Influence of High-Frequency Charge–Discharge Cycling Induced by Cell Voltage Equalizers on the Life Performance of Lithium-Ion Cells

Various types of cell voltage equalizers that exchange charges among cells have been proposed for series-connected lithium-ion cells. During equalization, cells are possibly cycled not only over a conventional period that ranges from seconds to hours but at a high frequency that is equal to the operating frequency of the equalizers as well. This paper investigates how the high-frequency cycling occurs and the factor that affects the life performance of the cells during high-frequency cycling. Experiments on charge-discharge cycling at frequencies of 1-100 kHz were performed for lithium-ion cells. The capacity of cells cycled at frequencies below 10 Hz significantly deteriorated, whereas the capacity decay of cells cycled at higher frequencies were identical to the corresponding calendar degradation. Simulation analyses revealed that the cycling current at high frequencies served only to charge and discharge the double-layer capacitance C_dl, whereas the current that flows through the charge-transfer resistance R_ct, indicating the charge-transfer process, dominated at low frequencies. A correlation between the degradation trend and current distribution to C_dl and R_ct was observed, implying that the calendar degradation and the degradation due to the charge-transfer process were the major causes of capacity decay in the frequency range below 10 Hz.