Charging Characteristics And Basci Control Problems Of Lithium-ion Batteries For Electric Vehicle

Li-ion battery has become a perfect power source of the electric vehicles for its high specific energy and specific power. However, the complexity of its own characteristics and the sensitivity of the abuse power restrict the power battery performance cannot be used reasonably. In this dissertation, the following key issues are studied aiming at the typical commercial Li-ion power battery in the market, according to a large number of experimental data, the working characteristics of power battery are researched based on the Macro-Micro-Macro analysis route in order to improve the reliability of power battery system and to promote the development of electric vehicle industrialization. The specific contents and conclusions are as followings:(1) Based on the experimental results of the charge and discharge of power battery under the different stress, the change law of polarization voltage is obtained, which indicates that the accumulation of polarization voltage is obvious at low temperature and high ratio. With the change of temperature, the composition and the value of the polarization voltage are different obviously. The completely different voltage changes are obtained by using the charge mode test at different frequency, which is found that the working voltage is gentle, the temperature rise slowly and the battery can accept the high charge and discharge current when the current frequency is located in the low impedance area.(2)By analyzing the variation of the specific heat capacity of the power battery with the temperature, the internal heat generation rate can be obtained. Combined with the change of the dQ/dV curve, results indicate that the temperature rise rate is consistent with the dQ/dV curve at the different SOC. The thermal-electric model is established to expound the process of Li-ion intercalation and removal in different modes from the microscopic point of view, which could analysis emphatically the change of concentration gradient and internal temperature field to explain the principle of the charge and discharge characteristics in macro aspects. Experimental results show that the pulse mode is to change the dispersion of liquid phase ion concentration and has little effect on the solid-phase concentration gradient; the forced convection can reduce the battery temperature rise, but the temperature difference between inside and outside of the battery is increased, and the aging degree of the internal and external active materials is enlarged.(3) A three-electrode charge and discharge test model of power battery is established. The electrode potential and working voltage in the different charging current are analyzed deeply and indicate that the rising rate of voltage is an effective parameter to determine the charge cutoff condition. Based on the theory of the cell, it analysises the characteristics of different connection modes for power battery system, and the experimental data shown that when the charge control parameters of the series mode are the same as that of the single control mode, it can avoid the over charge phenomenon of the cell in the system effectively; The control mode of the paralles system mainly based on the temperature gradient of cell in the system and can effectively avoid the high rate loading phenomenon at the end of the charging, which can prolong the service life and improve the security of the system.(4) Based on the voltage and current variation of power battery pack, it summaries the current cell state and adopts a filtering method using discrete cosine filter method to analyze the characteristics of voltage data collected, and which reduce the noise interference and keep the real situation of the voltage fluctuation. Then the voltage data under different vibration frequency can be gained. Experimental data processed by filtering is analyzed based on local shannon entropy, ensemble shannon entropy and sample entropy to obtain the optimal range of application, and through experimental comparison, it is found that the ensemble Shannon entropy can timely and accurately predict the location of the fault and has strong anti-interference ability to improve the safety and reliability for the power battery system.