A controller for an electrical energy supply unit, comprising: a) a first filling level input, to which a first filling level of a first energy store of the electrical energy supply unit is transmittable, b) a further filling level input, to which a further filling level of an optional further energy store of a further electrical energy supply unit is transmittable, c) a nominal alternating voltage determiner, which is designed to determine a nominal alternating voltage in consideration of the first filling level and/or of the further filling level, wherein 1) the nominal alternating voltage determiner is designed to determine a nominal frequency of the nominal alternating voltage in consideration of the first filling level and/or of the further filling level, and/or 2) the controller is designed, from a further frequency of a further alternating voltage on a further alternating voltage generator of the further electrical energy supply unit, to determine the further filling level of the further energy store, d) a nominal alternating voltage output, from which the nominal alternating voltage is transmittable to an alternating voltage generator of the electrical energy supply unit characterized in that the nominal alternating voltage determiner is configured such that the nominal frequency e) essentially corresponds to the further frequency, if the further electrical energy supply unit is connected to the electrical energy supply unit in an electrically conductive manner, and f) is otherwise determined in consideration of the first filling level. a) a first filling level input, to which a first filling level of a first energy store of the electrical energy supply unit is transmittable, b) a further filling level input, to which a further filling level of an optional further energy store of a further electrical energy supply unit is transmittable, c) a nominal alternating voltage determiner, which is designed to determine a nominal alternating voltage in consideration of the first filling level and/or of the further filling level, wherein 1) the nominal alternating voltage determiner is designed to determine a nominal frequency of the nominal alternating voltage in consideration of the first filling level and/or of the further filling level, and/or 2) the controller is designed, from a further frequency of a further alternating voltage on a further alternating voltage generator of the further electrical energy supply unit, to determine the further filling level of the further energy store, 1) the nominal alternating voltage determiner is designed to determine a nominal frequency of the nominal alternating voltage in consideration of the first filling level and/or of the further filling level, and/or 2) the controller is designed, from a further frequency of a further alternating voltage on a further alternating voltage generator of the further electrical energy supply unit, to determine the further filling level of the further energy store, d) a nominal alternating voltage output, from which the nominal alternating voltage is transmittable to an alternating voltage generator of the electrical energy supply unit characterized in that the nominal alternating voltage determiner is configured such that the nominal frequency e) essentially corresponds to the further frequency, if the further electrical energy supply unit is connected to the electrical energy supply unit in an electrically conductive manner, and f) is otherwise determined in consideration of the first filling level.