A discrete-Time nonlinear observer for state of charge estimation of lithium-ion batteries

This paper presents a novel state of charge (SOC) estimation algorithm for lithium-ion batteries (LIBs) using a discrete-Time nonlinear observer (DNLO) and a second-order resistor-capacitor (2RC) equivalent circuit model. Considering the hysteresis characteristic of battery, the parameters of the 2RC equivalent circuit model depend on the SOC and the direction of battery current simultaneously. Then the exponential-function fitting method identifies the offline parameters of the battery model. The convergence and stability of the proposed observer is proved by the Lyapunov inequality equation. The performance of the proposed method is also verified by the experiments based on the hybrid pulse power characteristic (HPPC) test. The experiment results show that the proposed observer has better performance in reducing the computation cost, improving the estimation accuracy and enhancing the convergence capability, than the EKF algorithm and the discrete-Time sliding mode observer (DSMO) algorithm.