Research On Key Technologies For The Management Of Hev Lithium-Ion Battery Group

Along with rapid development of automobile industry, the energy problem and the environment problem are more prominent. All countries pay attention closely to them, and take positively the measure for those problems. The electric automobiles have been researched for reducing the pollution emissioned by fuel oil automobile and the consumption of energy. SOC of batteries used in HEV is the important data of battery management system.SOC requires to be estimated real-time, on-line, accurately. This is the premise of energy control strategy of HEV too, the guarantee which prevents batteries damage from overcharging or over-discharging. Battery Group of HEV has its own characteristics when it works:large interval of the temperature, the interval of temperature can reach 60 degrees Celsius; charge and discharge frequently and the large variation of current. However, many estimation algorithms of SOC which people often used in HEV don't take temperature factor into account or accept it as a constant. The relationship between SOC and the terminal voltage of batteries is non-linear, but many algorithms deal with them directly as a linear relationship, and result in large estimation error of SOC. In addition, the problem of energy storing device becomes the bottleneck in the development of HEV. Energy storing device equipped on HEV generally uses series battery stack. When batteries are connected in series, its performance and life span are not a circumstance to single cell. Studies have shown that it's primarily caused by imbalance between cells in series battery stack. Compared to the research of battery materials, mathematical models, charging methods, the balance between batteries is a new research field and has a bright future in application. The imbalance between cells can't be eliminated completely because of the outwardness of it. It will be intensified during the course of being used, so the control of it becomes a dynamic process.In view of the above situation, regarding several key technologies of HEV lithium ion battery management system, the main work of this dissertation includes:First, this dissertation introduces the development trend of the vehicle, key technologies of HEV and the working principle of lithium ion battery, analyses completely the affecting factors of SOC, studies on battery model in common use, sets up improved EMF battery model based on traditional EMF battery model, which takes into account the impact of temperature to the SOC and puts forward the way of parameter recognizing of improved EMF battery model.Second, this dissertation analyses the estimate algorithm of SOC existing, studies on the estimate algorithm of SOC based on EKF in order to fit the non-linear system, puts forward a kind of hybrid estimate algorithm of SOC based on EKF-AH, and verifies its effect with experiments. The results of experiment have indicated that estimate algorithm of SOC based on EKF-AH may meet the accuracy requirement of HEV, the temperature adaptability of improved battery model is obviously better than the model which has not been modified.Third, the dissertation also discusses the reasons for the extension of equalization and several existing balanced approach, puts forward the multiple winding magnetic model, and has improved the balancing method of the traditional DC-DC isolation. Against the application occasions of large number cells and high charge and discharge current on electric vehicles, according to the multiple winding magnetic balancing method and the improved DC-DC balancing method, the thesis proposes one kind of second-level balancing method, describes the detail design process, including the hardware module design principle, the circuit schematic diagram and the main modules of the detailed process of implementation. Finally, gives the experimental verification for the second-level balancing method which has proposed, the experimental result shows the equalizing charge which replace graduation balanced system by the improved DC-DC system is effective and the system has its superiority.Forth, at present, the battery management system takes insufficiently the battery energy management into account, or lacks the energy management strategy, thus causes drop of battery's state of health, affects the using and the service life of battery's energy. In view of this problem, the dissertation proposes the strategy based on multiple grouping of batteries power energy management and the strategy based on backup power energy management, analysises the applicability of the strategy based on backup power energy management, it mainly includes the principle of the battery groups, the control method of power feedback and the method of the charging and discharging control, and so on. The date of experiment indicates that those BMS which adopts energy management are better than those systems which have not adopted the energy management.Fifth, this dissertation proposes a functional frame of HEV lithium ion battery group management system and designs hardware and software of the system. The system uses the MCU as the core controller of every functional model. The system possesses functions of estimating battery groups' SOC, controlling and balancing charging and discharging of battery group, energy management. It will lead us use the battery group more properly and enhance the lifetime effectively.The main work of this dissertation has probably successful experiences of those who engage research of EV or HEV and especially those who engage research of HEV lithium ion battery group management to consult.