Parameter Identification And SOC Estimation Of Series Lithium Batteries

The stability of microgrid bus voltage is easily affected by photovoltaic,wind and other intermittent new energy generation and load fluctuations,and the energy storage system can adjust the bus voltage of microgrid by "cutting peak and filling valley",which has gradually become a research hotspot.In this paper,the series lithium-ion battery pack is used as the energy storage unit of microgrid,and it is taken as the research object to estimate the SOC of each cell online,so that the battery management system can be designed reasonably to manage the battery and the basis for online SOC equalization in series lithium batteries is provided.Based on the second-order RC model,the SOC of the single lithium-ion battery is estimated firstly in this paper.VFFRLS is used to identify the parameters of the model.Through the analysis of the algorithm,it can be seen that the noise will affect the size of the forgetting factor,thus affecting the overall convergence and tracking ability of the algorithm.Based on this,this paper proposes to build WVFFRLS by introducing WT for parameter identification.Then,the problem of the interaction between state variable estimation and error estimation in SOC estimation using EKF is pointed out.The nonlinear characteristics of the output equation of lithium battery are analyzed and the piecewise linearization of the equation is carried out.On the basis of parameter identification by WVFFRLS,KF-EKF is selected for SOC joint estimation.The simulation results verify the rationality of the designed algorithm.In the application of SOC real-time estimation of single battery in series lithium battery pack,the second-order RC model and its corresponding algorithm have the problems of complex calculation and long operation time.In this paper,the second-order RC model is simplified to obtain the simplified second-order model,which is applied to series lithium battery pack when the battery state is stable.By reducing the number of parameters identification,the operation time of the parameter identification and SOC estimation algorithm can be greatly reduced.Then the model is applied to the series lithium batteries with different SOC initial values for simulation verification.According to the simulation results,the feasibility of the simplified second-order model is verified.At the same time,it can be seen that the model can improve the real-time performance of SOC online estimation of lithium batteries when it is applied to series lithium batteries.Finally,the experimental prototype platform is built to construct the DC microgrid with series lithium battery energy storage unit.The control strategy is designed to stabilize the DC bus voltage fluctuation in the microgrid.At the same time,the parameter identification and SOC estimation of the series battery pack are carried out online.The experimental results show that the control strategy of bi-directional energy storage converter is feasible and the simplified second-order model can realize the on-line SOC estimation of series lithium battery pack with high accuracy.