An electric power converter transmits and receives, via a transformer, electric power between alternating current power source and second battery or between first battery and second battery. The test program displays a representation of a quality of service message on a user interface. The temperature sensor may, include a plurality of stages which are located from the center of the power semiconductor component to an edge of the latter in the gap between two cells. The balancing device includes a mechanical spring and a spring housing arranged enclosing the spring.