Secondary battery system

Since in existent charge-discharge control the battery direct current resistance of a lithium ion battery increases at a low temperature or in a low state of charge (SOC), the battery voltage may possibly exceed greatly the maximum allowable voltage instantaneously due to over-voltage during charging or may possibly decrease greatly to less than the minimum allowable voltage during discharging. If the lithium ion battery continuously lies in such a state the battery performance is degraded abruptly. The battery resistance of a lithium ion battery module (41) is estimated in accordance with SOC and temperature of the lithium ion battery module (41), and current to charge is set properly based on the estimated battery resistance.