Knee-Point Identification of Battery Degradation Trajectory Based on Constant Voltage Charging Capacity Variation

The turning point in the process of nonlinear aging is a key feature to identify the nonlinear aging behavior of lithium-ion batteries. In order to identify the knee-point online, this paper studies the capacity “diving” phenomenon of the battery during the experiment and the regulation of the appearance of the turning point during the nonlinear aging process. Then, a knee-point identification method based on constant voltage charging capacity is proposed, and the linear and nonlinear stages of battery decay are redefined. Based on the change of constant voltage charging capacity in the constant current and constant voltage charging strategy, the method defines the aging process in which the constant voltage charging capacity remains invariant as the linear decay stage of the battery, and the aging process in which the constant voltage charging capacity rises rapidly as the nonlinear decay stage. The intersection of linear and nonlinear decay is the knee-point of the battery’s aging trajectory. This method is tested on nickel manganese cobalt oxide (NCM) and LiFePO₄ batteries, and both achieve the knee-point identification very well. The method can achieve online capacity estimation without obtaining complete battery aging data.