Research And Design Of Electric Vehicle Power Battery Equalization Control System

As the power source of electric vehicles,the power batteries are the premise and foundation for the stable operation of electric vehicles.However,batteries are susceptible to the working environment and complex operating conditions,resulting in unavoidable inconsistencies within and between battery packs,which not only decrease the available capacity and energy efficiency of battery packs,but also seriously affect the cycle life of electric vehicle battery packs.Therefore,this thesis takes lithium iron phosphate battery,which is frequently used for electric vehicles,as the research object,and studies and designs the electric vehicle power battery equalization system in terms of equalization topology and equalization control strategy,aiming to improve the inconsistency problem,enhance the energy efficiency of the battery pack,extend the lifetime of electric vehicles,and improve the driving range of electric vehicles.The specific research contents of this thesis are as follows:Firstly,this thesis explains the working principle of lithium iron phosphate batteries,analyzes the causes of inconsistency and the essential of equalization management.Incorporating the requirements of the equalization system,the open circuit voltage,operating voltage and capacity characteristics of lithium iron phosphate batteries are tested experimentally and their characteristics curves are studied in depth to establish the basis for the next research of the equalization control system.Secondly,based on the analysis of commonly used equalization topologies,this thesis selects the Cuk circuit with bidirectional conversion,simple topology,convenient control,and flexible selection of equalization variables as the equalization topology circuit,and proposes a cascaded Cuk equalization topology.The circuit can simultaneously achieve intra-cell and intercell energy transfer,which solves the problem that the non-cascaded topology can only equalize two adjacent batteries with low equalization efficiency.Simulation results show that the cascaded Cuk equalization circuit has significantly higher equalization speed and good convergence.Then,according to the characteristics of OCV（Open Circuit Voltage）-SOC（State of Charge）curve,which is "steep at both ends and slow in the middle",a multivariate coordinated equalization scheme is proposed,in which SOC is selected as the equalization variable when 20%≤SOC≤90%,and voltage is selected as the equalization variable when SOC<20% or SOC>90%.A dual closed-loop equalization control strategy based on multivariable coordination mechanism is designed,with variable universe fuzzy control in the outer loop and fuzzy PID control in the inner loop,to dynamically adjust the equalization current by controlling the duty cycle of MOSFETs in the equalization circuit.The simulation results show that the equalization control strategy is able to precisely control the equalization current under three different conditions of resting,charging and discharging,and achieve effective equalization of the battery pack.Finally,this thesis designs the equalization control system for hardware and software,and the designed equalization system is tested and proved under three states of resting,charging and discharging,respectively.The experimental results show that the battery pack equalization threshold is able to satisfy under different working conditions: when 20%≤SOC≤90%,the SOC threshold is less than 0.5%;when SOC<20% or SOC>90%,the voltage threshold is less than0.01 V.The equalization system designed in this thesis has high equalization accuracy and good equalization effect,which can effectively reduce the inconsistency of the battery pack.