Secondary battery state of charge determination apparatus, and method of determining state of charge of secondary battery

Abstract

A technology described herein is for accurately determining the SOC of a secondary battery. An open-circuit-voltage-to-state-of-charge (OCV-SOC) characteristic has a capacity decrease estimation OCV region in which a relationship between the OCV and the SOC is assumed not to change due to degradation of the secondary batterysince a reference time point. Charge currents or discharge currents in the capacity decrease estimation OCV region are accumulated in a process in which the secondary battery SOC changes among OCVs. From the accumulation, an accumulated current is obtained. A secondary battery capacity decrease is estimated based on the accumulated current. An OCV-SOC characteristic is specified based on the estimated secondary battery capacity decrease and a predetermined relationship between the battery capacity decrease of the secondary battery since the reference time point and the OCV-SOC characteristic. A secondary battery SOC is determined based on the OCV and the specified OCV-SOC characteristic.