Identifying a root cause of battery cell degradation in a battery pack

An electric vehicle comprising:

• a battery pack having a plurality of lithium-ion battery cells;
• and
• a controller configured to monitor a voltage level of each of the plurality of lithium-ion battery cells, wherein the controller is further configured to:
• collect voltage data for each of a plurality of battery cells in the battery pack before and after each charging event of the battery pack;
• identify a first cell of the plurality of battery cells as a defective battery cell based on a determination that a voltage of the first cell is more than a threshold value below an average voltage of the plurality of battery cells;
• identify a second cell of the plurality of battery cells as a nominal battery cell based on a determination that a voltage of the second cell is within a threshold value of an average voltage of the plurality of battery cells;
• calculate a self-discharge of the defective battery cell based on the voltage data of the defective battery cell and the nominal battery cell, wherein the self-discharge is calculated based on a state-of-charge (SOC) of the defective battery cell and the SOC of the nominal battery cell from before and after a charging event;
• calculate a capacity fade of the defective battery cell based on the voltage data of the defective battery cell and the nominal battery cell, wherein the capacity fade is a loss of a charge storing capacity of the defective battery cell that is calculated based on the SOC of the defective battery cell and the SOC of the nominal battery cell from before and after a charging event;
• determine a root cause of a defect of the defective battery cell based on the self-discharge and the capacity fade of the defective battery cell;
• and
• deactivate the defective battery cell in the battery pack based on the root cause of the battery cell degradation.