How can the specific gravity of a lead-acid battery’s electrolyte show its state of charge?

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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 sulfuric acid electrolyte is a key component of the chemical reaction that generates power. During the discharge process, the lead dioxide at the positive electrode and the lead at the negative electrode both react with the electrolyte to produce lead sulfate and water.1

This chemical reaction consumes sulfate ions from the acid and produces water, which effectively dilutes the electrolyte solution. As the concentration of sulfuric acid decreases, the density of the electrolyte drops. Because the specific gravity of the liquid is directly related to its density, measuring this value provides a reliable indicator of the battery's state of charge. A higher specific gravity generally indicates a more fully charged battery, while a lower specific gravity indicates that the battery has been discharged and the electrolyte has become more dilute.1