Control-Oriented Modification Method Of Electrochemical Model And System Balancing Technique For Vehicular Li-Ion Power Battery

Li-ion power battery has high energy density,high power density and other characteristics,widely used in pure electric drive or hybrid electric vehicles.Li-ion power batterys as the energy source of new energy vehicles are made up of thousands of single or modular cells,which pose challenges to efficient,safe and reliable operation in complex on-board environments,so Battery Management Systems(BMSs)are critical.Building an accurate li-ion battery model suitable for control is crucial.Compared with the equivalent circuit model,the electrochemical model can better reflect the internal state of the battery from the perspective of mechanism.However,such traditional P2 D electrochemical model has the problem of reduced accuracy under the condition of high rate discharge.Therefore,based on the average electrode model of li-ion power battery,the key parameter analysis of the model,SOC estimation based on the variable parameter model and the strategy and design of the equilibrium system are carried out in this paper.Firstly,based on analyzing the structure and working principle of li-ion power battery,the reaction mechanism and li-ion diffusion process inside the battery are completely described by relevant electrochemistry and physical chemistry theories.Then,according to the idea of average electrode,the average electrode model is established.Moreover,the control-oriented electrochemical model is preliminarily completed by reducing the order of the most complex and critical solid phase li-ion diffusion equation.However,the experiment shows that the model error of the average electrode model increases under the condition of high rate discharge,which is not suitable for the condition monitoring of li-ion battery under complex conditions.Therefore,the model needs to be modified before it can be applied in the real-time control system.Secondly,aim to improve the accuracy of P2 D model under the condition of high discharge rate,a modified method based on the average electrode model is proposed.The effects of sensitivity parameters-solid phase diffusion coefficient and particle size on the average electrode model are analyzed.Based on the Ratio of Potentio-charge capacity to Galvano-charge capacity(RPG),the coupling relationship between current and solid phase diffusion coefficient is established.The particle size distribution characteristics of electrode particles of li-ion power battery are obtained by charging and discharging tests and concluded as the weight coefficients according to the maximum particle size,medium particle size and minimum particle size.Then,a three-particle electrochemical model with variable solid phase diffusion coefficient is given.Meanwhile,the multi-power discharge test for single battery and the NEDC cycle test platform for battery pack are carried out.Compared with the traditional P2 D electrochemical model,the accuracy of the proposed variable parameter model is increased by 80%,the error of average output voltage is no more than 0.02 V,and the maximum deviation is about 0.05 V.The experiment verifies the accuracy of the variable parameter model.Thirdly,considering the advantages and disadvantages of the existing equilibrium variables,the SOC value of the battery is selected as the equilibrium variable,and a method for estimating SOC value of battery is proposed based on the variable-parameter electrochemical model and the Kalman filter algorithm.Then the accuracy of the estimation method is verified by the pulse discharge experiment of single battery.The experimental results show that the overall error is less than 4%.At the same time,analyzing the advantages and disadvantages of the existing equalization methods,a new equalization circuit is designed,which takes capacitance as energy storage element and combines with inductance to filter the peak current in capacitance circuit.According to the designed equalization circuit,put forward an active balancing control strategy,which takes the vehicle-mounted 12 V power supply as the equalization energy source and supplements the energy of each single battery with the lowest power.In order to verify the feasibility of the proposed equalization circuit and equalization strategy,a li-ion equalization control simulating model is built on the Matlab/Simulink platform.The simulating results show that the proposed equalization control system can rapidly improve the consistency of the battery pack.Finally,in order to check the actual operation effect of the proposed balance control system,the testing bench for battery pack active balancing system is built.The electrochemical modified model and equilibrium control strategy proposed in this paper was transplanted and tested.The final test results show that the designed balance control system can effectively improve the consistency of the battery pack,and the energy transfer efficiency can reach about 85% in the balance process,which verifies the validity of li-ion power battery balancing system proposed in this paper.