Research On The State Evaluation Technology For Second Use Lithium-ion Battery Packs In The Whole Life Cycle

With the rapid growth of the number of electric vehicles(EVs),the problem of the way to deal with the batteries when they retire from EVs has attracted more and more attention.When the power lithium-ion batteries retire from EVs,they still possess 70%-80% of their nominal capacity and be used in the applications with lower performance requirements.The second use of lithium-ion batteries can not only reduce the pressure of battery recycling,but also can play out the residual value,extend the service life of batteries,reduce the cost of batteries in the whole life cycle,and contribute to the promotion and popularization of EVs.The consistency of second use batteries is worse than that of new batteries,and the inconsistency among batteries brings a great challenge to the state evaluation of battery packs.If the state of the battery pack cannot be accurately evaluated,the battery pack cannot be utilized to the maximum extent,which will further reduce the economic benefits of the battery pack and make the battery second use no longer have economic advantages.Aiming at the problem of battery pack state evaluation,the research work in this paper focuses on the state evaluation of second use lithium-ion battery packs in the whole life cycle including the battery screening process and the following second use process.The detailed research content are as follow:(1)Aiming at the problems of long battery test time and high cost in the process of battery screening,the consistency evaluation and screening methods are studied in this dissertation.Based on the test results of a retired battery pack and battery cells,the distribution characteristics of battery cell parameters are analyzed,and the contribution of battery cell aging and battery inconsistency to the aging of battery pack is analyzed.By adopting incremental capacity analysis,the aging variations of battery cells are studied.Based on the analysis results of battery aging variations,a rapid screening strategy for second use batteries is proposed by replacing battery capacity with characteristic parameters of incremental capacity curves,which shortens the test time compared with traditional screening strategy.(2)The inconsistency of battery cells will cause inaccurate evaluation of battery cell state of health(SOH)in the second use applications.Therefore,this dissertation conducts research on the batter cell SOH evaluation.Three types of loading profiles are designed and cycle life tests of second use batteries are conducted.By adopting incremental capacity analysis,the aging mechanisms of batteries as well as the aging variations of battery cell under different load profiles are analyzed.Based on the evolution of the feature parameters on the incremental capacity curves,three battery cell capacity estimation methods are developed.The maximum error,mean absolute error and root mean square error are used to compare the accuracy of the three methods,and then the application condition of the three methods are analyzed.Then the difficulties of capacity estimation of series connected batteries are analyzed.On the basis of summarizing the evolution of battery incremental capacity curve,a capacity estimation method suitable for series connected battery cells is proposed,which can accurately estimate the SOH of battery cells.(3)To improve the estimation accuracy of battery pack performance,the research on the battery pack consistency modeling and energy utilization efficiency estimation are studied in the dissertation.According to the characteristics of battery consistency parameters,Copula function is adopted to depict the coupling mechanisms of the parameters.Then the Copula based battery pack modeling method is put forward,which provides a model basis for subsequent battery pack performance evaluation.The proposed Copula based battery pack modeling method is then compared with the two existing modeling methods,and the superiority of the proposed method is illustrated from the aspects of depicting the statistical characteristics of parameters as well as depicting the correlation coefficient among parameters.The energy utilization efficiency is used as the index to evaluate the performance of the battery pack.Based on the battery pack consistency model,the framework of battery pack energy utilization efficiency is developed,realizing the accurate modeling of battery pack consistency and the evaluating of energy utilization efficiency of battery packs.(4)The aging of the battery pack is affected by the aging of battery cells and deterioration of the consistency,which leads to the inaccurate estimation of the battery pack SOH estimation,and thus cannot provide effective guidance for the optimized use of the battery pack.Aiming at the problem that the aging of battery pack is affected by the aging of battery cells and the deterioration of the consistency,which leads to the inaccurate estimation of the battery pack health state and the inability to provide effective guidance for the optimized use of battery pack,the research on the prediction of the evolution of battery pack SOH is carried out in this dissertation.A method to predict the evolution of battery cell parameters based on Gaussian process regression is firstly proposed.Then the definition of battery SOH is put forward,realizing the quantitative evaluation of battery aging and consistency deterioration.Finally the method to predict the evolution of battery pack SOH is put forward,which includes the battery cell parameter estimation,the prediction of the evolution of battery cell parameters,and the evaluation of battery pack SOH.The battery pack cycle life test is designed and conducted to verify the accuracy of the proposed method.The accurate prediction of the evolution of battery pack SOH could provide effective guidance for the battery pack balancing and maintenance,as well as realize the maximum utilization of the battery pack energy.The proposed battery screening strategy,battery cell SOH estimation method,battery pack consistency modeling and energy utilization efficiency estimation method,and the method to predict the evolution of battery pack SOH could help solve the key technical problems of state evaluation of the second use battery packs in the whole life cycle,as well as increase the economic benefits of second use batteries,which is of great values in the engineering field and future popularization.