Thermal gap fillers for EV battery packs
Two-component gap filler is a paste-like thermally conductive material dispensed into the gap between the bottom of the battery cells and the cooling plate. Once cured, it forms a continuous, elastic thermal interface layer that withstands vibration throughout the life of an electric vehicle.
Temperature is the decisive factor in battery pack life. Lithium cells perform best between 20 and 40 °C, and the smaller the temperature difference between cells, the more durable the pack. Thermally conductive adhesive is the bridge that carries heat from the cells to the cooling system.
The role of gap filler in a battery pack
The bottom surfaces of the cells and the cooling plate are never perfectly flat. Assembly tolerances can create gaps of 0.5 to 3 mm, and if those gaps are filled with air, almost no heat gets through. Gap filler fills these gaps completely while also compensating for tolerances across hundreds of cells in a module.
Unlike pre-cut thermal pads, liquid gap filler is applied with metering and dispensing equipment, making it well suited to automated lines and placing less compressive stress on the cells.
Key parameters to consider when selecting a material
| Parameter | Typical range | Significance |
|---|---|---|
| Thermal conductivity | 1.5 – 4.0 W/m·K | Determines how much heat passes through the adhesive layer |
| Density | 2.0 – 3.2 g/cm³ | Directly affects pack weight |
| Hardness after cure | Shore OO 40 – 80 | Softer material reduces stress and is easier to remove for repair |
| Dielectric strength | Above 8 kV/mm | Ensures electrical insulation between cells and housing |
| Working time | 30 – 90 minutes | Matches the assembly line takt time |
A notable trend is low-density gap fillers, which cut several kilograms from each pack while maintaining thermal conductivity above 2 W/m·K.
Vibration resistance and aging
Electric vehicles are subjected to constant road vibration, combined with thermal cycling from -30 °C to 85 °C. Silicone-based gap fillers stay elastic across a wide temperature range without cracking or delaminating. Some silicone-free systems are used when plants are concerned that volatile siloxanes could affect electrical contacts.
Dispensing tips
- Control the 1:1 mix ratio with metering equipment that alarms on deviations.
- Use static mixers and wear-resistant nozzles, as the material contains a high load of ceramic fillers.
- Design wave-pattern or dot dispensing paths to avoid air entrapment when cells are pressed into place.
Before finalizing a material, run trials on a sample module and measure the temperature of each cell during fast charging. A difference of less than 5 °C between the hottest and coolest cell is generally considered acceptable. These results also help determine the minimum amount of material required, avoiding waste of an expensive material.
To request samples and a comparison chart of gap fillers from Henkel, Dow and Bergquist, contact AMS Vietnam.
