Research On Design Of Battery Pack Equalization System And Control Strategy For Pure Electric Vehicles

With the growing challenges of energy crisis and environmental pollution,countries around the world are committed to advancing the development of new energy vehicles.Currently,lithium-ion batteries are widely used as power batteries for pure electric vehicles due to their advantages such as high energy density and low self-discharge rate.In practical applications,a large number of battery cells are often required to be connected in series and parallel to meet the high voltage and high power requirements of pure electric vehicles.However,after many cycles of charge and discharge,the power battery pack tends to be inconsistent.Inconsistent means that the voltage or state of charge(SOC)between the battery cells is not exactly the same.In use,overcharge or overdischarge may occur.This phenomenon will cause irreparable damage to the battery,which will seriously affect the life of the battery.Currently,equalization technology is the most effective way to solve the problem of inconsistent battery packs.Therefore,this dissertation focuses on lithium iron phosphate battery and studies and designs an equalization system to achieve battery pack equalization.The main work of this paper is as follows:First,a bidirectional isolated C?k equalizer based on energy bus network is chosen as the equilibrium topology,and a sufficient condition for synchronization equalization that is easier to implement is derived.The bidirectional isolated C?k equalizer uses the bus as a carrier for energy transfer to achieve energy exchange between the cells.The use of synchronous equalization strategy to improve the equalizer's equalization efficiency,through the analysis of the equalization process and mathematical model,derivation of a sufficient condition to meet the synchronous equalization strategy is: the duty cycle of the PWM control signal is greater than 50%.Second,a multivariate coordinated equalization scheme is proposed,and a dual closed-loop Fuzzy-PI control strategy with adaptive fuzzy coordinator is designed.The multivariate coordinated equalization scheme may select voltage or SOC or the ratio of voltage and SOC as the final equalization variable according to the specific state,thereby overcoming the insufficiencies of a single equilibrium variable failing to completely and fully characterize the inconsistency of the battery.The designed adaptive fuzzy coordinator implements the multivariate coordinated equalization scheme.Its control strategy combines the equalization variables selected by the coordinator to establish inconsistent standards,and then eliminates inconsistencies by adjusting the equalization currents to achieve balanced control of the battery pack.Third,build a equalization system prototype platform and passed various tests.The test results show that the sufficient condition for synchronization equalization is correct and easy to implement.The coordinator always selects the variable with faster equilibrium speed as the final equilibrium variable.The equilibrium strategy can achieve equilibrium in different states and ensure that the maximum voltage difference is less than 0.005 V.The equalization topology selected in this dissertation has the characteristics of high equilibrium efficiency and strong scalability.The designed multivariate coordinated equalization scheme is effective and the equalization control strategy is correct and feasible.