Design And Verification Of Active Balancing Control For Vehicle Powered Seriers-connnected Lithium-ion Battery Pack

With the continuous intensification of global climate deterioration and the increasing shortage of oil resources,electric vehicles(EVs)have developed rapidly in the automotive industry because of their advantages of energy saving and environmental protection.Lithium-ion battieris(LIBs)are widely used in EVs due to their good performance in terms of high energy density and long cycle life.Limited by the voltage of a single lithium-ion battery,for the lithiumion battery pack,which serves as the energy supplier for EVs,it is consisted of mutiple single cells connected in series or parallel.However,there often exists an inconsistency in each single cell due to the consistency among the materials production,assemble and fabrication process cannot be ensured by the existing technology.And this inconsistency can be further aggravated in daily operation with the increasing period of use,which further affects the life cycle of the battery pack and poses a threat to the overall performance and safety of EVs.Therefore,it is necessary to propose an appropriate and effective equalization technology to reduce the inconsistency,so as to ensure the battery pack work safely and effectively.Based on ternary lithium batteries,this paper models and analyzes the power series lithiumion battery pack of EVs,and studies the equalization technology from the two aspects of equalization strategy(ES)and equalization circuit(EC).The main work is as follows:First,several commonly used lithium-ion battery equivalent circuit models are summarized,based on the summarization the second-order RC equivalent circuit model closer to the real battery characteristics is selected to construct the single cell model,and the model parameters are identified by exponential function fitting method.After that the battery model is built in MATLAB/Simulink to validate its accuracy.Second,according to the the characteristics of open circuit voltage(OCV)-state of charge(SOC)identification curve of the battery,an ES based on a dual threshold trigger mechanism is presented,which uses battery voltage and SOC together as the equilibrium variables to achieve accurate control in the whole SOC range.For the LIBs with SOC between 20%-80%,SOC is served as the equilibrium variable to balance.Conversely,when SOC is absent from 20%-80%,voltage is adopted as the equilibrium variable to balancethe.Additionally,in this strategy,the improved particle filter algorithm(EKPF)is used to estimate SOC,which upsurges the equalization control precision.Third,a modified dual-layer inductor EC is constructed on the basis of the traditional duallayer inductor EC,which overcomes the difficulty that when the energy deviation between two non-adjacent cells is huge,the energy only flows between contiguous cells.Meanwhile,particle swarm optimization(PSO)algorithm is employed to optimize the energy transfer path in the modified circuit,so as to improve the equaliation efficiency.Combined with the above the second-order RC equivalent circuit model,the EC model of the 8-cell series ternary LIB pack is simultate,thus the efficiency of the modified EC is proved.Finally,based on the circuit simulation model,the corresponding EC board and the equalization system software interface are developed and designed.Addtionally,the equalization experiment platform is built to further validate the effectiveness of the proposed ES and the proposrd EC.