1 Challenges in Battery Assembly
Modern battery packs for electric vehicles combine hundreds of individual cells into compact, crash-resistant units. The joining technology must meet five key requirements 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 leads to corrosion, leakage currents, and cell damage. Sealing systems must meet IP67 or IP68 standards—even after years of use and across a wide range of temperatures.
- Electrical insulation: High-voltage components (up to 800 V) require flame-retardant potting compounds with high dielectric strength 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 applied?
A typical battery pack consists of several hierarchical levels. Specific adhesive systems are used at each level.
Cell-to-cell bonding
Cylindrical cells (18650, 21700, 4680) or pouch cells are assembled into modules. Flexible adhesives accommodate thermal expansion while simultaneously dissipating heat. Two-component silicones with thermal conductivities ranging from 1 to 3 W/m·K are standard. For prismatic cells, acrylate foam adhesive tapes are often used to compensate for tolerances.
Cell-to-module and module-to-housing
Structural adhesives are required to secure cell modules within the battery housing. Epoxy-based systems, such as the Permabond ET500 series, offer shear strengths exceeding 20 MPa and cure even at room temperature. Alternatively, fast-curing polyurethanes are used, which reach full strength after just 24 hours.
BMS fixation
The battery management system, including circuit boards, sensors, and control units, must be installed in a vibration-resistant manner. Thixotropic silicones do not run after dispensing and provide electrical insulation.
Cooling plate bonding
Gap fillers must minimize thermal contact resistance between cell modules and aluminum heat sinks. Highly filled silicones (with aluminum oxide or boron nitride) achieve thermal conductivities of up to 5 W/m·K and compensate for unevenness ranging from 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 their structural integrity and prevent cell damage. Structural adhesives distribute forces over large areas and absorb energy through controlled plastic deformation.
Epoxy structural adhesives
Two-component epoxies are the top choice for high-stress bonding applications:
- Shear strengths ranging from 20 to 35 MPa (according to DIN EN 1465)
- Temperature resistance up to 150 °C (up to 180 °C for short periods)
- 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 curing 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 ranging from 60 A to 70 D, they are particularly well-suited for material combinations with different coefficients of thermal expansion. Their elongation at break of over 100% enables energy absorption during crash impacts.
Crash requirements according to UN ECE R100
UN Regulation No. 100 requires that high-voltage components remain protected in the event of frontal, side, and rear collisions. 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 ranging from 0.5 to 5 mm. After curing, they remain elastic and accommodate thermal expansion. Typical thermal conductivities:
- Standard: 1.5 to 2.5 W/m·K
- Thermal conductivity: 3 to 5 W/m·K
- Ceramic-filled: up to 7 W/m·K
thermal pastes
For thin layers (less than 0.3 mm), thermal interface materials offer lower thermal resistance than gap fillers. They remain paste-like over time 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 Encapsulants 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 dominate BMS encapsulation. Their advantages:
- Temperature resistance from −60 °C to +200 °C
- Dielectric strength greater than 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 a range of viscosities for manual and automated application. Pot life: 30 to 90 minutes. Cures at room temperature in 24 to 48 hours; curing is accelerated at 60 °C to 2 to 4 hours.
Polyurethane casting compounds
PU potting compounds cure into harder systems (Shore A 70 to Shore D 60) and offer higher mechanical strength. They are more cost-effective than silicones but have a lower temperature resistance range (−40 °C to +120 °C). Application: Potting of sensors and low-voltage electronics.
IP67/68 protection
Battery packs require a minimum IP67 rating. Encapsulants must therefore:
- Be cast completely void-free (vacuum casting recommended)
- Permanently adhere to housing bushings
- Show no water absorption (less than 0.5% according to DIN EN 60068)
- Remain tight across the entire temperature range
6 Comparison of Materials by Application
| Application | Material | property | Typical value |
|---|---|---|---|
| Cell-to-cell | 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–heat sink) | High-performance silicone | Thermal Conductivity / Shore | 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 / 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 sealants are applied robotically in beads and cure into elastic seals. One-component silicones (RTV-1) cure in 24 hours through exposure to atmospheric moisture. Advantages: No need to stock different seal geometries, can be automated, consistent quality. Precise dosing (bead width 3 to 5 mm) and defined joint gaps (0.5 to 1.5 mm) are crucial.
