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This paper presents a method to control the current of each battery cell in a serially connected battery stack according to each cell capacity. With this method, the performance of a battery stack can be increased significantly. In a second life concept, battery cells with different capacities and even battery cells with different chemistries can be connected together. Moreover, if one or more battery cells become inoperative, then the battery stack can still be used in the limp-home operation mode. The required power converter structure and the calculation of the individual cell currents are presented, and a simulation for two different power converters during discharging and charging is performed. A current equalization prototype is used to validate the simulation results and to show the performance on a real battery stack with 12 serially connected cells. Finally, the influence of the equalization currents and the capacity variance in the battery stack on the discharging energy are investigated.