Research On Equalization Control Based On SOC Estimation For Battery Pack Of Electric Vehicle

Nowadays,electric vehicle(EV)is a good alternative to solve the problems of energy shortage and environment pollution because it has the advantages of low noise and nearly zero emissions.Lithium-ion battery is widely used in EV because of the advantages of high energy density,long life,low self-discharge rate and good safety performance.The operating voltage of EV is between 100V-500 V.However,the nominal voltage of Lithium-ion battery is less than 4V,which is determined by the anode and cathode materials of battery.Therefore,the battery packs with hundreds or thousands of cells connected in parallel and/or in series are required to meet the power and energy requirements of EV.However,the inconsistency of battery is inherent and inevitable during production.Moreover,vehicles usually operate in various conditions such as starting/stopping,acceleration climbing or deceleration braking according to demand.In such case,the charging/discharging mode,current magnitude and duration of battery pack are uncertain and thus the internal heat distribution is uneven,which also aggravates inconsistency especially for the high-capacity battery pack connected in series.The inconsistency leads to the reduction of capacity for battery pack,the attenuation of cycle life and the decrease of driving range eventually.Therefore,the equalization is necessary for battery management system in EV.The main research contents include the following aspects:1)The experiment is designed for the inconsistency analysis of power battery pack.Then the lithium-ion battery pack is tested under different charge and discharge conditions.And the inconsistency of voltage,capacity and stage of charge(SOC)is analyzed for finding the influencing factors of power battery inconsistency.Then the SOC is chosen as the evaluation standard of the battery inconsistency characteristic according to the combination of the national standard and experimental results.2)The Thevinan equivalent circuit model is built,and the parameters of resistance and capacitance in the model are identified.Then the accuracy of the model is verified under the custom condition.The Extended Kalman Filter(EKF)is used to estimate SOC accurately,which provides an effective evaluation index for the battery equalization.Then the connection mode is chosen through the analysis of the reliability of the different connection modes in battery pack.And the number of cell is determined by the requirement of power in EV.In addition,the module of battery pack is built in the Matlab/Sim Power Systems in this paper.3)The two-stage bidirectional equalization circuit with energy transferring inductors is designed to implement the equalization.The equalization circuit paves the way for equalization of hardware.In addition,the two-stage bidirectional equalization circuit is built in the Matlab/Sim Power Systems.4)An equalization control method based on fuzzy logic control(FLC)is proposed,and equalization current is controlled by adjusting the duty cycle of MOSFET tube.In addition,the equalization control method is compared with the mean-difference algorithm.The experiment results show that the equalization with two-stage bidirectional equalization circuit based on FLC can improve the inconsistency of battery,reduce energy consumption and save equalization time under same equalization conditions compared with mean-difference algorithm.