Introduction and application of formation methods based on serial-connected lithium-ion battery cells

Abstract The process step of formation is one key process to guarantee high performance, long-lasting and safe automotive lithium-ion cells. Since the formation of the cell is the most expensive process in cell manufacturing, reducing process cost and time is advanced. The state-of-the-art formation process includes the cycling of lithium-ion cells each on its own power electronic channel which amounts to about 38% of the total formation costs. Therefore, this paper proposes an optimized formation method by serial-connected lithium-ion cells. Due to small resistance and capacity deviations conditioned by manufacturing tolerances, charge balancing is necessary for formation of serial-connected cells. This paper introduces several serial interconnection circuits for serial cell formation, like passive balancing or drop out system. Furthermore, the associated influences on the formation process parameters, e.g. the charging profile and the charge current control are investigated. The comprehensive comparison of serial formation techniques reveals cost reduction potentials and challenges regarding the process control.