Research On Modular Multi-level Lithium Battery Equalizer And Its Control Strategy For Electric Vehicle

In today’s world,ecological degradation and energy depletion are serious constraints on sustainable social and economic development,and have become huge problems that all countries and regions around the word must face.The large-scale development of electric vehicles is an important measure to alleviate the problem.The vast majority of electric vehicle manufacturers choose lithium batteries as the power battery of electric vehicles,and it is necessary to connect multiple battery cells are grouped in series and parallel to meet the voltage level required by electric vehicles.However,the difference in the performance of each battery cell will lead to inconsistency in the voltage of each battery cell,which greatly affects the service life and use safety of the battery pack.Therefore,it is positive and practical significant to study an equalizer with fast equalization speed,high modularization and low cost to reduce the voltage inconsistency between battery cells.Traditional non-energy-consuming equalizers have many problems such as complex circuit topology,large size and high cost,a dual active half-bridge equalizer is studied.Based on the detailed introduction of its working principle,an equivalent circuit model is established,and the key factors for the balanced energy flow between intra-group and inter-group battery cells under the open-loop control strategy were analyzed emphatically.The research results show that the dual active half-bridge equalizer with open-loop controlled has a satisfactory equalization speed only when the switching frequency is low,but the lower switching frequency leads to an increase in the size and weight of the transformer in the equalizer,when the switching frequency is high or the voltage difference between inter-group battery cells is small,the equalization speed between inter-group battery cells is extremely slow.The phase shift control strategy is proposed to solve the shortcoming of dual active half-bridge equalizer under open-loop control.The key factors for equalization energy flow between intra-group and inter-group battery cells under phase shift control are analyzed.The research results show that the dual active half-bridge equalizer based on phase shift control can achieve fast voltage equalization between inter-group battery cells at high frequency operating conditions,thereby reducing the size and weight of the transformer in the equalizer.In view of large errors in traditional continuous voltage sampling,a time-sharing voltage sampling is adopted to collect the voltage of the battery cells when they are in a static state,which improves the voltage sampling accuracy of the battery cells.For dual active half-bridge equalizer can only achieve voltage equalization between four battery cells,while the number of battery cells connected in series in an electric vehicle is as many as hundreds.Therefore,the equalizer is constructed from different levels,and using dual active half-bridge equalizers to build a lower voltage equalization module with cascaded voltage doubler rectifier equalizer to from a modular multi-level equalizer.The working principle of the cascaded voltage doubler rectifier equalizer is introduced in detail,and its equalization characteristics are analyzed.The influence of the equalizer parameters on the equalization current is analyzed,and the design range of the main parameters of the equalizer is given.The research results shows that the cascaded voltage doubler rectifier equalizer can realize the voltage equalization between each equalization module,and the modular multi-level equalizer can realize the voltage equalization between all battery cells.The corresponding simulation and experiment platform is built,and relevant simulation and experiment verifications are carried out.The results of simulation and experiment verify the correctness of the theoretical analysis.