BATTERY CONTROL DEVICE

PROBLEM TO BE SOLVED: To suppress battery degradation, maintain vehicle performance for a long time, and reduce the cost burden of a user accompanied with battery replacement, in an electric vehicle including an EV.SOLUTION: A battery control device includes a main battery 301 which is of large capacity and is used in the region of small degradation that is narrower than the conventional SOC use region and a sub battery 302 which is of small capacity and is used in the range between the upper limit value and the lower limit value of the conventional SOC use region, and power is transferred properly between the main battery 301 and the sub battery 302 so that the SOC of the main battery 301 is a preferable value from the view point of achieving a long lifetime. Then, it is assumed to replace the sub battery 302 periodically, and a SOC setting value of the sub battery 302 is set as a setting value at which the lifetime of the main battery 301 is preferred. Thus, although the attendant cost is increased by installation of the sub battery, the lifetime cost as the system total is made cheap by achieving the long lifetime of the main battery 301, resulting in reducing the burden of a user.