A hydrometer is used to measure the:

• A. Voltage of a battery cell
• B. Specific gravity of a battery cell
• C. Charge state of battery
• D. Electrolyte temperature

Answer: (B)

A hydrometer measures the density of a liquid. In a lead–acid battery, the electrolyte is a mix of water and sulfuric acid, so its density (specific gravity) depends on how concentrated the acid is. When the battery is charged, there is more sulfuric acid in solution and the electrolyte has higher specific gravity; as it discharges, the acid concentration decreases and the specific gravity falls. Therefore reading the electrolyte’s specific gravity with a hydrometer gives an indication of the battery’s charge state. It doesn’t measure voltage or temperature directly, and while electrolyte temperature can affect the reading, the primary use here is assessing specific gravity to infer charge.