Study Of Bi-Directional Balance System For Lithium Ion Batteries

Under the situation of shortage of mineral resources and serious environmental pollution,all countries are developing the electric vehicle powered by battery to reduce the consumption of non-renewable resources and mitigate the impact of environmental pollution.Battery is the heart of electric vehicle,and determines the safety,power and mileage of electric vehicle.However,battery pack is usually connected in series and parallel,the inconsistency between monomers will lead to a decrease in energy utilization and safety,the battery management system can monitor the state parameters of the battery in real time,and control the battery in a safe,stable and efficient state.Therefore,battery management system is very necessary for battery pack application.The article takes lithium ion battery as the research object,the research revolves around the research of the functions of data acquisition,security protection,balance control and estimation of charge state(SOC)and so on,and designs a battery management system suitable for lithium ion batteries.First of all,the principle and performance of lithium ion battery are introduced and analyzed,and the internal resistance characteristics,discharge characteristics and hysteretic characteristics of lithium ion battery are further studied.In view of the characteristics of lithium ion battery,this article discusses the applicability of the SOC estimation method based on the existing battery equivalent model.In order to describe the working characteristics of lithium ion battery and improve calculation accuracy of the battery SOC,we put forward a correction algorithm considering hysteresis characteristics based on the improved Thevenin equivalent circuit model.We also establish SOC estimation model on the Simulink platform.By comparing the actual data with the simulation results,we find that the improved Thevenin model has high precision,and the new SOC algorithm has a strong correction effect on the hysteresis characteristics.Then we analyzed the reasons for the inconsistency of the battery,and demonstrated the importance of equalization to the battery pack.By studying the characteristics and practicability of the existing different balance methods,we choose the energy storage inductor bi-directional balance which is suitable for the experiment object,and establish the simulation model to analyze the balance process,and verify the function of the balance.Finally,we use the control chip of Freescale 12 XD series to realize the system hardware and software design.Through the EDA software Altium Designer,we completed the design of schematic diagram and PCB diagram.In the Labview environment,a host computer virtual instrument is set up to monitor the signals obtained from the data acquisition circuit.A balanced experiment of a group of Li Fe PO4 batteries has been carried out,the experimental results verify the expected effect of the balance system.