A Hybrid State of Charge Estimation Method of a LiFePO4/Graphite Cell Using a Reduced Order Model with an Extended Kalman Filter

Estimation of state of charge (SOC) is imperative in the battery management system that ensures safe and reliable operations. Lithium iron phosphate cells are preferred for high power applications because of their electrochemical and thermal stability. On the other hand, these cells are characterized by voltage plateau and path dependence, which impede the accurate estimation of SOC. In this work, a new method is proposed to estimate the SOC using a physics-based reduced order model (ROM), where the two-phase transition of cathode is modeled and validated. In addition, an extended Kalman filter (EKF) is used to minimize the estimation errors of the SOC caused by the inaccuracy of the ROM or the initial errors. The insensitivity of voltage at the plateau is further compensated by combining Coulomb counting. This method is then validated against experimental data obtained with different C-rates at both single and multiple cycles. The effect of EKF on SOC estimation is also analyzed. The results show that the absolute SOC error is kept less than 3% for all the tested conditions.