Research On Equalizer Based On Buck-Boost Converter For Series-connected Batteries

As one of the power sources of electric vehicles(EVs),the battery is restricted by the cell voltage and capacity.Thus,cell cannot meet the voltage and capacity requirements of EVs.Therefore,cell need to be used in series and parallel to meet voltage and capacity requirements.However,there are inevitably differences in the factory parameters and operating environment of cell,and this difference will continue to expand as the number of monomer cycles increases.Furthermore,the ever-expanding difference will lead to decrease capacity of the battery pack,and even the risk of explosion in serious cases.There,reducing the difference between cells and increasing the available capacity of battery pack by equalization circuit has become a research hotspot.In this paper,the balancing circuit based on Buck-Boost converter is studied deeply,and three different equalization circuits based on Buck-Boost converter are proposed,which are compared with the existing equalization circuits based on buck-boost converter.This paper first analyzes the series structure and parallel structure equalization circuit based on Buck-Boost converters.The series architecture equalization circuit has the disadvantages of slow equalization speed,low efficiency and poor accuracy.And the parallel architecture equalization circuit improves the balancing speed and efficiency compared to series architecture,but still has the disadvantages of poor scalability.Thus,a double-layer E-structure equalizer based on Buck-Boost converter is proposed.The proposed equalizer can take into account balancing speed,efficiency and scalability,but still has the disadvantage of poor accuracy.In order to improve the equalization precision,a two-stage control strategy is proposed.Finally,Simulation and experiment results show that the proposed equalizer can improve equalization performance of battery cells compared with series architecture equalization circuit.Secondly,although the distributed equalization circuit based on Buck-Boost converter has the advantage of easy modularization,but the charge of the unbalanced cell cannot be exchanged with the whole battery,which limits the equalization speed.In order to further reduce the number of diodes and increase the equalization speed,the centralized equalization circuit based on Buck-Booost converter was improved.A bidirectional cell-to-pack equalizer based on buck-boost converter is proposed.The proposed equalizer not only reduces the number of components by exchanging the connection,but also realizes the bidirectional charge flow.Finally,the working principle of the proposed equalization circuit is analyzed in detail.Simulation and experiment results show that the proposed equalization circuit has faster equalization speed unidirectional cell-to-pack equalization circuits are compared,.Finally,the bidirectional cell-to-pack based on Buck-Boost converter has the advantages of less components and bidirectional charge flow.However,in the process of freewheel flow,the current will pass through the parasitic diode of the switch,and the charge can not be reasonably flowed,which will decrease the balancing speed and efficiency.Moreover,the equalizer based on Buck-Boost converter has different performance for different voltage distribution.Therefore,a multi-mode equalizer based on Buck-Boost converter is proposed.By designing a reasonable control strategy,the proposed equalizer not only realize the optimal flow of charge,improve the balancing speed and efficiency,but also ensure that the equalization performance is same under different voltage distribution.Finally,simulations and experiments results show that the proposed equalization circuit with control strategies 3 not only achieve the optimal flow of charge,but also ensure the accuracy of the balance.