A battery control system is used with a battery, and has a converter to charge and/or discharge the battery. The converter is thermally coupled to the battery, and the temperature of the battery is sensed. A control circuit is provided for controlling the efficiency or power loss of the converter according to the temperature of the battery sensed by the sensor, thereby to alter the level of thermal heat generation, and in turn, transfer from the converter to the battery. In this way, the battery temperature is controlled so that the life time of the battery can be extended. It makes use of the existing converter to deliver thermal energy, so that a separate heater is avoided.