The quality of heat transfer from batteries to their outer casing has a significant impact on an electric vehicle's performance and service life. An ongoing research project aims to develop a new generation of gap fillers with improved thermal conductivity and reduced density and to make them suitable for use in series production.
A heat-conductive gap filler is applied before the battery modules are inserted into the compartmentalised battery tray.
Audi AG / Stefan Warter
Gap fillers are pasty materials with high thermal conductivity. They transfer heat and compensate for dimensional tolerances between the battery module and the battery casing. During battery assembly, the gap filler is applied in the battery tray compartments by a robot. The battery modules are then inserted and gradually compress the gap filler into the areas that are to be filled. The battery cells are sensitive to pressure, so it is important not to damage them during this step. The gap filler's consistency is therefore formulated so that the compression forces do not damage the battery. At the same time, the gap between the battery and the base must be completely filled and contain no air bubbles.
Similar to combustion engines in modern vehicles, the battery is the key component in electric vehicles and therefore determines the price. For reasons of operational reliability, all current designs assume that the individual battery cells will be integrated into a lightweight battery casing. The casing is fully integrated into the vehicle body, providing the advantage of the lowest possible centre of gravity for the entire vehicle.
The pastes that are currently available can reach thermal conductivity levels of 3 W/mK, which is two orders of magnitude below the thermal conductivity of aluminium (220 W/mK), for example. In addition to limited thermal conductivity, the use of these gap fillers leads to other conflicts with the requirements of automotive construction. For example, pastes with a high filling material content have a high density and increase the weight of the vehicle, which is incompatible with lightweight design.
Aims of the research project
The OWES project (Optimised Heat Dissipation from Energy Storage Devices for Series Electric Vehicles) run by the Federal Ministry for Economic Affairs and Energy focuses on research and development approaches for production engineering, with the following priorities:
- Optimisation of previous gap filler designs through improved filling material.
- Use of alternative (more cost-effective, lighter) filling materials.
- Innovative heat transfer designs, e.g. conductive fabrics.
- Development of testing and simulation methods which can be used as a basis for assessing whether materials are suitable for the process.
- Establishment of known processing methods necessary for series production of compact pastes and the development of new, innovative procedures such as gap filler injection.
The appropriate work is being carried out by a project consortium led by Audi. The consortium consists of material manufacturers Wacker Chemie and Polytec PT, filling materials supplier Quarzwerke, plant manufacturer Atlas Copco IAS and Fraunhofer IFAM research institute.