Research On Hierarchical Equalization Topology Based On Graph Theory And Equalization Control Strategy

Lithium-ion batteries have been widely used in electric vehicles due to their good environmental performance,long cycle life,and no memory.However,problems such as frequent charging due to the short battery life,large number and high cost due to frequent replacement,making charging problem become one of the most fundamental problems in the development of electric vehicles.At the same time,after hundreds or thousands of lithium-ion batteries are used in series and parallel,because of the difference between the individual cells,it is necessary to balance the battery pack in order to ensure the safety of the battery and to improve battery pack energy utilization,Firstly,the dissertation summarizes the research status of equilization technology,which is divided into two aspects: equilization topology and equilization strategy.According to the flow direction of energy in the equalization system,the circuit topology is divided into four types.Analysis the application characteristics of different topologies in detail,as well as the advantages and disadvantages of working methods.The analysis of the equilization strategy includes the equilization indicator and the indicator-based equalization algorithm.The indicators focus on the terminal voltage,the remaining capacity and the state of charge(SOC)for comparison,and analyze the accuracy and stability of various strategies in determining the energy transfer path and the amount of transfer energy.Then the dissertation focuses on the current problem of slow equalization speed and high energy loss of large-scale series circuit,starting from the graph theory model of circuit structure,ignoring the influence of each component and considering the specific connection mode inside the circuit only.Then the graph theory models of the four different topologies are established,and the theoretical optimal equalization circuit structure can be obtained based on their advantages and disadvantages.However,considering that the energy transfer speed will be slower due to the influence of switching elements,This dissertation designs a layered structure based on the theoretical structure.Based on the graph theory model,the hierarchical equalization topology based on Buck-Boost circuit is designed.The structure can shorten equalization time and path,and can be synchronized in multiple ways.According to the topology characteristics of the layered equalization circuit,equalization strategy is formulated.SOC is selected as the equalization indicator at first.The first-order RC model is combined with the unscented Kalman filter algorithm to estimate the SOC to ensure the real-time and accuracy of the estimated value.According to the characteristics of constant voltage before constant voltage in the charging process,the double-loop control strategy is adopted in the equalization process.and the equalization efficiency is improved.The experimental results show that the overall equalization scheme has good operability,short equilization time and high efficiency.