Thermal Analysis and Modeling of Lithium Iron Phosphate Batteries and Battery Packs

Lithium iron phosphate (LFP) batteries are becoming widely used in electric vehicles (EV) and hybrid electric vehicles (HEV) owing to its capability of high power output and its safety characteristics. Generally, the temperature of LFP batteries rises while discharging and if it goes beyond the appropriate temperature range, it will lead to a shorter cycle life. Besides, for a battery pack, we not only need to sustain temperature in an appropriate range, but we also need to ensure the consistency among batteries, which hugely influences the discharge rate of a battery pack. But, it is time consuming and expensive to optimize battery pack design through trial-and-error tests and experiments. Hence, this study aims to develop an accurate model to facilitate the analysis of battery pack performance and further, to provide a few insight into thermal management of a battery pack. This study analyzes two different types of LFP battery models. One is the Newman model, which incorporates the properties of material and physical processes occur in batteries. Another is the equivalent circuit model, ECM, which resembles the physical processes of a battery with equivalent circuits such that it reduces the complexity of analysis. In comparison to experimental data, we find that the ECM model simulates more accurate results than Newman model. Therefore, the battery pack model is developed on the basis of the ECM, single cell model. The thermal management strategies analyzed include changing the adjacent battery distance under natural convection, adjusting the number of fans under at fixed total flow rate (force convection) and varying the locations of inlet and outlet for flow arrangement. In conclusion, for a battery pack, the most important thing is to maintain consistency among individual cells. Based on our simulation results, we find that reducing the adjacent battery distance and increasing the number of fans are feasible strategies to achieve that. Apart from that the aforementioned strategies, we also learn that flow arrangement via exchanging the location of inlet and outlet may achieve better performance in certain cases.