Research On Balancing Topology And Control Strategy Of Lithium-ion Battery Pack

Lithium-ion batteries are widely used in alternative-fuel vehicles and chemical energy storage systems due to high voltage,high energy density and memoryless properties.To match with the requirements of power and capacity,hundreds of Lithium-ion cells must be grouped.The capacity,voltage,internal resistance and other properties of the cells may be inconsistent.The inconsistency will seriously affect the performance of the pack and shorten the cycle life of the pack.These phenomena are mainly manifested as barrel effect,overcharge,overdischarge and different charge and discharge rate.These phenomena will result in very bad consequences such as fire and explosion under extreme conditions.Balancing technology is an important means to improve the consistency in the use of lithium-ion battery cells in groups.The main researches are currently focusing on lithium-ion battery modelling,estimation of SOC,balancing topology and control strategies.The main contents of the dissertation are as follows:1)Based on the comparison of the characteristics of electrochemical model,mathematical model and equivalent circuit model,a model combined with equivalent circuit model and hysteresis voltage is proposed to improve the accuracy and provide the basis for estimation of SOC.2)From the perspective of influencing factors of single cell inconsistency,the advantages and disadvantages of different balancing indexes are compared and SOC is selected as the balancing index.The performance of different SOC estimation methods is analyzed.Combined with the battery model,a double Kalman filter algorithm based on Frisch scheme and a dual time scale extended Kalman particle filter algorithm are proposed,which are of great significance for improving the accuracy and the efficiency of battery estimation of SOC.3)Based on structural classification,multiple evaluation indexes are extracted,and then the grey relational degree theory is used to establish a comprehensive evaluation system of balancing topology performance according to the indexes membership function,which provides the theoretical basis for evaluation and selecting of balancing topology.Based on the analysis from multiple perspectives of existing balancing topologies,a new topology based on Cuk chopper circuit is proposed.4)Based on Buck-Boost circuit,a new hierarchical balancing method on the basis of multi-layer balancing circuit is proposed.To realize cells dynamic grouping,a fuzzy c-means clustering algorithm based on density function for the grouping balancing topology circuit is proposed.The efficiency and rationality of the scheme are verified by experiments.5)Based on the performance analysis of energy transfer balancing between any cells and the comparison research of the advantages and disadvantages of various topologies,a balancing topology on the basis of Flyback circuit is developed.The dynamic adjustment method for equilibrium threshold with correction factor and the control strategy of energy transfer based on the solution of matrix sparsity were established.The efficiency and rationality of the scheme are verified by comparative experiments.The conclusions of the dissertation provide a solid theoretical basis for improving the balancing effect and efficiency of lithium-ion battery packs,which has broad application prospects in battery management systems.