STATE-OF-CHARGE ESTIMATION METHOD AND STATE-OF-CHARGE ESTIMATION DEVICE

A voltage-change convergence time Ä from termination of charge and discharge of a secondary battery until convergence of change in open-circuit voltage is measured beforehand for each combination of a state of charge of the secondary battery at the time of termination of charge and discharge and the temperature of the secondary battery at the time of termination of charge and discharge. Upon termination of the charge and discharge of the secondary battery in actual use, a voltage-change convergence time Ä is selected that corresponds to the temperature and the state of charge based on an integrated current value at the time of termination of charge and discharge, and the selected voltage-change convergence time Ä is used in a voltage characteristic expression.