Reseach On Impedance Model Of Lithium-ion Battery Based On EIS

At present, although the detection technology and device of storage battery have greatly improved on the precision, normal projects in general still tends to use relatively backward battery detection method restricted by high cost of equipments, complexity of the detection algorithm. For this situation, this paper regards the electrochemical impedance spectroscopy(EIS) of lithium iron phosphate battery as the foundation, have a research mainly focused on simplifying and establishing battery model in the low frequency region, discussing the correctness and application of the model with simulation verification, which provides a simple and feasible method for detecting the performance of battery in actual engineering.In the term of establishing model, this paper is based on the EIS of lithium iron phosphate batteries, by fitting comparison and correction, a three order RC circuit is established as the simplified battery impedance model in the low-frequency region. Analysis shows that, there is a solid electrolyte membrane in the battery cathode interface(SEI), the impedance response in EIS band has not been displayed and reflected in the low frequency region of simplified model. After in-depth study of the formation mechanism of SEI and its impedance characteristic, modified the model by adding a corresponding equivalent elements. Through the fitting test, the modified model response in the low frequency region and impedance frequency band can agree very well with the EIS spectra of graphs, which proves that this model is accuracy in the impedance response.According to the established equivalent model, this study accomplish the simulation verification work of battery discharge characteristics. Based on the third order RC circuit theory, the parameters in the circuit model have been identified, and established the function relationship between the battery SOC and element parameters.On this basis, using Simulink platform to set up a simulation model for the lithium ion battery and accomplishing the simulation work in two kinds of working conditions,which are constant current discharge and electric pulse discharge in cycle.The errors of the two cases are concentrate in the start and end stage of discharge.The values of error show that the established simplified model imitates the characteristics of battery well under the dc or low frequency charging and discharging.In application of the model, the study has carried on the work of the battery SOC estimation combined with extended kalman filter(EKF).According to the relationship between model parameter identification, the system state equation and observation equation and estimation recursive process have been built. Then regarding the SOC value calculated by Ah method as a real value in theory, process the simulation of SOC estimation in Matlab under periodic pulse discharge. The results show that the estimate error value of SOC is mainly concentrated at the end of the discharge phase. From overall, the estimation precision of SOC by using this model can satisfy the general requirement for engineering application.The research and verification of this paper show that simplified battery impedance model in low frequency region of EIS can simulate the dynamic characteristics of battery well, and this model also has a certain application. This method is less demanding on equipments, and processing method is simple and practicable, therefore its application in actual project is feasible.