Thermal-Electrochemical Modeling Of Li-ion Battery For Electric Vehicle Application

Lithium ion battery is a promising electrical power source for the new energy vehicle in the near future. The electrochemical properties of the lithium ion battery must be well acknowledged in order to control the battery to output moderated power at pratical vehicle condition. However, just conducting calibration test to acquire the know-how of the electrochemical properties is inefficient. An electrochemical model must be built to improve the efficiency to simulate the electrochemical properties at variant conditions. Simultaneously, the lithium ion battery releases heat during cycling. A thermal management system is essential to dissipate the heat generated during cycling to the environment. Similarly, the design of a thermal management system again needs a thermal model rather than massive experimental calibrations. The electrochemical process and thermal process are always coupled together. Therefore there is a need to build an electrochemical-thermal coupled model to guide the design of the battery management system.8Ah lithium ion battery with Alumium plastic package was selected to be tested under different temperature and current rates for the sake of foundation of the electrochemical-thermal model. The experimental data were analyzed to acquire the electrochemical characteristics and the temperature distribution of the battery cell. Additionaly, frequently varing load was applied on the battery cell to test the instantaneous properties of the lithium ion battery.Utilizing the well-known pseudo-two-dimensional model, an electrochemical model was builted based on the experimental data including variant temperature and current rates. The simulation results of the electrochemical model can fit the experimental data well at different kinds of working conditions, the errors of discharge capacity and voltage are both within 3%. The model can get good accuracy, with a voltage deviation of 1%, when applying a time-variant load, which leads to a confirmation that the model has high fidelity to simulate the electrochemical behavior of the lithium ion battery.Modeling analysis was performed on 12 variable critical parameters that may influence the electrochemical properties of the model. According to the modeling analysis, the length of the electrodes, the maximum intercalacable lithium, the initial intercalacable lithium, and the average current density have great influence on the simulated voltage and capacity for the electrochemical model. Moreover, the average current density, the length of the separator and the initial concentration of lithium ion in the electrolyte are the three parameters that have boundary effect on the electrochemical properties of the battery model.The thermal balance equation was coupled with those electrochemical equations to build the electrochemical-thermal coupled model. The temerpature rise simulated by the thermal model during discharge under different temperature and current rates can fit well with the experimental data. And modeling analysis was also performed to find critical parameters that may have great influence of the battery thermal properties. It was found that the surface coefficient of heat transfer, the average current density, the length of the electrodes and the maximum intercalacable lithium of negative electrode have greet influence on cumulative temperature rise of battery. The maximum intercalacable lithium of positive electrode, the density and the specific heat capacity influence the transient temperature rise rate a lot, but they almost don’t affect the steady cumulative temperature rise.