Performance Analysis Of SOC Online Estimator Of Lithium-Ion Battery Based On Kalman Filter

Since the 21st century,electrochemical energy storage has been widely used in power systems and has become the largest type of energy storage in the power system except for pumped energy storage.Among various types of electrochemical energy storage technologies,the cumulative installed capacity of lithium-ion batteries is the largest.Due to the high cost of lithium-ion batteries,to ensure the safe and efficient operation of lithium-ion batteries,it is necessary to estimate their State of Charge(SOC).The Kalman Filter is widely used in SOC estimation of lithium-ion batteries due to its high accuracy and moderate calculation complexity.However,the SOC estimation effect based on the Kalman Filter is easily affected by factors such as filter parameter settings,voltage and current measurement accuracy,and battery model accuracy.There,it is important to clarify the quantitative relationship between the SOC estimation accuracy and each factor.This is not only an important link for the application of the method but also a prerequisite for further improvement of the method.Under the above background,starting with the calculation process when Kalman Filter is applied to SOC estimation,this paper gives the error sources that cause SOC estimation errors and deduces the time-domain expression and z domain transfer function of SOC estimation errors under each error source.Furthermore,the influences of initial SOC error,voltage measurement error,current measurement error and battery parameter error on SOC estimation results are analyzed.The main work and conclusions of this paper are as follows:(1)The basic theory of Kalman Filter is briefly introduced,and the equivalent mathematical model,as well as circuit structure of the lithium-ion battery,are introduced.Furthermore,The calculation process of SOC estimation based on extended Kalman Filter(EKF)and 2RC model of lithium-ion battery is given.And SOC is estimated based on HPPC experimental data.The experimental results show that when there are large initial SOC error as well as voltage and current measurement error in the form of Gaussian white noise,the SOC estimation results based on EKF can quickly converge to the true value,and the steady-state error is within 2%.(2)The anti-interference performance analysis method for SOC estimation using EKF is given.Starting from the calculation process of SOC estimation based on EKF and 2RC model,an error transfer system is established by recursion formula.It takes voltage and current measurement errors as input as well as SOC estimation errors as the state.Then the time-domain expression and Z domain transfer function of SOC estimation error under initial SOC error,current measurement error and voltage measurement error are derived.Furthermore,the factors that determine the convergence of the error system,as well as the time constant describing the convergence rate and the value of SOC estimation error in steady-state are given.Finally,the effectiveness of the proposed method and the obtained conclusions are verified based on the data of HPPC test.(3)The robustness analysis method of SOC estimation using EKF is given.Starting from the calculation process of SOC estimation based on EKF and 2RC model,the z domain transfer function from output deviation of battery state equation(deviation of battery state predicted value)and output deviation of observation equation(deviation of battery terminal voltage predicted value)to SOC estimation error is derived by recurrence formula.Then,under the assumption that the current changes slowly,the z domain transfer function from each battery parameter deviation to SOC estimation error is deduced.Finally,the effectiveness of the proposed method and the conclusion are verified based on the data of the constant current discharge experiment.