Patent ID: 7429436

Claim:
A method of determining a state of charge (SOC) of a lead-acid battery having an unknown specific gravity, the method comprising the steps of: applying a current ramp to the battery while measuring battery voltage at terminals of the battery; measuring presence of gas points during application of the current ramp; determining a maximum voltage per cell (V pc ) for the battery; estimating the SOC of the battery by applying a default low specific gravity of 1.28 having a known correlation curve; when gas points are present, comparing the V pc to an upper limit of a maximum measured battery voltage, measured during the step of applying the current ramp to the battery, divided by a number of cells in the battery (SG LIM ); when gas points are absent, comparing the estimated SOC to an SOC threshold value representing a maximum state of charge for which gas points should not be detectable; determining a specific gravity of a lead-acid battery from one of the two comparing steps; determining a rested state open circuit voltage of the lead-acid battery at a rested state; and determining the SOC of the lead-acid battery at a rested state from the rested state open circuit voltage using a correlation function, wherein the correlation function depends on the specific gravity of the lead-acid battery.