Cylindrical Cell Thermal Modeling for an Automotive Battery Pack

Lithium-ion batteries are widely used in electric vehicles (EVs) and hybrid electric vehicles (HEVs) today, these batteries advantageous compared to other forms of energy storage due to their high energy density and long cycle life. The object of this thesis is to develop the model based on the internal electrochemical states of the battery for battery monitoring and control in advanced battery management systems (BMSs). The investigated battery is 2.5Ah LiFePO4 cylindrical 26650 type from company A123. Pseudo-two-dimensional (P2D) model developed by John Newman and his colleagues is probably the most widely used Physics based lithium-ion battery model in the world, however, the complexity of P2D model makes on-line simulation difficult in automotive applications. Hence, it is necessary to obtain a simplified model under all operating conditions of the batteries. In this paper, simplification of electrochemical models of Li-ion battery to improve simulation and computational efficiency will be proposed, the idea is to reduce the number of partial differential equations (PDEs), so a simplified Pseudo-two-dimensional (SP2D) model is established in this work. Moreover, the numerical method used to solve the system of equations can have a significant impact on the computational cost of the simulation. Two different methods called ‘parameters polynomial approximation’ and ‘orthogonal collocation’ are used in this paper to solve the PDEs, also the results obtained by applying these two mathematical methods are discussed and compared. A thermal model is also developed because the thermal accumulation of the battery strongly affects its performance and durability. The heat generation rate and the average temperature of the battery under different working state can be obtained by the thermal model. At the same time, the average temperature will be the additional input of the electrochemical model because the temperature will influence the chemical reaction within the battery. As a result, a combined-electrochemical-thermal model is obtained. The modeling of the battery for both electrochemical and thermal part is realized in MATLAB, so this paper also contains an instruction of the MATLAB code. Furthermore, A high-fidelity COMSOL model is created to compare the results with the MATLAB’s. Another advantage of using COMSOL is that the 3D plot of the average temperature of the battery is available. Finally, the comparison of the results among three different approaches (MATLAB, COMSOL, and experiment) is performed. The SP2D model solved by ‘orthogonal collocation’ enable fast and accurate simulation of the behavior of battery at different discharge/charge rate while ‘parameters polynomial approximation’ shows a larger error at the relatively high rate. However, the SP2D model itself is not reliable at very high discharge/charge rate, overall it is a good model for studying systems operating under long charge or discharge time when fast computation time is required.