Hydrometers measure the specific gravity of liquids, usually in industrial batteries. This tool features markings or graduations on its surface to determine the specific gravity reading. Instruments floating high in the fluid are lighter and have a higher specific gravity. Conversely, instruments that sink lower in the liquid have lower specific gravity readings.
The electrolyte used in car batteries is 65% water and 35% sulfuric acid. Electrolytes in a fully charged battery have a specific gravity of 1.265 at an operating temperature of 80 degrees Fahrenheit.