The present method relates to a method of charging a traction battery of a vehicle from an external AC power supply system. The traction battery is coupled, via an inverter, to an electric alternating-field machine such that the AC power supply can be connected to the phase windings of the alternating-field machine. The method includes a step of synchronizing the rotational speed of the alternating-field machine with the frequency of the AC power supply. Then the AC power supply is connected to the phase windings of the alternating-field machine and the inverter, which is electrically coupled to the alternating-field machine, is operated as a step-up converter.