Research On SOC Estimation Method For Equivalent Circuit Model And Simplified Electrochemical Model Of Lithium-ion Battery

For battery management systems,accurate State of Charge(SOC)estimation is the key to the safe and reliable operation of battery systems.As an important decision-making factor of the BMS system,SOC plays an important role in preventing battery abuse,improving the utilization rate of battery capacity,and ensuring the safe and reliable operation of the battery system.However,due to the strong time deformation and nonlinearity of batteries,accurate SOC estimation still faces challenges.In addition,as the battery ages,the capacity of the battery will also change significantly,the capacity of the battery will continue to decay with aging,and the capacity of the battery is difficult to directly measure during actual use.At the same time,the P2 D model has been widely used in the simulation of battery characteristics.The P2 D model is the most widely studied electrochemical model so far.However,the electrochemical model needs to solve complex partial differential equations,which is difficult to directly apply to real vehicles.Therefore,this paper conducts research on P2 D model simplification and SOC estimation algorithm.The specific research contents are as follows:(1)Basic performance experiments: battery experiments such as capacity calibration experiments and HPPC experiments were carried out,and the charge-discharge characteristics of batteries under different aging and different rates were studied.Based on the second-order RC model,the model parameters were identified.In addition,the change rule of battery capacity with aging process is analyzed.(2)SOC and capacity estimation: The second-order RC circuit model is adopted,and the online identification of the model parameters using the least square method is analyzed.Next,the cycle times of the battery are obtained through the DTW algorithm,and the maximum available capacity of the battery is calculated on the capacity cycle curve.At the same time,it is proposed to use the error of the battery terminal voltage to adjust the forgetting factor through the PI observer,thereby improving the battery SOC estimation accuracy.Finally,the SOC estimation method of unscented Kalman filter is introduced in detail,and the SOC estimation results are analyzed.Comparing the SOC estimation results of 20-cycle and 200-cycle batteries respectively,it shows that the proposed method has higher accuracy and convergence.(3)Simplified electrochemical model: The P2 D model is introduced,the solid-phase lithium ion concentration and liquid-phase lithium ion concentration are simplified,and the pade approximation is used to solve the transfer function of solid-phase lithium ion concentration and current density.The relationship between liquid-phase lithium ion concentration and current density was solved by polynomial fitting method.The reaction overpotential and electrolyte potential were further deduced.In addition,the thermal model is coupled on the basis of the simplified electrochemical model.And based on the simplified electrochemical thermal coupling model,the SOC estimation of the battery is realized.Finally,the errors of the simplified electrochemical model and the circuit model in terms of voltage are compared,and the accuracy of the simplified electrochemical model is shown.At the same time,the performance of the two models in SOC estimation is compared,and the results show that the simplified electrochemical model has better accuracy and robustness in SOC estimation.