Research On Battery Management System For Electric Vehicles

Due to the increased environmental pollution and energy shortages in recent years,electric vehicles which are free of pollution and consume less energy have developed rapidly.The key to the development of electric vehicles is battery and battery management systems(BMS).The maturity of battery management technology directly relates to the safety and rational use of batteries,and the merits of electric vehicles performance as well.The battery management system for electric vehicles has been designed by this thesis,and a distributed structure with a main control board and multiple unit boards has been adopted,then the hardware and software for the main control board and unit board have been designed.The main control board is responsible for managing the overall work of the battery pack,including total voltage and current acquisition,the control of charge and discharge,the control of temperature,and information exchange with other systems of the electric vehicle.The unit board is responsible for managing each battery,including the acquisition of single battery voltage,battery box temperature and power balance implementation.The state of charge(SOC)and charge balance strategy of the battery pack has been focused on research by this thesis,and some improvements has been proposed based on previous achievements.The battery SOC is an important parameter for the electric vehicle.In order to estimate the battery SOC more accurately,a second-order RC equivalent circuit has been used to reflect the internal working state of the battery,and a recursive least square method has been used to identify the model parameters online,and the improved extended kalman filter algorithm has been used to estimate the battery SOC on the basis.In this improved algorithm,the ampere-hour integration method has been used to establish the state equation of the SOC,the EMF method has been used to calculate the initial value of the SOC,and extended kalman filter algorithm has been used to perform recursive calculations,then the charge of the battery has been obtained.In addition,the accuracy of the algorithm has been verified by MATLAB simulation.The charge balance management can control the inconsistency effectively in the charging process and reduce the harm caused by the inconsistency.In order to complete the charge balance more quickly and accurately,a DC-DC power supply charging balance strategy has been proposed by this thesis,and power balance has been achieved by transferring thebattery pack’s power to the lowest voltage battery.At the end of this thesis,some functions of the battery management system have been verified by experiment,and a battery management system unit board and four 18650 lithium iron phosphate batteries have been used to simulate the test of single-cell voltage acquisition accuracy,the test of battery pack voltage acquisition accuracy,the test of temperature acquisition accuracy,and the test of battery charge balance.Both the acquisition circuit and the charge balance strategy have been verified by experiment.