Patent ID: 7805264

Claim:
A method of calculating an internal resistance of a secondary battery mounted on a vehicle, comprising steps of: detecting a current of a secondary battery by a current sensor, and a voltage of the secondary battery by an internal state detection device when a large current discharge occurs in the secondary battery, and the detected voltage and current of the secondary battery are stored into a buffer unit in the internal state detection device; calculating a regression characteristic curve by an arithmetic processing unit in the internal state detection device based on the detected voltage and current, stored in the buffer unit, within a detectable current range of the current sensor; setting a maximum discharge coordinate, by the arithmetic processing unit, on an extended curve extended from the regression characteristic curve toward the large current discharge side which is greater than the detectable current range of the current sensor; setting a pre-discharge coordinate, by the arithmetic processing unit, designated by the detected voltage and current of the secondary battery detected before the secondary battery discharges the large current; and calculating an internal resistance of the secondary battery by the arithmetic processing unit based on an inclination of a line which connects the maximum discharge coordinate and the pre-discharge coordinate, wherein the maximum discharge coordinate is set by the arithmetic processing unit, which approximately corresponds to a coordinate designated by an estimate maximum discharge current and a voltage at the estimated maximum discharge current of the secondary battery when the vehicle engine starts.