Research And Design Of Centralized Battery Management System For Electric Vehicles

In recent years,with the promotion of the national strategic plan,the electric vehicle industry has ushered in vigorous development.With the rapid development of electric vehicles,there are some problems in the key part of electric vehicles--power batteries,which also need to be solved.Lithium-ion power batteries used in electric vehicles usually require a large number of cell series-parallel connected to form a battery pack to meet the automotive power requirements.However,due to the difference between the series of cell,and once this difference has been increasing over time,which easily cause battery fault even dangerous,so the battery management system(BMS)has played a crucial role in protecting the battery energy output,working safely and efficiently.Domestic and foreign universities and technology companies have invested in the development of electric vehicle battery management system.Currently,there are some problems in the battery management system on the market.For example,the low sampling accuracy of the battery data leads to a large measurement error,which directly influences the battery's state estimation and management protection;the single SOC(State of Charge)algorithm causes the error is getting more and more bigger after electric vehicle to run for a period of time.These problems have a direct impact on the effective use of power batteries.In order to solve these problems,this thesis researches and designs the battery management system based on the school-enterprise cooperation project for 144 V Li-ion power battery.In the end,it's developed a BMS prototype that meets the project's technical specifications.The specific research content of this paper is as follows:Firstly,this thesis introduces the development status of electric vehicles and BMS at home and abroad,raises several exist key issues currently.Then,starting from the working characteristics of lithium-ion batteries,the equivalent circuit model of lithiumion batteries was studied,the existing SOC estimation algorithms and equalization control schemes were analyzed,and designs a SOC estimation combination algorithm,which uses the ampere hour integral method and open-circuit voltage method then corrected by the Kalman filter,and puts forward the passive equilibrium control scheme with cell voltage as the equalization variable.Finally,the current hardware and software solutions of the battery management system are analyzed.The experience is summarized from the existing solutions.A new software and hardware solution is designed based on the project requirements and engineering practice requirements.The hardware platform of this project is a centralized hardware topology,and NXP's MC9S12XET256 is selected as the main controller.The specific analysis and design of each function module circuit of the battery management system hardware includes the micro control unit(MCU)minimum system,power supply module,detection module,CAN(Controller Area Network)communication module,RTC(Real-time clock)module,data storage module,relay control,and equalization control circuit.The software system chooses to the current popular real-time operating system—FreeRTOS.It first elaborates the design of the underlying driver program,then introduces the FreeRTOS operating system in the HCS12 X hardware platform.In the end,by exploiting the operating system's thread scheduling mechanism,the BMS function is subdivided into a number of threads(tasks),the corresponding program design for each subtask,and each task is independent Work,in addition to the need for synchronous communication between tasks,the rest do not interfere with each other,thus ensuring the stability of the overall system and real-time.Finally,in order to verify the function and reliability of the BMS prototype,a bench test experiment,battery pack charge and discharge cycle test experiment was designed in this paper.The experimental results show that the BMS prototype voltage and current temperature detection function is normal,the measurement error is within allowable range,and the SOC estimation error Within 5%,below 8% of the specifications,communication functions and management control functions are normal,and the comprehensive test results basically meet the technical specifications.