Research On Equilibrium System Of Lithium-ion Battery Pack Based On Electric Vehicle

Power battery system is one of the three-power-systems of new energy vehicles.With the support and promotion of the national "863 plan" and relevant policies,China’s new energy vehicle industry is booming,the quantity demand and quality requirements for power battery packs are greatly improved.After the power battery cells are connected in series and in parallel,the initial inconsistency will be magnified with the increase of cycle usage times,and obvious imbalance will occur,which will seriously affect the available capacity of the battery pack and even cause potential safety hazards.This paper takes three-component lithium-ion battery as the object,conducts in-depth research on active balance technology of power battery pack,and the main work contents are as follows:(1)Analyze the dynamic characteristics of lithium-ion batteries,such as capacity,open-circuit voltage and internal resistance experimentally,summarize the existing mathematical model of the lithium battery,and establish a second-order RC equivalent circuit model for lithium battery considering hysteresis effect.The off-line method is used to accurately identify the model parameters,and the Matlab/Simulink simulation model is built to verify the exactness of the model using custom dynamic conditions.(2)Perform directed graph analysis on traditional equalization circuit,summarize the characteristics of ideal equalization structure,a multilevel bidirectional active balancing circuit is proposed combining the advantages of Cuk and flyback DC/DC converter,and the working mode of the balancing circuit is analyzed i n detail.Based on the mean-difference method,fuzzy control was used to design the equalization strategy,and the equalization current was dynamically adjusted according to the parameters of the battery state to optimize the time efficiency and energy efficiency of the equalization process.Matlab/Simulink was used to build the simulation model,and compared with the traditional active equilibrium structure.The results show that the active balancing topology can achieve high precision balancing goal with high speed and energy utilization efficiency.(3)Using master-slave distributed structure,the software and hardware scheme of active equalization system are designed.The hardware structure uses Freescale MC9SXDP512 as the main control chip and AD7280 A as th e front-end chip,and the peripheral circuits of various parts such as acquisition,isolation,drive,balance and communication are designed.Then design the main and module program based on CodeWarrior environment,and design balance system PC for real-time balance process display based on LabVIEW.(4)Design and test the equalization system PCB,and build an active verification system experimental verification platform for the 6-cell battery pack.For the battery pack with random initial SOC value,equilibrium experiments are performed under static,constant current charging and constant current discharge condition,the results show that the active equalization system can achieve the expected equilibrium target with high equilibrium efficiency.