Algorithm to Determine the Knee Point on Capacity Fade Curves of Lithium-Ion Cells

Lithium-ion batteries typically exhibit a transition to a more rapid capacity fade trend when subjected to extended charge-discharge cycles and storage conditions. The identification of the knee point can be valuable to identify the more severe degradation trend, and to provide guidance when scheduling battery replacements and planning secondary uses of the battery. However, a concise and repeatable determination of a knee point has not been documented. This paper provides a definition of the knee point which can be used as a degradation metric, and develops an algorithm to identify it. The algorithm is implemented on various data cases, and the results indicate that the approach provides repeatable knee point identification.