E-mobility battery assembly: Adhesives and potting compounds for battery packs
The assembly of lithium-ion battery packs for electric vehicles places the highest demands on joining technologies. Adhesives, potting compounds, and seals do much more than just provide mechanical fixation: they ensure thermal management, crash safety, electrical insulation, and protection against moisture. This technical article shows which materials are used where and what battery engineers need to consider when making their selection.
1. Challenges in battery assembly
Modern battery packs for electric vehicles combine hundreds of individual cells into compact, crash-proof units. In doing so, numerous requirements must be met simultaneously:
- Thermal management: Lithium-ion cells generate heat during charging and discharging. Without efficient heat dissipation, thermal runaway can occur. Gap fillers and thermal pastes must minimize contact resistance and reliably dissipate heat to cooling plates.
- Vibration and crash resistance: Battery packs are exposed to continuous vibrations and extreme forces in the event of a crash. Structural adhesives must offer high shear strength and energy absorption without becoming brittle.
- Weight optimization: Every kilogram counts. Adhesive bonds replace heavy mechanical fasteners and enable lightweight construction concepts using aluminum and composites.
- IP protection: Moisture ingress can lead to corrosion, leakage currents, and cell damage. Sealing systems must guarantee IP67 or IP68 – even after years of use and across wide temperature ranges.
- Electrical insulation: High-voltage components (up to 800 V) require dielectric-strength potting compounds with high voltage resistance and defined creepage distances.
European battery production is growing rapidly: gigafactories in Germany, Hungary, and France rely on automated manufacturing processes. Adhesive systems must therefore not only be technically convincing, but also reproducibly dosable and fast-curing.
2. Adhesives in the battery pack: Where are they used?
A typical battery pack consists of several hierarchical levels. Specific adhesive systems are used at each stage:
Cell-to-cell bonding
Cylindrical cells (18650, 21700, 4680) or pouch cells are combined into modules. Elastic adhesives are used here, which tolerate thermal expansion and dissipate heat at the same time. Two-component silicones with thermal conductivities of 1 to 3 W/m·K are standard. Acrylate foam adhesive tapes are often used for prismatic cells to compensate for tolerances.
Cell-to-module and module-to-housing
Structural adhesives are required to secure cell modules in the battery housing. Epoxy-based systems such as the Permabond ET500 series offer shear strengths of over 20 MPa and cure even at room temperature. Alternatively, fast-curing polyurethanes can be used, which reach their full strength after just 24 hours.
BMS fixation
The battery management system (BMS) with circuit boards, sensors, and control units must be mounted in a vibration-proof manner. Thixotropic silicones are suitable for this purpose, as they do not run after dispensing and provide electrical insulation. Important: No corrosive outgassing that could damage electronic components.
Cooling plate bonding
Gap fillers must minimize thermal contact resistance between cell modules and aluminum cooling plates. These highly filled silicones (with aluminum oxide or boron nitride) achieve thermal conductivities of up to 5 W/m·K and compensate for unevenness of 0.5 to 3 mm. Products such as Bluesil TCS 4525 cure at room temperature to form elastic, non-adhesive layers.
3. Structural adhesives for crash safety
In the event of a crash, battery packs must maintain structural integrity and prevent cell damage. Structural adhesives transfer forces over large areas and absorb energy through controlled plastic deformation.
Epoxy structural adhesives
Two-component epoxies are the first choice for high-stress bonding applications. They offer:
- Shear strengths from 20 to 35 MPa (according to DIN EN 1465)
- Temperature resistance up to 150 °C (briefly up to 180 °C)
- Excellent adhesion to aluminum, steel, and composites
- Low shrinkage during curing
The Permabond ET5145 series combines high strength with flexibility and is specifically qualified for automotive applications. Typical cure profiles: 1 hour at 80°C or 7 days at 23°C.
Polyurethane structural adhesives
PU adhesives offer a balance between strength and elasticity. With Shore hardnesses between 60 A and 70 D, they are particularly suitable for material combinations with different expansion coefficients. The elongation at break of over 100 percent enables energy absorption in the event of crash loads.
Crash requirements according to UN ECE R100
UN Regulation No. 100 requires that high-voltage components remain protected in the event of a frontal, side, or rear impact. Battery packs must withstand defined acceleration profiles without electrolyte leakage or short circuits. Adhesive bonds are part of the crash structure and must be taken into account in FEM simulations.
