Study On State Estimation Of Ternary Lithium Ion Battery Based On Electrochemical Impedance Spectroscopy

In recent years,global car ownership has increased rapidly,but at the same time,it also poses a great threat to global climate change and environment.For this reason,governments around the world have begun to study the ban on the sale of fuel vehicles,and vigorously carry out the research of electric vehicle technology.However,the popularity of electric vehicles is still constrained by battery technology.Currently,ternary lithium ion batteries(NCA)are becoming the mainstream of automotive power battery market due to its advantages such as the highest specific energy,low cost,and good cycle performance.However,due to the late development of the NCA battery,the current study on state estimation of battery is still relatively limited.The battery state generally refers to state of health(SOH)and state of charge(SOC).The accurate estimation of them is of great significance for the promotion of ternary lithium ion batteries.In current battery state estimation methods,electrode reaction kinetic information is generally not considered.However,when the lithium ion battery is charged and discharged,the electrochemical reaction occurring at the internal electrode interface is closely related to SOH and SOC.Therefore,it is feasible to estimate battery state by using electrode response information.The electrochemical impedance spectroscopy(EIS)method can obtain the EIS curve which can reveal the electrode reaction kinetic information through the non-destructive measurement of the battery impedance.By studying the relationship between the EIS and the state of battery,it is possible to accurately estimate the SOH and the SOC.Therefore,the EIS method can be used as an effective method for the state estimation of battery.Based on the EIS technology,this paper studies the state estimation of ternary lithium ion battery,and the main study contents and conclusions are as follows:(1)Through the battery cycle aging test,the EIS of different cycle times and different SOC was measured,thereby an equivalent circuit model based on the information of battery internal mechanism was established.By analyzing the relationship among model parameters and low frequency impedance and SOH,it is found that both the charge transfer internal resistance Rt and the low frequency impedance amplitude have the law of increasing with decreasing SOH.Considering the practical application,finally,the low frequency impedance amplitude of battery was chosen to estimate the SOH,and the SOH estimation method based on the low frequency impedance amplitude was established.After verification,the root mean square error(RMSE)between the estimated value and the true value is 3.16%,indicating that the SOH estimation method is effective.(2)Through experimentally measured battery EIS data at different SOC,the parameters of the equivalent model circuit are identified.By analyzing the relationship among model parameters and low frequency impedance and SOC,it is found that the model parameters and the amplitude of the low frequency impedance both have no regular changes with the change of SOC.This shows that for the ternary lithium ion battery tested in this paper,it is difficult to estimate the SOC by using EIS alone.(3)In order to estimate the battery SOC,the method of combining the equivalent circuit model established by the EIS with the Extended Kalman Filter(EKF)is adopted.Firstly,the battery model is simplified reasonably,and its parameters are identified by impulse test.It has been verified that the RMSE between the model voltage and the true value is 0.062 V,indicating that the battery model is effective.Then using the EKF algorithm based on this model to estimate the battery SOC,it is found that when the SOC initial value is accurate,the RMSE between the estimated value and the true value is 2.7%,which verifies the estimation effect of the method;When the deviation of the SOC initial value is large(50%),the method can correct the initial deviation and quickly approach the true value.Finally,the estimation result is compared with the estimated value using the traditional Thevenin model algorithm,the result shows that the estimation error of the proposed algorithm is smaller and the estimation effect is better than the latter.