Research On Theory And Key Methods Of Detecting Power Battery Pack Consistency Based On Magnetic Field Sensing

The performance consistency of power lithium-ion battery packs is one of the key factors influencing operation safety and reliability of new energy electric vehicles and electrochemical energy storage power stations.However,under the complex operating conditions and nonlinear battery degradation mechanisms,it is difficult for existing technical means to ensure the performance consistency of battery pack during its entire service lifetime.Therefore,it is necessary to accurately evaluate the performance consistency between cells within the pack using non-destructive testing,from which appropriate maintenance strategies can be taken to ensure safe operation of battery packs,but evaluation of battery performance consistency at cell level is still at its infancy.To solve the above issue,this thesis investigates the generation mechanism and influencing factors of induced magnetic field of the battery pack during charging and discharging process,and reveals the correlation between capacity consistency of the pack and its external magnetic field.From the findings,an in-situ non-destructive evaluation method based on magnetic field sensing is proposed,which derives the small imbalance current within the pack by measuring induced magnetic field outside the pack,then realizes capacity consistency evaluation of the pack.For this purpose,this thesis mainly focuses on the battery pack magnetic field detection theory and the corresponding magnetic field evaluation method:1.The induced magnetic field generation mechanism of lithium-ion battery is analyzed using the battery electrochemical reaction principle and electromagnetism theory.A test system for measuring battery induced magnetic field is designed,and signal acquiring and processing method for battery induced magnetic field are established.A battery simulation model is built based on the induced magnetic field generation principle of lithium-ion battery,then the model effectiveness is verified with experimental results.The effect of battery magnetic susceptibility variation on analyzing current-induced magnetic field of the pack during charging and discharging is analyzed.Based on simulated and experimental results,a magnetic field measuring strategy using magnetic field component By for the pack is proposed,which can highlight the current-induced magnetic field variation and minimize the effect of susceptibility variation.The results can be used as theoretical foundation for the subsequent construction of battery pack magnetic field evaluation methods.2.A non-equilibrium current change analysis method for the packs based on two-dimensional magnetic field images is proposed to evaluate the battery pack capacity consistency.The effectiveness of the proposed method is verified by simulated and experimental data.Firstly,an evaluation method based on magnetic field image features is proposed,magnetic field image feature extraction and principal component analysis are used to achieve qualitative evaluation of battery pack capacity consistency and locate low-capacity batteries.In addition,a current inversion calculation method based on the superposition principle of the battery pack current-induced magnetic field is proposed.Battery pack current inversion calculation model is constructed based on the magnetic field distribution function of a single cell,the current value of each battery is then solved by particle swarm optimization algorithm to achieve quantitative detect current change in the pack,which can be used to evaluate capacity consistency of the pack.Based on the experimental analysis results,the solved current deviation can be kept within 5 mA,which is better than the existing battery pack current detection methods.3.An evaluation method of battery pack capacity consistency based on one-dimensional magnetic field scanning measurement is developed to realize fast and accurate detection of battery pack performance consistency.Firstly,the relationship between battery capacity and steady-state current distribution is established through equivalent circuit model to study the rapid detection principle of battery pack capacity consistency.A one-dimensional magnetic field scanning measurement scheme of the pack is then proposed by simulated calculation and experimental measurement results.Subsequently,a battery pack capacity consistency evaluation method based on one-dimensional magnetic field data feature extraction and k-nearest neighbors algorithm is proposed,and detection deviation of the method is verified to be within 4.99%by experimental data.Finally,the factors affecting magnetic field distribution characteristics and the practicality of the method are discussed through a multi-cell simulation model.This thesis adopts a research scheme combining simulation calculation and experimental validation to study the magnetic field detection theory and key evaluation methods for power lithium-ion battery packs,the results can provide a new technical means for in-situ non-destructive testing of battery pack performance consistency and lay a theoretical and methodological foundation for practical application of magnetic field sensing methods in battery pack detection.