An output power controller in a system having two or more secondary batteries connected in parallel. A battery ECU transfers all the stored electric charge from one secondary battery to another secondary battery when battery temperature and state of charge (SOC) are so low that an output power requirement cannot be satisfied. The SOC of the secondary battery increases by the transfer of the stored electric charge and output power sufficient for the output power requirement can be obtained. Further, the secondary batteries are heated by thermal energy generated by the transfer of the stored electric charge.