4. Thermal interface materials (TIM)
Heat dissipation from the battery pack is crucial for service life and safety. Thermal interface materials close air gaps between the heat source and heat sink and reduce thermal resistance.
gap filler
Gap fillers are paste-like or thixotropic silicones that fill gaps of 0.5 to 5 mm. After curing, they remain elastic and compensate for thermal expansion. Typical thermal conductivities:
- Standard gap filler: 1.5 to 2.5 W/m·K
- High-performance gap filler: 3 to 5 W/m·K
- Ceramic-filled special products: up to 7 W/m·K
Processability is important: gap fillers must be dosable (stencil printing, dispensing) and must not form air pockets. Bluesil TCS products are specially developed for automated battery assembly lines.
thermal pastes
For thin layers (less than 0.3 mm), thermal interface materials offer lower thermal resistance than gap fillers. They remain permanently paste-like and require mechanical contact pressure. Application: Between BMS components and metal housings.
thermal pads
Pre-assembled silicone pads (phase-change materials) melt at operating temperature and adapt to the surface. Advantage: Clean processing, no dosing. Disadvantage: Higher thermal resistance than gap fillers of the same thickness.
5. Encapsulation compounds for BMS and power electronics
Electronic control units, high-voltage connectors, and power distribution rails are encapsulated to protect them from moisture, vibrations, and mechanical stress.
Silicone casting compounds
Two-component silicones are the dominant technology for BMS encapsulation. Advantages:
- Temperature resistance from -60 °C to +200 °C
- Excellent electrical insulation (dielectric strength over 20 kV/mm)
- Permanently elastic (Shore A 20 to 60)
- No corrosive emissions
- Repairability: Silicone can be removed mechanically
The Bluesil RTV 3400 series offers different viscosities for manual and automated processing. Typical pot life: 30 to 90 minutes. Curing at room temperature in 24 to 48 hours, accelerated at 60 °C in 2 to 4 hours.
Polyurethane casting compounds
PU casting compounds cure to form harder systems (Shore A 70 to Shore D 60) and offer higher mechanical strength. They are more cost-effective than silicones, but less temperature-resistant (typically -40 °C to +120 °C). Application: Casting of sensors and low-voltage electronics.
IP67/68 protection
Battery packs require at least IP67 (protection against temporary submersion). Encapsulants must therefore:
- Be cast completely void-free (vacuum casting recommended)
- Permanently adhere to housing bushings
- No water absorption (less than 0.5 percent according to DIN EN 60068)
- Remain tight across the entire temperature range
6. Material comparison by application
| Application | Material | property | Typical value |
|---|---|---|---|
| Cell-to-cell bonding | Thermal conductive silicone (2K) | thermal conductivity | 2.0 - 3.0 W/m·K |
| Structural bonding of module housing | Epoxy structural adhesive | shear strength | 25 - 35 MPa |
| Gap filler (cell cooling plate) | High-performance silicone | Thermal conductivity / Shore hardness | 3.5 - 5.0 W/m·K / Shore A 40 |
| BMS encapsulation | silicone potting compound | Dielectric strength / Temp. | > 20 kV/mm / -60 to +200 °C |
| Housing seal | FIPG silicone (1K) | IP protection class / Curing | IP67/68 / 24 hours at 23 °C |
| high-voltage connector | polyurethane casting | Shore hardness / Tear resistance | Shore D 50 / 15 MPa |
7. Seals and gaskets for battery enclosures
The battery housing must be permanently sealed against moisture, dust, and splashing water. Three technologies dominate:
FIPG (formed-in-place gasket)
Liquid seals are applied robotically as beads and cure to form elastic seals. Single-component silicones (RTV-1) cure in 24 hours through exposure to air humidity. Advantages: No need to stock different seal geometries, can be automated, consistent quality. Important: Precise dosing (bead width 3 to 5 mm) and defined joint gaps (0.5 to 1.5 mm).
Butyl seals
Pre-assembled butyl cords remain permanently sticky and seal through mechanical pressure. They are inexpensive and quick to process, but offer lower temperature resistance than silicones (typically -30 °C to +90 °C).
Hybrid sealing systems
Combination of mechanical seal (O-ring) and additional liquid seal for the most demanding requirements. Used in IP68 enclosures for underfloor battery packs.
8. Standards and qualifications for battery materials
Adhesives and potting compounds for battery packs must pass extensive testing:
UN ECE R100
UN regulation for electrical safety of high-voltage vehicles. Requires testing for mechanical strength, electrical insulation, and fire behavior. Adhesives are part of the crash structure and must be included in the overall certification.
GB/T 31467 (China)
Chinese standard for lithium-ion battery systems. Requires, among other things, thermal shock tests (-40 °C to +85 °C) and vibration tests in accordance with ISO 16750-3.
