Abstract: Method and apparatus for accurately determining the charge state of a battery is disclosed. The charge state is derived from the battery voltage, which is corrected for errors introduced by temperature and series resistance. Error from series resistance is minimized by either making an open circuit voltage measurement, or by making a plurality of voltage measurements under known load conditions, calculating the series resistance from these measurements, and calculating an equivalent open circuit voltage, compensating for the voltage drop caused by the series resistance of the battery. Errors introduced by temperature induced shifts in battery voltage are corrected by reading the battery temperature and correcting the battery voltage to a reference temperature. The battery voltage corrected for temperature and series resistance effects is used to compute charge state by table look up or algebraically.