Observability and State Estimation Accuracy Analysis of LiFePO4 Batteries With Hysteresis Model

The structural stability of lithium iron phosphate (LFP) batteries makes them highly suitable for applications where fire safety is a critical concern. However, the wide open-circuit voltage (OCV) plateau characteristic of LFP batteries significantly increases the difficulty of state of charge (SOC) estimation when the equivalent circuit model relies solely on the SOC-OCV relationship. To maintain observability within the OCV plateau region, this paper proposes a nonlinear battery cell model with voltage hysteresis dynamics associated with the SOC. Because SOC-related hysteresis dynamics provides additional information regarding the SOC in the output equation, the proposed model remains locally observable in OCV plateau regions, where conventional methods suffer from rank deficiency problems. Theoretical validation based on an observability ranking test confirms that the proposed model is locally observable within the OCV plateau. The effectiveness of the proposed method is further demonstrated through experiments using a 280 Ah prismatic LFP battery cell under an urban dynamometer driving schedule current profile.