There is some demand for high-capacity and high-energy density secondary batteries as power supplies for mobile devices and electric vehicles. Secondary batteries degrade by, for example, being repeatedly charged and discharged. As secondary batteries degrade, the capacity of discharge from a fully charged state (hereinafter, simply referred to as the “capacity”) decreases. As a result, in the case of electric vehicles, the distance that can be traveled per charge becomes shorter. Accordingly, to make it possible to treat degradation of a secondary battery, for example, by replacing the secondary battery, at an appropriate time, there is some need to develop a method for sensing secondary battery degradation in a simplified and correct manner.
In view of this, there has been proposed a conventional determination of secondary battery degradation on the basis of an open-circuit voltage of a secondary battery and a charge capacity obtained by current integration (see Patent Document 1). Here, a capacity change is calculated from a pre-charge open-circuit voltage and a post-charge open-circuit voltage of the secondary battery, and the calculated capacity change is compared with a charge capacity obtained by current integration.