The invention relates to a hybrid drive system for a vehicle; comprising an intermediate circuit (3) and an energy accumulator (5). According to the invention, the energy accumulator (5) is connected to the intermediate circuit (3) by means of a diode (6), whereby a transistor (7) enabling current to flow in an opposite direction is connected parallel thereto. The transistor (7) is placed in a blocking position in order to discharge the energy accumulator (5) and the diode (6) enables current to pass. If the energy accumulator (5) is not used, the diode (6) is also blocked by increasing the voltage (8) of the intermediate circuit (3) with the aid of the voltage (14) on the energy accumulator (5). In order to charge the energy accumulator (5), the transistor (7) is conductingly switched. The diode (6) does not allow the current to pass in this direction.