Patent ID: 8036839

Claim:
A battery state determining apparatus for determining a battery state of a lead battery mounted on a vehicle, comprising: an open-circuit voltage measuring section which measures an open-circuit voltage OCV of the lead battery; an engine starting voltage measuring section which measures an engine starting voltage Vst representing a voltage at an engine starting time of the lead battery; a temperature measuring section which measures a temperature of at least one of the vehicle and the lead battery; an engine starting voltage correcting section which calculates an engine starting voltage Vst 1 obtained by correcting the engine starting voltage Vst measured at the engine starting voltage measuring section based upon the open-circuit voltage OCV measured at the open-circuit voltage measuring section and the temperature measured at the temperature measuring section; a determination voltage arithmetic section which calculates, from the open-circuit voltage OCV measured at the open-circuit voltage measuring section, a lead battery determination voltage Vst_th representing a voltage at an engine starting time of the lead battery when an increase rate of internal resistance of the lead battery reaches a predetermined value for lead battery replacement; and a battery state determining section which determines whether or not the engine starting voltage Vst 1 calculated at the engine starting voltage correcting section is equal to or more than the lead battery determination voltage Vst_th calculated at the determination voltage arithmetic section, wherein the battery state determining section determines a degraded state of the lead battery by determining a region to which the OCV measured at the open-circuit voltage measuring section and Vst 1 calculated at the engine starting voltage correcting section belong among a plurality of regions in a pre-determined characteristic map which defines a relationship between OCV and Vst 1 of the lead battery and which is divided into the plurality of regions representing the degraded state of the lead battery.