the constituent lead reacts with sulfuric acid ions to produce lead sulfate and hydrogen ions, which effectively replaces the hydrogen ions consumed at the positive terminal.1

Beyond its role as a chemical reactant, the electrolyte serves to carry electric current by moving ions between the electrodes. Because the concentration of the acid changes as it is consumed and water is produced, the specific gravity of the electrolyte also provides a reliable way to determine the battery's state of charge.1