#1 | 2019
SYN] Magazine
13/18

What you need to know about joints in high-voltage batteries

The right joining procedure has a considerable impact on the performance, safety and durability of the high-voltage batteries used in electric vehicles.

1. Cell bonding: bubble-free joints are essential for safety

To provide the energy required, prismatic cells must be joined firmly to form stacks. However, these cells are highly sensitive and must not be exposed to heat or severe forces during assembly. If two-component (2C) adhesives are used, no external heat needs to be applied for curing and the joint meets the highest possible crash protection requirements. Elastic adhesives absorb the vibrations which occur during operation, extending the service life of the battery. These adhesives mean that cells can easily expand and contract during charging and discharging. The adhesive must be applied with a high degree of precision to prevent air inclusions and to ensure full contact and insulation. In the event of an accident, air inclusions may cause short circuits – a severe risk for high-voltage systems.

2. Reinforcement of cell stacks: cold joining essential   

To protect the battery in the event of a collision, cell stacks may be equipped with braces at their sides. Conventional joining methods such as spot welding are not suitable for this assembly step because they expose the sensitive cell to heat and welding splatter which may cause damage. The solution is to use a cold joining procedure such as self-pierce riveting. This clean, purely mechanical joining procedure does not expose the cells to any heat and does not generate any hazardous fumes or weld splatter. Self-pierce riveting can be used for joining several layers of different materials such as aluminium or steel. The joining process is highly reliable, with short cycle times, allowing designers considerable leeway and ensuring maximum safety and high productivity.

3. Gap filler: application of thermal conduction paste is a challenge  

Temperature management is a major challenge in battery production. Battery cells must be operated within a specified temperature range in order to maintain their performance and avoid overheating. For this reason, a thermal conduction paste, the “gap filler”, is applied. To ensure thermal conductivity, any bubbles in the paste must be avoided. This poses a challenge because the liquid material is applied in large quantities. Precise metering and application technology is called for. Additional quality monitoring functions may also be beneficial. For example, camera-based systems can monitor the position of the bead in order to ensure precise results. Application errors are detected and can be corrected immediately. This approach keeps cycle times short and reduces the cost of reworking and quality assurance. Some gap fillers are highly abrasive, resulting in rapid wear on the system. The system components such as material supply and metering systems must be designed to handle large quantities of demanding materials with high productivity. 

4. Module assembly: controlled tightening with soft materials 

The battery modules must be installed on the liquid gap filler. This challenge can be solved using nutrunners. However, the soft joining behaviour of the gap filler is problematical. In the case of poorly tightened joints, the paste may be squeezed out or bubbles may remain. To ensure even distribution and full contact between the battery modules and the paste, the tightening process must be fully controllable. Electronically controlled multiple-spindle solutions are especially well-suited to ensure homogeneous tightening. Synchronized operation in the final tightening stage reduces the cycle time and each module is installed homogeneously in the battery compartment. Parameters must be set to take account of the behaviour of the liquid thermal conduction paste with a view to establishing optimum contact.

5. Sealing: protection against moisture and gases is decisive  

As soon as all the modules have been mounted and the battery management system has been installed, the battery casing must be sealed. Moisture must not penetrate into the battery as it could severely impair battery performance and cause damage and corrosion. In addition, the battery produces gases which may be detrimental to vehicle passengers. The space inside the battery must be fully sealed from the inside and outside. This calls for precise, seamless application of the liquid sealant. Sealant may either be applied to the cover or the casing. As the battery must not be exposed to any heat, materials such as single-component hot butyl, two-component polyurethane or two-component silicone sealants are suitable. Hot butyl sealant can also be removed, for example for maintenance work. Irrespective of the sealant used, it must be applied evenly. Especially at the beginning and end of the sealant bead, high precision is required in order to ensure a tight seal.

6.   Cover installation: joints must be disconnectable

Finally, the cover is installed on the battery housing and the battery is only accessible from the outside. This must be taken into consideration when selecting the joining procedure. It must be possible to disconnect the joint in order to facilitate maintenance and dismantling. Flow drill fastening is ideally well-suited to meet these requirements. The fastener is rotated at high speed with high pressure to heat up the surface. The material is softened, allowing the fastener to flow into the metal and form a thread, a highly efficient and flexible joining method for a variety of materials. This process ensures reliable mechanical joining. The joint is also disconnectable and only requires one-sided access. The metal components form a conductive connection and create a Faraday shield that prevents electromagnetic interference.