Research On Variable Fractional-order Index Based Evaluation Method Of Comprehensive Performance For Lithium-ion Batteries

With the problems of environment pollution and global energy shortage becoming more and more serious,the new energy vehicle has become the development direction of global automobile industry.China's electric vehicle industry meets great development opportunities under the support of relevant national policies.As the power source of electric vehicle,battery is an indispensable core component of electric vehicle,and its performance directly affects the power and safety performance of electric vehicle.However,most of the evaluation indexes applied in battery management system(BMS)can only evaluate single-side performance of battery,and there is no way to evaluate the overall performance of battery.Lithium-ion battery is a typical nonlinear time-varying system,the states and parameters in different scales are highly coupled,which makes it difficult to evaluate the battery performance comprehensively for battery in real-time.Therefore,by means of fractional-order data,mechanism modeling and its realization technology,this paper researches a novel method to evaluate lithium-ion battery performance comprehensively.This paper is organized as follows:Firstly,based on the internal electrochemical reaction mechanism of lithium-ion battery,the charging/discharging characteristics are studied by the standard characteristic tests of battery.Meanwhile,fractional-order elements and electrochemical impedance spectroscopy(EIS)are combined to research the electrochemical reactions with fractional-order dynamics in lithium-ion battery.The constant phase element(CPE)in fractional-order equivalent circuit is used to describe and analyze the fractional-order characteristics of lithium-ion battery,which lays the foundation for the variable fractional-order method to comprehensively evaluate the performance of lithium-ion battery.Then,considering that the existing evaluation indexes are not comprehensive enough,this paper develops the fractional-order order a by analyzing the fractional-order characteristics of battery from the capacitive arc part in mid-frequency region of EIS.And the variable fractional-order ??is proposed as a novel index to comprehensively evaluate the performance of lithium-ion battery.Moreover,the Steiglitz-McBride iterative algorithm is introduced to solve the problem that the Nyquist plot of lithium-ion battery is difficult to be captured in real-time.Therefore,the on-line calculation of the variable fractional-order index ?? is realized,which provides theoretical support for its application in the on-line evaluation for lithium-ion battery comprehensive performance.Finally,in order to verify the proposed variable fractional-order indexa,a battery test platform is built.The relationships between the variable fractional-order index??and SOC,aging degree,discharging rate and temperature of lithium-ion c?battery are analyzed by a series of experiments with LiFePO4 cells and modules.The results show that the fractional-order index??is monotonically related to the above parameters of lithium-ion battery in different frequency ranges,which proves that the variable fractional-order index aw is an effective method to evaluate the performance of lithium-ion battery comprehensively.