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Why does the electrolyte’s density change as a lead-acid battery discharges?

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Cross-section diagram of an alkaline-manganese dioxide battery showing labeled internal components, including the steel-plated covers, cathode, anode gel, current collector, separator, steel can, and brass rivet. — battery Cutaway view of an alkaline–manganese dioxide power cell.

In a lead-acid battery, the electrolyte’s density changes during discharge because of the chemical reactions occurring at the electrodes. Specifically, the lead dioxide at the positive electrode and the lead at the negative electrode both react with the sulfuric acid electrolyte to produce lead sulfate and water.1

As the battery discharges, the consumption of sulfate ions from the acid and the simultaneous production of water dilute the electrolyte. This reduction in the concentration of sulfuric acid causes the overall density of the electrolyte to decrease. Because of this direct relationship, the specific gravity of the electrolyte serves as a reliable indicator of the battery's state of charge.1