Hydrometers (density meters) measure the specific gravity of liquids. Specific gravity is the ratio of the density of the liquid being tested to the density of water. In the case of battery testing, the hydrometer is measuring the specific gravity of the battery’s electrolyte. The higher the acid concentration in the electrolyte, the higher the specific gravity.

Based on the specific gravity, the user can determine the state of charge of the battery.

Retaining hydrometer readings and data over time is recommended by IEEE as part of any Battery Maintenance Program