Study On The State-of-Charge Estimation And Management System Design For Li-ion Battery

Battery management system(BMS) is an important part of electric vehicle,which not only could improve the efficiency of the battery but also ensure the safety of the battery according to real-time management.Accurately estimate the state of charge(SOC) is one of the key technology of BMS,high precision of SOC estimation not only conductive to the life but also could improve the energy efficiency and reduce operating costs.After analysising the advantages and disadvantages of various models based on introducing the working theory,we choose the second-order RC equivalent circuit to simulate the external characteristics.According the offline parameter identification method and calling fitting tool of Matlab,we realize the parameter identification of battery model. The second-order RC equivalent circuit builded could simulate battery characteristics well after experimental verification.Introducing the work theory of linear kalman filter and the whole recursive algorithm of estimating SOC.Extended kalman filter(EKF) is the nonlinear part of battery model which could unfold by first-order Taylor to complete linearization to adapt the linear kalman filter.We complete the design of sliding mode observer to estimate the SOC based on the state space equation of the battery model.First of all,judging the system state variables are observable by the rank matrix.Then complete the design of feedback gain parameters and slip gain parameters of sliding mode observer by Lyapunov theorem.Finally,analysis the effects and error of EKF and sliding mode observer by simulation in Matlab/Simulink.Experiments show that the error of sliding mode observer is smaller.Finish the design of hardware circuit and software programs of BMS.BMS consists of battery control unit(BCU) and local electric control unit(LECU) by distributed thoughts. BCU used to deal with the information of LECU and communicate with LECU and external unit module. The function modules of BCU consist of power modules,communication module,total voltage detection module and charging and discharging current detection module.LECU used to collect battery data and complete inter-cell voltage balance which realized by cell voltage collection chip. The function modules of LECU consist of cell voltage collection module and temperature detection module. Fulfill the design of software by the same thougnt of hardware. According to system debugging and experimental verification, the system could work safely and the accuracy of data collection meet the relevant requirements.