What is the primary function of a hydrometer in battery maintenance?

• A. To measure electrolyte temperature
• B. To measure the specific gravity of the electrolyte
• C. To measure the battery voltage
• D. To measure electrolyte pH

Answer: (B)

A hydrometer is used to determine the specific gravity of the electrolyte. In a lead-acid battery, the electrolyte is a sulfuric acid–water solution, and its density changes with how much acid is dissolved. When the battery is charged, the electrolyte becomes denser because more sulfuric acid is present; when the battery is discharged, the density decreases as the acid concentration drops. The hydrometer’s float rises or sinks with density, giving a direct reading of specific gravity. By comparing that reading to the manufacturer’s specifications and applying temperature compensation, you can assess the battery’s state of charge and overall health. This tool isn’t used to measure electrolyte temperature, voltage, or pH.