Abstract: A control system for a vehicle-mounted battery which, in accordance with the state of a battery, can make a polyphase alternating current rotating machine in a hybrid car assist the driving gear (the internal combustion engine) or charge the battery. The current and voltage during discharge and charging of a battery mounted on a vehicle are measured, and on the basis of this measured current and voltage information, a program control circuit controls the discharge current when an auxiliary driving force is given to the driving gear. This control is performed via an inverter which performs AC-to-DC or DC-to-AC conversion between the battery and the polyphase alternating current rotating machine coupled to the driving gear of the vehicle. Because the charging and discharging currents are limited in accordance with the state of charging of the battery, it is possible to avoid charging and discharging uniformly irrespective of the amount of charging of the battery.