LV 123 (Volkswagen Group)
Test specification for electrical and electronic components. Defines climate change tests, corrosion tests, and outgassing tests. Adhesives must demonstrate approval in accordance with LV 123 K01 (climate change test).
UL94 V-0 (flame retardant)
Requirement for self-extinguishing materials. Casting compounds and adhesives must be classified as at least V-1, ideally V-0, according to UL94. Important: Halogen-free for reduced smoke emission.
REACH and RoHS
European Chemicals Regulation and Restriction of Hazardous Substances. All materials must be REACH-compliant and RoHS-compliant. Special note: SVHC substances (Substances of Very High Concern) must be declared.
9. Processing tips for battery assembly
automated dosing
Modern battery production is fully automated. Adhesives and casting compounds are applied using dosing systems:
- Gear pumps: For low-viscosity materials (below 10,000 mPa·s)
- Progressive cavity pumps: For highly filled gap fillers and thixotropic materials
- Pneumatic cartridges: For manual processing and prototypes
- Vacuum potting systems: For cavity-free BMS potting
Important: Materials must have consistent viscosities across batches. Tolerances of ±10 percent are acceptable; larger fluctuations require adjustment of the dosing parameters.
Curing times and cycle times
Fast curing is critical for high throughput rates. Strategies:
- Heating chambers (60 to 80 °C) for accelerated curing of epoxies and silicones
- UV-curing acrylates for instant casting (niche: sensor fixation)
- Fast epoxies with a 15-minute fixture time at room temperature
Caution: Too rapid curing can lead to tension. Process validation via temperature profiles is essential.
surface preparation
Aluminum surfaces should be degreased (isopropanol) and possibly pretreated with plasma or corona. Composite materials often require primer for optimal adhesion. Critically inspect painted surfaces: Adhesion failure can occur on the paint, not on the adhesive.
Rework and repairability
Silicone bonds and castings can be removed mechanically (cutting, milling). Epoxy bonds are practically impossible to remove and require destructive disassembly. Design for rework: Provide separation joints, design modules to be interchangeable.
10. Frequently asked questions (FAQ)
Qualified adhesives and potting compounds are designed to last for the entire service life of the vehicle (typically 10 to 15 years or 3,000 to 5,000 charging cycles). Material selection and process quality are crucial. Silicones show no embrittlement even after 20 years in acceleration tests (Arrhenius). Epoxies can cure and become brittle when exposed to continuous high temperatures above 120 °C, which is why temperature profiles are critical in the specifications.
That depends on the adhesive system. Silicone bonds can be separated mechanically (cutting with wire or a blade). Epoxy-based structural adhesives are practically impossible to remove—destructive disassembly is necessary in this case. In modern cell-to-pack (CTP) designs, cells are integrated directly into the housing and are not intended to be replaced. Modular designs with screw connections plus adhesive allow for better repairability but require more installation space.
The minimum requirement is UL94 V-1 (vertical burning test, self-extinguishing within 30 seconds). Premium applications require UL94 V-0 (self-extinguishing within 10 seconds, no burning dripping). In addition, LOI (Limiting Oxygen Index) above 28 percent is increasingly required. Halogen-free formulations are standard in order to avoid toxic gases in the event of a fire. Important: Flame retardancy must not impair thermal and electrical properties.
Battery cells expand during charging and temperature changes (typically 0.5 to 2 mm for large formats). Elastic adhesives with a low modulus of elasticity (below 10 MPa) compensate for these movements. Gap fillers with Shore A 20 to 40 are ideal. For structural bonding with rigid epoxies, the adhesive joint must be dimensioned to absorb shear stresses. Rule of thumb: the greater the temperature difference and the more different the materials (aluminum vs. composite), the more elastic the adhesive must be.
Yes, modern battery production is highly automated. Dosing systems with robot-guided applicators achieve accuracies of ±1 percent. Inline control via camera systems (Beadvision) checks bead geometry and positioning. Material consistency (viscosity, mixing ratio) and environmental conditions (temperature, humidity for RTV-1 silicones) are critical. Large manufacturers rely on closed material supply systems with temperature control and continuous mixing. Cycle times of less than 60 seconds per battery pack are state of the art.
Safety note: Working on high-voltage battery packs
Battery packs contain voltages up to 800 V with high currents. Work may only be carried out by qualified personnel with high-voltage training (HV-1, HV-2, HV-3 according to DGUV Information 200-005). Before opening the pack: Check that there is no voltage, observe the 5 safety rules, use personal protective equipment (insulated tools, face protection). Lithium-ion cells can suffer thermal runaway if damaged – never subject them to mechanical stress or short-circuit them.
