Research On Lithium Battery Balancing Strategy Based On Multiple Indicators

Due to its advantages of energy saving and environmental friendliness,the development of electric vehicles has become an important way for countries to solve energy and environmental problems in the field of transportation.Lithium-ion power batteries are widely used in pure electric vehicles because of their good performance in terms of self-discharge rate,energy density,service life and safety.As the working voltage platform of pure electric vehicles is getting higher and higher,the single lithium-ion battery with a voltage of about 4V cannot meet the needs of electric vehicles.Generally,a large number of single cells are designed in series and parallel to provide power for electric vehicles.When single cells are used in groups,the production process will lead to initial inconsistency.The change of start,acceleration and deceleration of electric vehicles leads to complicated discharge conditions of power battery packs,which will cause temperature differences in the battery packs.And exacerbate the inconsistency between single cells.The increased inconsistency between single cells will reduce the actual capacity and safety performance of the battery pack,which in turn affects the mileage of electric vehicles.In order to solve this problem,improve the overall performance of electric vehicles and increase market competitiveness.Based on the Panasonic 18650 PF lithium battery,this paper models and analyzes the power battery pack system,and studies the equalization technology.The main contents are as follows:(1)Battery pack inconsistency analysis and single cell modelingThe manifestations and causes of the inconsistency of the power battery pack are analyzed,and the characteristic curve of the battery in the working process is drawn according to the HPPC experimental data.The OCV characteristics of the battery are experimentally analyzed,and a second-order RC equivalent circuit model that meets the requirements of complexity and accuracy is established.The internal parameters of the battery are accurately identified offline,and a simulation model based on Matlab/Simulink is built.The self-defined dynamic conditions are compared with simulation and experiments to verify the accuracy of the model.(2)Estimation of battery pack SOC and open circuit voltageSince the used open circuit voltage and battery state of charge SOC(State of Charge,SOC)input quantities cannot be directly measured,online estimation is required.Therefore,this paper estimates the open circuit voltage of the battery based on the load voltage method based on the current correction coefficient,and uses the extended Kalman filter(EKF)based on time-varying parameters to estimate the SOC of the lithium-ion battery,which overcomes the cumulative error of the traditional estimation method and the difficulty in online use.The online estimation model is established in Simulink,and the estimation with high precision is completed,which provides an effective evaluation index for the balanced system.(3)Equalization circuit designAn active equalization circuit based on Buck-Boost power inductor is designed as an energy transfer path between single cells.Optimize the design of circuit parameters,determine the control method of the working current and control the balance current within a safe and efficient range.Combined with the actual hardware working characteristics,the inductor is demagnetized to improve the working reliability.(4)Design of balanced control systemUsing the algorithm based on fuzzy control,the optimal value of the battery SOC and the open circuit voltage in the statistical sense is the control signal of the balance control system,so as to adjust the size of the balance current.And compare the equilibrium efficiency with the traditional control method.It shows that the active equalization circuit and fuzzy control strategy based on Buck-Boost power inductor proposed in this paper can effectively improve the equalization efficiency,reduce the unbalanced degree of the battery pack,reduce the loss of the battery pack,and prolong the battery life.