Nonlinear Observer Design for the State of Charge of Lithium-Ion Batteries

As rechargeable lithium-ion batteries are widely used in many applications nowadays, how to accurately evaluate the battery’s state of charge(SOC) becomes a more and more important issue. A new method for estimating the SOC of lithium-ion batteries based on an inclusive equivalent circuit model is proposed in this paper. To the best of our knowledge, the parameters in the model are usually considered as constants to simplify the problem of the SOC estimation, which may lead to some estimation error. In order to get more accurate estimation results, the capacitances and resistances in the battery model are considered as nonlinear functions of the SOC and the temperature of the battery. The resistances also depend on the battery’s charging or discharging mode. Nonlinear relationship between the open circuit voltage(OCV) and the SOC is considered and a nonlinear observer is designed to estimate the inner characteristics of the battery. Lyapunov stability analysis is utilized to prove its performance and simulation results are provided to illustrate the performance of the proposed approach.