Butyl seals
Pre-cut butyl cords remain permanently tacky and seal through mechanical compression. Cost-effective and quick to install, but with lower temperature resistance than silicones (−30 °C to +90 °C).
Hybrid sealing systems
A combination of a mechanical seal (O-ring) and an additional liquid seal for the most demanding applications. Used in IP68 enclosures for underbody battery packs.
8 Standards and Qualifications
| Standard | Scope | Core requirements |
|---|---|---|
| UN ECE R100 | Electrical Safety of High-Voltage Vehicles | Mechanical strength, electrical insulation, fire resistance |
| GB/T 31467 | Li-ion battery systems (China) | Thermal shock (–40 to +85 °C), vibration according to ISO 16750-3 |
| LV 123 | VW Group E/E Components | Environmental testing, corrosion testing, outgassing tests (K01) |
| UL94 V-0 | Flame retardancy | Selbstverlöschend < 10 s, kein brennendes Abtropfen, halogenfrei |
| REACH / RoHS | EU Chemicals Regulation | SVHC Declaration, Restriction of Hazardous Substances |
9 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 (less than 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
Curing times and cycle times
- Heating chambers (60 to 80 °C) for accelerated curing of epoxies and silicones
- UV-curing acrylates for instant casting (niche: sensor fixation)
- Fast-curing epoxies with a 15-minute cure time at room temperature
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
How long do adhesive bonds last in battery packs?
Qualified adhesives are designed to last for the entire vehicle lifespan (10 to 15 years, 3,000 to 5,000 charging cycles). Silicones show no signs of embrittlement even after 20 years in accelerated testing. Epoxies can continue to cure and become brittle under sustained loads above 120 °C—therefore, temperature profiles in the specifications are critical.
Can bonded cells and modules be replaced later?
Silicone bonds can be separated mechanically (by cutting with wire or a blade). Epoxy structural adhesives are virtually indissoluble—destructive disassembly is necessary in this case. In cell-to-pack (CTP) designs, cells are integrated directly into the housing, and replacement is not intended. Modular designs with screw connections plus adhesive allow for better repairability.
What are the applicable flame-retardant requirements?
At least UL94 V-1 (self-extinguishing within 30 seconds). Premium applications require V-0 (self-extinguishing within 10 seconds, no burning droplets). LOI above 28% is increasingly required. Halogen-free formulations are standard. Flame retardants must not impair thermal or electrical properties.
How is thermal expansion compensated for?
Battery cells expand by 0.5 to 2 mm during charging. Elastic adhesives with a low modulus of elasticity (below 10 MPa) compensate for these movements. Gap fillers with a Shore A hardness of 20 to 40 are ideal. With rigid epoxies, the adhesive joint must be dimensioned to accommodate shear stresses. Rule of thumb: The greater the temperature difference and the more dissimilar the materials, the more elastic the adhesive must be.
Can adhesive processes in gigafactories be automated?
Yes. Dosing systems with robot-guided applicators achieve accuracies of ±1%. Inline inspection using camera systems checks the bead geometry and positioning. Material consistency and environmental conditions are critical factors. Cycle times of less than 60 seconds per battery pack are state of the art.
Battery packs contain voltages of up to 800 V. Work may only be performed by qualified personnel who have completed high-voltage training (HV-1, HV-2, HV-3 in accordance with DGUV Information 200-005). Before opening the pack: Check for absence of voltage, observe the 5 safety rules, and use personal protective equipment. Lithium-ion cells may experience thermal runaway if damaged.
Still unsure about the material?
Whether it's sealing, encapsulation, or bonding—when the application is critical, the choice of material is no trivial matter.
Discuss the application →