The Influence of Thermal Management Concepts on the Battery Behavior - Insights from a New Coupled Numerical Modelling Approach on the Battery Module Scale and Corresponding Experiments
Authors: Nyhues Thomas , Kalmbach Thomas , Kansy Jessica , Loges André , Wetzel Thomas , Wiebelt Achim 
Published: December 2024

Battery electric vehicles (BEVs) are the future mainstream drivetrain solution for the individual mobility sector. However, the fast charging capability of BEVs, which is seen as a key enabler for a broad customer acceptance and a convenient e-mobility is a remaining challenge in the development of battery systems. During fast charging, increased heat losses occur in the battery cells and the electrical connectors of a battery module. The battery thermal management then plays a crucial role to keep the battery temperature in the recommended range and to avoid accelerated degradation of the cells. Most of the conventional battery thermal management concepts in today’s BEVs are not efficient in managing the increased heat losses which leads to currently limited fast charging rates. Therefore, new thermal management solutions are being discussed and developed. A promising approach is the direct liquid cooling of batteries, also known as immersion cooling. With immersion cooling, the cells, cell tabs and electrical connectors are in contact with a dielectric fluid. Immersion cooling has the potential to increase the temperature homogeneity within and between the cells and to decrease or even prevent subsequent cell degradation.

For the development of efficient thermal management systems for future BEVs and to use the full potential of concepts like immersion cooling, the influence of the thermal management on the battery cells must be understood and evaluated in detail. Considering that, a deep understanding of the interacting electrical, electrochemical, thermal and fluid dynamic phenomena occurring inside a battery module is required. Therefore, in this contribution, numerical simulations and corresponding experiments with battery modules in application-related setups are combined to study the effects of the thermal management on the battery behavior. The two fundamentally different concepts of immersion cooling and state-of-the-art bottom cooling are investigated as well as variations of each of those concepts. In the immersion cooled module, particular attention is paid to the fluid flow distribution and the effect different flow concepts on the thermal and electrochemical conditions in the cell.

The numerical studies are based on a new coupled modelling approach (Fig. 1). Detailed thermal models of the 65 Ah automotive NMC/Si-Gr pouch cells are set up in a 3D CFD environment based on the analysis of the disassembly of the cell and the measurements of its thermophysical properties. Then, networks of electrochemical models of the cell are electrically and thermally coupled to the 3D models and the module components. This holistic approach allows for the investigation of e.g., the interaction of the cooling fluid flow with the current and temperature distribution and its effect on the local electrochemical cell behavior like the anode potential during fast charging.

The corresponding experimental battery modules in a 3s2p configuration (Fig. 2) are explicitly designed and built for the validation of the numerical models. The cells and module components are equipped with extensive temperature measurements to capture thermal gradients within and between the cells. The modules are then installed and operated in separate climate chambers and connected to individual cooling circuits (Fig. 3) to ensure defined and equal boundary conditions as basis for the comparison of concepts and concept variations.

The performance differences in various operating points including fast charging are demonstrated and analyzed. Furthermore, the variation of boundary conditions e.g., the volume flow rate or the fluid temperature allows for the analysis of relevant differences in the cooling concepts’ operating principles.

The results from simulation and experiment illustrate the weak spots of the conventional concepts that are used in today’s BEVs. Furthermore, the numerical models reveal the critical local conditions inside the cells that are provoked by high loads in combination with inefficient thermal management concepts. Furthermore, the results show the benefits of immersion cooling over conventional concepts and its potential to increase the lifetime of battery systems by steering the battery temperature and temperature homogeneity with suitable fluid flow conditions. At the same time, the fast charging capability of BEVs can be increased significantly with immersion cooling.

The good agreement of numerical and experimental results (Fig. 4) show that the presented modelling approach can be used as a tool for the design and the optimization of thermal management concepts. The application-related setup in this study ensures transferability of the findings from experiment and simulation to automotive applications and will help researchers and developers from industry and academia towards developing better thermal management solutions for BEVs.

Figure 1