Research On Consistency Screening Indexes Of Cells In Lithium-ion Battery Pack

With the popularity of electric vehicles,the number of retired batteries is increasing.Echelon utilization of retired batteries is an important means to reduce the use cost of batteries and extend the life cycle of batteries.Due to the different use scenarios before battery retirement,there are obvious differences in the degree of aging.In addition,in the process of battery production,the production equipment error and production process will also lead to congenital differences in battery performance.The state and parameters of each unit in the decommissioned battery pack are inconsistent,which will lead to the degradation of the performance of the reconstituted battery pack and even the danger.At present,the research on screening methods and screening indicators is still in its infancy,and the application scope of the results is relatively single,which can not meet the growing market demand for consistency screening of retired batteries.In view of the above problems,this paper takes eight retired LiFePO4 batteries on the patrol sightseeing bus as the research object,and studies the monomer consistency screening index of the reconstituted battery pack.Firstly,the main indexes related to the battery performance,the state parameters and internal parameters of the battery are introduced.A convenient and practical method to obtain the state parameters of the battery is proposed,including the measurement method of the battery health state SOH,the estimation method of the power state SOP,and the calculation method of the battery state of charge SOC.Relying on the existing equipment in the laboratory,the battery test platform is built to test the performance of single battery and battery pack,and the equivalent circuit model of the battery is established.The parameters of the battery model are identified through the model reference adaptive algorithm,and the internal parameters of the single battery are obtained.The relationship between the capacity and power characteristics of the reconfigurable battery pack and the state parameters and internal parameters of the cell is derived.It is concluded that the capacity characteristics of the reconfigurable battery pack are limited by the cell with the smallest residual charge and the cell with the smallest spare capacity.The peak power of the battery pack is related to the open circuit voltage and internal resistance of the cell,and is limited by the cell with the smallest peak power.The results are verified by experiments.Furthermore,the influence of different cell parameters on the capacity and power characteristics of the battery pack was studied,and the main index of consistency screening of the reconstituted battery pack was proposed.Because the parameters of the battery can not be completely consistent,in order to improve the performance of the reconfigurable battery pack,according to different use scenarios and requirements,the optimization strategies of the capacity,power and aging rate of the reconfigurable battery pack are proposed.