A thermoelectric generator includes a thermal generator to which thermal energy is provided from a heat source. In order to keep the output voltage of the thermal generator in a range suitable for charging a battery or for directly feeding into the vehicle's electrical system, the rate of heat production of the heat source and/or the cooling power of a cooling device for the thermal generator is controlled by a regulator. For this purpose, the regulator is connected to a temperature sensor which is mounted on the thermal generator. The regulator can be more cost-effectively configured than a D.C. converter between the output side of the thermal generator and the battery so that this converter can be dispensed with.