Effect Of Lithium-ion Battery Parameters Variation On Performance Aging Of Battery Pack

Lithium ion batteries are widely used in electrochemical energy storage systems such as electric vehicles.Due to the limited capacity and voltage,battery cells must be used in series and parallel to form a battery pack.Due to the variation of production,sorting,grouping and use,there is an irresistible variation among the cells in the battery pack,which leads to the loss of battery pack capacity and the acceleration of cell aging,and even dangerous working conditions and safety accidents.Lithium ion battery pack is a complex electrochemical system,its variation degradation is a complex and slow process,and the influence mechanism in series and parallel battery packs is different,and the capacity degradation and aging acceleration are coupled with each other,which makes the research of battery pack inconsistency more complex.Starting from the qualitative analysis of performance degradation mechanism,this paper explores the battery parameters and influence mechanism of performance degradation caused by series and parallel batteries,and gives the quantitative experimental results.Firstly,based on the Capacity-SOC diagram,the performance degradation in the soting and use process is analyzed qualitatively.It is considered that in the series battery pack,the variation of initial parameters leads to the difference of discharge rate and cycle depth,which leads to the increase of capacity loss and cell aging difference.In the parallel battery pack,the variation of initial parameters leads to the uneven current distribution,which leads to the aging difference increases and the safety decrease;Secondly,the initial parameters variation of 200 lithium iron phosphate cylindrical batteries of the same batch were measured,and the initial distribution characteristics and correlations of several battery parameters including capacity,ohmic internal resistance and self-discharge rate were obtained,which provided data basis for inconsistency simulation and experimental research;Thirdly,based on Thevenin model and Bernardi model,a comprehensive battery pack model is established to describe the electrical,thermal and aging behavior of battery pack with arbitrary topology.The influence of SOC-OCV curve on the uneven current distribution of parallel battery pack is analyzed,and a two-stage linear fitting method is proposed to obtain smooth and accurate SOC-OCV curve.The effectiveness of the model is verified by simulation and experiment.The analysis shows that the change of polarization resistance is the main reason for the error;Finally,based on the above experimental data and battery PACK model,the aging simulation of series and parallel batteries is carried out,and the influence of operating conditions and initial variation of parameters on the performance variation of battery is further investigated,and the quantitative experimental results are obtained.The results show that the variation of capacity,self-discharge rate and initial SOC leads to the increase of capacity loss and aging difference in series battery pack;the variation of capacity and internal resistance leads to the increase of aging difference in parallel battery pack.Furthermore,the effect of variation of capacity and internal resistance on the increase of aging difference in parallel battery pack is verified by accelerated aging experiment.