Research On The Inconsistency And Balancing Management Of Series-Connected Lithium-ion Battery Strings

The energy crisis and environmental pollution have become more prominently in recent years.Electric vehicles(EVs),as the new forces in the energy background,have many advantages such as low emissions,zero pollution advantages,and so on.And they are considered as the critical path to solve the above problems.In order to meet the electric vehicle driving power and mileage requirements,the cells are generally connected in serial and parallel.But the cells have different parameters innately,and as the applied in the process of recycling,this inconsistency continues to increase reducing seriously battery capacity and cycle life.This paper presents an equalizer and implementation method for improving the consistency and extending the cycle life of the Lithium-ion battery pack.And the main works are listed as follows:(1)The causes of the inconsistency are analyzed based on Lithium-ion batteries multiform phenomena.The test platform and inconsistency evaluation system of Lithium-ion battery pack has been set up.(2)The battery equalizer based on Buck-Boost and bidirectional LC resonant converters is proposed through comparing the battery equalization methods of the domestic and abroad.In this equalizer,the chopper technology and principle of resonance are applied to achieve the bidirectional energy transfer between the pack and the cell.(3)A balanced system is designed based on the main equalization circuit,including voltage collection circuit current collection circuit,driver circuit and other peripheral hardware circuit.The "dSPACE+FPGA" control system which loading the Matlab/Simulink battery models are used to be the software part of this system.(4)In order to verify the equalization validity of the balanced circuit,a series of equalization experiments are carried out,including single equalization mode,mixed alternately equalization mode and adaptive equalization mode.Experimental results show that the equalizer can achieve the balancing for a battery string.Different control modes have different equalization results.The adaptive mode has the best result.The equalizer proposed in this paper has many advantages such as the bidirectional equalization between the pack and the cell,less energy loss,high efficiency and so on.It can be applied in the battery management system of the electric vehicles.This thesis has a very important practical significance for improving the battery performance,extending the battery life and increasing the battery safety factor.