By utilizing computational fluid dynamics, a model for a lithium-ion cell is developed and is eventually used to predict the spatial temperature distribution across battery packs comprising twelve cells. Instead of geometrically resolving the layered structure of the jelly roll, a substitute is introduced to replace the entire jelly roll and to enable cost-effective simulations. The jelly roll substitute takes into account the anisotropy of thermal conductivity caused by the layered structure, and provides production of waste heat by applying an electric load. As the electrical conductivity of the substitute material is dependent on temperature, the model includes thermal-electrical coupling. Applying appropriate methods enables the derivation of thermal material parameters from properties of the underlying layer materials.
In contrast, the electrical conductivity has to be parameterized in consideration of the dimensions and the electric boundary conditions of the substitute. For this purpose, a single cell is tested in several cycling experiments in order to acquire data on the electrical response as well as data on the temporal development of the surface temperature.
The experiments are performed using different load levels and different SoC-values. Sufficient runtime is chosen to achieve thermally steady conditions. To validate the CFD model in a first step, a chosen experiment is computed using steady state analyses employing both, a realistic model including an ambient air volume and a simplified model representing heat dissipation to the environment by heat flux boundary conditions. Subsequently, transient analyses are performed using the simplified model only. The results show that all heat transfer mechanisms contribute significantly to heat dissipation in a natural convection situation.
The pack models consider three different cooling concepts in which either air or coolant act as cooling fluid. The cooling effect is implemented by conducting the cooling fluids through a unit affixed to the bottom surfaces of the cells, or through gaps located in between the cells. Several inlet conditions of the cooling fluids and modifications of characterizing design features are investigated by means of parameter studies. The heat transfer capability of each individual cooling concept as well as its resulting temperature field are evaluated. In doing so, maximum temperature and temperature homogeneity inside the jelly roll volumes are of particular interest.