Research On Factors Affecting The Performance Decrease Of Power Battery Under Overcharge Condition

The development of electric technology has made electric vehicles widely concerned by the government and industry.However,due to the unsatisfactory consistency between the batteries and the errors in the state estimation of BMS,in the field of automotive power batteries,lithium-ion batteries may experience slight overcharging.Overcharging can cause problems such as battery performance degradation and battery stability degradation.Severe overcharging may even lead to dangerous accidents such as battery expansion and explosion.Therefore,it is necessary to understand the influencing factors of overcharging and their influencing mechanisms,so as to provide a basis for battery overcharging judgment for BMS and provide a reference for estimation of battery SOH.In this paper,lithium iron phosphate battery is taken as the research object.First,single-overcharge experiment of the battery is conducted by setting different overcharge paths and overcharge cut-off SOC to determine the influence of different factors on battery performance attenuation.By comparing the reference performance tests results before and after single-overcharge,the battery’s changes in cycle performance,internal resistance characteristics,and power output characteristics are obtained.In the experiment,it was found that the battery had a series of recovery phenomena after single-overcharge,including capacity recovery and battery internal resistance recovery.By comparing the changing trends of various parameters,the relationship between these recovery phenomena and the causes are initially determined.At the same time,an overcharge detection method based on internal resistance test is proposed which can be used to determine whether the battery has been overcharged by combining the performance of the CV segment of the battery.Secondly,the result of single-overcharge experiment is used to determine the slight-overcharge boundary of the battery.Taking five slight-overcharge cycles as a period,a slight-overcharge cycle experiment was performed on the battery.As a result,it was found that the slight-overcharge cycle caused the performance of the battery to decay significantly.Moreover,the capacity decay will also appear the capacity"diving" phenomenon,which usually occurs at the end of life under normal cycling conditions,and the performance of the battery has obvious stage characteristics.Fitting the battery capacity decay data,a semi-empirical capacity decay model of the battery under slight-overcharge conditions is obtained.Finally,the wavelet denoising were used for data processing to obtain the battery capacity increase curve.Analyze through the obtained capacity increment curve,it is found that the series of recovery phenomena after the battery undergoes single-overcharge is mainly caused by the recovery of lithium deposition.The performance degradation of the battery caused by the slight-overcharge cycle can be divided into two stages.The first stage is mainly the loss of active materials and the growth of the SEI film.The second stage is dominated by the loss of reversible lithium.The work in this paper supplements the research on the aging mechanism of lithium-ion batteries under different overcharge conditions,and provides suggestions and references for BMS overcharge judgment and SOH estimation.