Research On Efficient Active Equalization Technology Of Lithium-ion Batteries Based On Reconfigurable Topology

Since the "dual carbon" goal was proposed,the penetration rate of new energy power generation in China has been increasing rapidly.Renewable energy generation has random fluctuation,intermittence,and no damping characteristics.The safe and stable operation of the power system has been severely challenged.Energy storage systems have the advantages of energy time shifting,rapid response,and flexible arrangement,which is an important technical to promote the consumption of new energy and ensure the safe and stable operation of power system.As the main carrier of new energy storage systems,the imbalance between lithium-ion battery cells will lead to the decrease of available capacity,shortening of cycle life,and decrease of safety of energy storage systems.Therefore,fast and efficient equalization of lithium-ion battery packs is important to improve the safety and economy of energy storage systems.In this paper,two active equalization methods of lithium-ion battery packs based on the consistency of state of charge(SOC)are proposed.The main research contents are as follows:(1)Comparative analyses of Li-ion battery modeling and SOC estimation methods are performed to establish a second-order RC equivalent model for Li-ion battery cells.The extended Kalman filter(EKF)method is used for the simulation of the SOC estimation of Liion batteries.The simulation results showed that the EKF-based SOC estimation is not affected by the initial value error of the input SOC,and can converge quickly to the vicinity of the true SOC value with 99.86% accuracy.(2)A double-layer reconfigurable equalization topology and its control strategy are proposed to address the problems of low equalization efficiency and high output fluctuation of existing active equalization circuits.The inner layer uses the reconfigurable topology to equalize the cells within a group,and the outer layer controls the DC/DC converters with different output powers to equalize the cells between groups.Efficiency calculation showed that the double-layer reconfigurable equalization method effectively improved the efficiency compared with the conventional converter equalization method.Simulation results showed that this method can effectively balance the battery pack and maintain the output voltage at the set value through closed-loop control with an error of less than 0.18%,which solves the problem that the output voltage fluctuates obviously when the reconfigurable topology is used alone.(3)An improved matrix-type reconfigurable equalization topology and its control strategy are proposed for the existing active equalization circuit with many energy storage elements and high cost.The battery pack is balanced between series and parallel topologies through switch matrix control,and speed up equalization by isolating the cell or group of cells with the lowest SOC.Since no energy storage element is used,the battery pack equalization is achieved by switching control only,which effectively reduces the cost of the circuit.Simulation results showed that the improved method can improve the equalization speed by 13.1%,27.1%,and30.1% for loads of 10 W,20 W,and 40 W.Compared with the original matrix-type reconfigurable equalization method;it can avoid SOC divergence when considering the switch on-resistance and successfully achieve the equalization target.(4)A hardware experiment is designed to verify the double-layer reconfigurable equalization method.An experimental platform based on LTC6804 voltage measurement chip and SC8701 DC/DC converter chip was developed,and the design ideas and working principles of each part of the system were analyzed.Discharge experiments were carried out on 12H18650 CIL lithium-ion battery cells and the measured voltage waveforms and SOC curves were analyzed.The average SOC difference between the batteries and the SOC difference within the groups became less than 1% after 1459 s.It has proved that this system can effectively balance the imbalance between the battery cells,thus improving the output capacity of the battery pack and prolonging the cycle life of the battery pack.