Editing Hydrometer (section)

===Battery hydrometer===
The state of charge of a [[lead-acid]] battery can be estimated from the density of the [[sulfuric acid]] solution used as [[electrolyte]]. A hydrometer calibrated to read specific gravity relative to water at {{Convert|60|F}} is a standard tool for servicing [[car battery|automobile batteries]].  Tables are used to correct the reading to the standard temperature. Hydrometers are also used for maintenance of wet-cell nickel-cadmium batteries to ensure the electrolyte is of the proper strength for the application; for this battery chemistry the specific gravity of the electrolyte is not related to the state of charge of the battery.

A battery hydrometer with thermometer (thermohydrometer) measures the temperature-compensated specific gravity and electrolyte temperature.