Analysis Of Lithium-ion Power Battery Capacity Fading Under Fast Charging Conditions And Optimization Of Multi-stage Charging Strategy

As the energy storage carrier of electric vehicle,the fast and efficient charging of lithium-ion power battery is beneficial to alleviate the ’mileage anxiety’ of electric vehicle,so as to promote the rapid development of electric vehicle.However,fast charging will cause excessive temperature rise of power battery and limited charging capacity while shortening charging time.In addition,it will accelerate power battery capacity fading and affect power battery life.Therefore,it is helpful to find effective suppression methods to prolong the service life of the power battery by clarifying the reason of power battery capacity fading under fast charging conditions.At the same time,combining multi-objective optimization of charging strategy is also an effective means to achieve comprehensive improvement of power battery charging time,temperature rise and electric quantity.Eliminating the negative impact of fast charging on power batteries can not only improve the driving experience of users,but also play an important role in promoting the large-scale application of electric vehicles.Firstly,by analyzing the decreasing trend of terminal voltage of LiFePO4(LFP)power battery from 20% state of charge(SOC)with 2 C fast constant current charging to the end of 80% SOC,the time of internal polarization disappearance of the power battery was found.This was used as the basis for dividing the different scales of resting time,and the different scales of resting time were combined with the2 C fast constant current charging strategy to develop a fast-charging condition for the cycle aging test of LFP power battery.Secondly,in order to clarify the factors affecting the aging of LFP power battery with different resting time after fast charging strategy,the paper combines capacity fading analysis,capacity increment analysis and electrochemical impedance spectrum analysis to investigate the aging mechanism of power batteries.The analysis results show that the longest scale of resting time affects the material changes inside the LFP power battery during cycle aging,and the other scales of resting time do not affect the material changes inside the battery.In addition to this,the material changes inside the LFP power battery affect its available capacity,and the resting time scale that can extend the battery life is found.Finally,charging time,charging power and charging temperature rise are used as coordinated optimization objectives.By designing a multi-objective coordinated orthogonal experimental scheme for stage current optimization,a 5-stage constant current charging optimization strategy is proposed after two orthogonal experiments.The proposed charging strategy can not only charge 80% of the available capacity of LFP power battery in a relatively short time,but also ensure that the power battery always maintains a safe charging temperature during the whole charging period.The analysis of the CA tests results shows that the proposed optimization strategy does not accelerate the capacity fading of the LFP power battery compared with the standard charging strategy.