SOLUTION

• A lead-acid battery uses sulfuric acid as the electrolyte. During the discharge process, the sulfuric acid reacts with the lead plates to form lead sulfate, which reduces the concentration of sulfuric acid and, consequently, the density of the electrolyte.
• When the battery is charged, the reverse reaction occurs. The lead sulfate is converted back into lead and sulfuric acid. This increases the concentration of sulfuric acid in the electrolyte, thereby increasing its density. With a fully charged battery, its specific gravity is about 1.28.
• Therefore, measuring the specific gravity (density) of the electrolyte is a common method to determine the state of charge of a lead-acid battery.