Research On Estimation Method Of SOH And Reliability Model And Equalization Control Of Battery Module For Lithium-ion Power Battery Of Vehicles

To ensure the stable,efficient and safe operation of new energy vehicles,it is necessary to conduct management and control of power batteries.State of health(SOH)estimation is one of the key technologies of the Battery Management System(BMS).SOH refers to the deviation degree between the current performance and the original performance of batteries.The quality of its evaluation method largely determines the performance of the BMS.The accurate estimation of the battery SOH is of great significance to reduce battery failures,reduce maintenance costs,extend service life and ensure the normal operation of new energy vehicles.At present,battery SOH estimation methods mainly include durability empirical model method,electrochemical impedance spectroscopy method and model-based parameter identification method.Although these methods can complete the estimation of the battery SOH,they also have many shortcomings,such as the low estimation accuracy and poor generalization ability of the durability empirical model method,the electrochemical impedance spectroscopy(EIS)method is mostly limited to laboratory research and difficult to apply in engineering,and the mechanism model parameter identification method is complex and computationally expensive.For this reason,this paper takes the ternary lithiumion battery(LiNii-x-yCoxMnyO2)as the research object,and conducts in-depth research from the following aspects:estimation of the SOH of lithium-ion battery cells,reliability model of battery modules in series,research on isoelectric balance control strategy and identification of failed cells in modules.The main contents are as follows:(1)A power battery test platform is built and battery performance test is conducted,and power battery performance analysis and evaluation research are carried out.The effects of ambient temperature and discharge rate on the discharge capacity of the battery are analyzed.The results show that the lower the ambient temperature,the greater the discharge rate,and the less the battery discharge.The relationship between the DC internal resistance of the battery and the state of charge(SOC)is analyzed.The results show that when SOC ≤20%,the change of the DC internal resistance during charging and discharging is sharp,and when 20%≤SOC ≤100%,the change of the DC internal resistance during charging and discharging is relatively flat;Finally,the influence of each factor on battery SOH is analyzed from three dimensions:discharge rate,depth of discharge(DOD)and ambient temperature.The results show that the greater the discharge rate,the deeper the discharge depth DOD,and the lower the ambient temperature,the faster the battery SOH decay rate.(2)An estimation method of the SOH based on the characteristic points of incremental capacity curve(IC)is proposed.Firstly,the change rule of characteristic point position parameters on IC curve closely related to battery aging with cycle life is analyzed.Secondly,to solve the problem that the noise in the traditional numerical analysis method brings difficulties to the identification of characteristic points,Savitzky Golay filter is used to smooth the IC curve.Finally,the change rule of each characteristic point position on the IC curve with the cycle life is discussed.Principal Component Analysis(PCA)is used to reduce the dimension of the coordinate data of the characteristic point position on the IC curve,and a regression model for battery capacity estimation is established.The estimation error is 0.63AH(3.15%).(3)A reliability model of series battery modules based on Weibull Distribution is proposed.Based on the characteristics of series battery modules(module capacity meets the principle of "cask effect"),two parameter Weibull distribution is selected as the failure data distribution model.The reliability function line of Weibull distribution is linearized,and the unknown parameters in the linear equation are determined by the least square method based on the failure data β、η identification.The correlation analysis of variables X and Y in the linear equation is carried out,and the value of correlation coefficient is ρX,Y=0.9725,indicating that variables X and Y are significantly correlated.The selected model is tested by Kolmogorov Smirnov test(K-S test).The maximum value of K-S test statistic D is Dmax=0.0371,far less than D obtained from K-S D critical value table DC=0.301,indicating that it is feasible to use Weibull distribution model to study the reliability of series battery modules.(4)The fault characteristics of series battery modules and the identification method of failed cells during charging are proposed.The equivalent circuit model with the connection resistance Rc is established,and the analytical equation of the monomer terminal voltage UN is obtained.The trend of the charging voltage curve is affected by three main factors which are the initial SOC of the battery,the battery capacity Q,and the battery internal resistance Rint.In the charging voltage data,take the initial open circuit voltage Uocv,the difference between the average voltage and the initial voltage U-UOCV,and voltage range UN-Uocv three parameters of OCV,and analyze the relationship between these three parameters and initial SOC,internal resistance Rint and the maximum available capacity Q.U-UOCV is used to determine the maximum value of Rint in the series module.Voltage range UN-UOCV is used to determine the minimum capacity Q in the module,and the conclusion is consistent with the test result.(5)An equalization control method based on isoelectric series battery modules is proposed.The principle of the isoelectric balance control method is analyzed by using the electric quantity scatter diagram.The residual charge and discharge quantity of each cell in the series module tends to be the same through charge transfer during the battery charging and discharging stages.The SOC of each cell in the module is obtained by looking up the OCV table,and the maximum usable capacity Q is calculated by SOC1,SOC2 and charge quantity Qc.The new battery capacity variation coefficient VS is selected as the balance control parameter,and the feasibility of this method is verified by designing experiments to simulate the isoelectric balance control process.Compared with the unbalanced series battery module,the module capacity is increased by 12.9%after equalization based on this method,effectively extending the service life of the series battery module.To sum up,SOH estimation method of single battery based on IC curve characteristic points and reliability model of series battery modules based on Weibull distribution are proposed,and an isoelectric series battery module equalization control method and charge process oriented series battery module failure single identification according to these two estimation methods,which provides research method ideas and technical support for Electric Vehicle power battery SOH estimation and fault diagnosis.