Abstract: The disclosure relates to a method for determination of a maximum allowable load current of an energy storage cell by means of a substitute model. The Parameters of the substitute model are adapted during the lifetime of the energy storage cell. The substitute model contains two or more RC-elements. The respective parameters of a RC-element preferentially are adapted during separate time intervals. The disclosure further relates to a method for determination of a maximum performance of an accumulator-unit having two or more energy storage cells. The performance is preferentially calculated from a maximum allowable load current of the weakest energy storage cell and the sum of the respective voltages being produced at the energy storage cells at appliance of this maximum current. The disclosure also relates to a control unit for performing the method.