Online SOC Estimation Of Series Connected Lithium-Ion Batteries In DC Microgrid

In order to alleviate environmental pollution,improve energy efficiency and meet voltage level requirements,retired lithium-ion batteries of electric vehicles connected in series are used in the DC microgrid.In this paper,the SOC estimation methods of single battery and series battery are studied.The specific contents of this paper are as follows:For a single lithium-ion battery,based on the analysis of the second-order RC equivalent circuit model,the recursive least square is discussed in the process of parameter identification,and the reason why RLS algorithm affects the accuracy of model parameter identification is pointed out.For this reason,an improved RLS algorithm is proposed.The improved RLS algorithm is to add an inner loop that updates the estimated parameter vector multiple times in the traditional RLS to improve the identification accuracy.Secondly,the process of unscented Kalman filter for SOC estimation of cell is analyzed.The unstable factors of UKF algorithm and the influence of UKF algorithm on SOC estimation accuracy and computation are pointed out.The KFSRUKF algorithm composed of linear Kalman filter and square root traceless Kalman filter is proposed for SOC estimation of cell.Finally,a test platform based on battery measurement equipment and special test software is built.The results show that two improved algorithms have better tracking ability,smaller prediction error and smaller calculation.For series-connected lithium-ion batteries,considering the differences between individual cells,the complexity of the algorithm and the real-time performance of the algorithm,the SOC estimation of the cells based on the Rint model is carried out.Firstly,the idea that the estimated value of terminal voltage is equal to the real value is used to identify the internal resistance of each cell,and KF algorithm is applied to estimate SOC.Secondly,the simulation results show that the model and algorithm can reduce the execution time of the algorithm and achieve high accuracy estimation of SOC of each cell.Finally,the hardware experiment platform of a small capacity DC microgrid with series lithium-ion batteries is built.The control strategy of restraining the voltage fluctuation of DC microgrid is designed,and the application of series lithium-ion batteries in the hardware platform is completed.The platform realizes on-line parameter identification and SOC estimation of series lithium battery.The correctness and effectiveness of the model and algorithm are verified by experiments.