Accurate state-of-charge (SOC) estimation of lithium-ion battery packs is technically challenging because of the cell-to-cell variability due to the manufacturing tolerance. In addition, there is no unanimous definition of the pack SOC since each cell has its own SOC and the pack can be configured in different ways. This study first adopts a suitable pack SOC definition among existing ones, then proposes uncertainty modeling and propagation analysis for pack SOC estimation considering the cell-to-cell variability, and finally conducts the SOC estimation for one serially connected battery pack using the one state hysteresis model with the extended Kalman filter (EKF). The results reveal that pack SOC variability is inevitable due to the cell-to-cell variability and accurate pack SOC estimation is challenging considering both the cell-level SOC accuracy and the pack-level estimation efficiency.

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