Tushar Pawar is Business Line Manager – ADH, Industrial Assembly Solutions Division at Atlas Copco India
The Industrial Technique Business Area (ITBA) within Atlas Copco Group offers world class products and solutions for all types of manufacturing. Multiple joining solutions are a few of these. They include highly automated (robotic) riveting, Industrial gluing and high precision fastening solutions in the manufacture of two-wheelers, cars, heavy commercial vehicles, battery - powertrain, aerospace, heavy fabrication, rail & trains, foundry, shipping, cranes, energy industry, white goods, computer, phones, industrial electronics, and medical.
Much has been said about the importance of electric vehicles (EV) in the fight against climate change, and rightly so. It is estimated that petrol and diesel cars are responsible for nearly 20 % of global carbon dioxide emissions. EVs in comparison have zero emissions and are key in tackling both climate change and poor air quality in our towns and cities.
Too little attention, however, has been paid to the EV batteries that are expected to power almost 30 million EVs globally by 2025. It is also imperative to note that the performance, safety, and durability of these batteries depend upon various comprehensive assembly processes. Choosing the right joining technology, as the special prerequisites of battery manufacturing demands, and aiming for an efficient joining process is crucial.
Mentioned below is a six-step process for assembly of an incomparable EV battery pack:
1. Cell-to-cell bonding: Bubble-free bonding ensures safety
To supply the required energy, the prismatic battery cells (cells encased in aluminium or steel) must be firmly attached to cell stacks. This is a major challenge as the cells are quite frangible and hence, no heat or force can be applied in the joining process.
Using 2C adhesive bonding ensures that no external heat is required for the curing, and the joint meets the highest quality in terms of rigidity and crash behaviour. The vibrations generated during the operations are absorbed by using light elastic adhesives, which also increases the battery lifetime. This also allows the cells to expand marginally while the process of charging and discharging. The precise and consistent adhesive solution helps to avoid any air pockets/bubbles. This is a crucial step for full contact and insulation in the batteries as otherwise, in case of a crash, air pockets can lead to short circuits – a huge safety issue in high voltage systems.
2. Cold Joining: For the reinforcement of cell stacks
To protect the battery in case of a crash, cell stacks can be reinforced with lateral braces. For this assembly step, common joining techniques like spot welding are not advisable; as such processes create heat and welding splatter that can harm the sensitive build of the cells. Therefore, the solution is a cold joining technique such as self-pierce riveting. This clean and purely mechanical joining process brings no heat into the cells and does not generate any hazardous vapours or weld spatters. Additionally, Self-pierce riveting can join multiple layers of different materials such as aluminium or steel, providing electrical conductivity for grounding. This joining process is also highly reliable with short cycle times, enabling design freedom and maximum safety while keeping productivity at a high level.
3. Gap filler: Dispensing the thermal-conductivity paste is a challenge
A major challenge in battery manufacturing is temperature management within the battery cells. The cells must be operated within a specific temperature range to safeguard their performance by avoiding overheating of the parts. For combating this, although a heat-conducting paste is applied, a bubble free result is crucial to guarantee seamless thermal conductivity. This is a challenge because the liquid gap filler material is applied at high volumes, which requires precise metering technology. Gap filler materials, in their nature, are highly abrasive and may quickly wear out the dispensing equipment. Atlas Copco offers additional monitoring with our Quiss vision system, which monitors the position of the bead to ensure a precise result. This way, the application errors are recognized and can be corrected immediately, assuring quality, and keeps the cycle time short.
4. Module Mounting: Soft joints need ‘controlled tightening’
The battery modules are required to be mounted on top of the liquid gap filler paste, which is at the bottom of the tray, and can be done with tightening. The challenge here is the “soft joint” behaviour of the gap filler – either the paste gets pinched out easily or air enclosures remain. To guarantee even distribution of the paste and full contact between the battery modules and the thermal compound, the tightening process needs to be entirely flexible and manageable. The programmed tightening strategy needs to consider the behaviour of the liquid conducting paste to create optimal contact. An electronically-controlled multi-spindle solution from Atlas Copco achieves an even tightening process. By working synchronously in the final tightening, the cycle time is reduced, and each module is fixed evenly in the tray.
5. Cover Sealing: Protection against moisture and gases is crucial
Once all modules are tightly fixed, and the battery management system is installed, the tray must be sealed. It is crucial to avoid any penetration of moisture; otherwise the battery power deteriorates dramatically, which can lead to damage and corrosion. Furthermore, the battery produces hazardous gases that can be harmful for the passengers. Hence, the internal space must be sealed securely not just internally, but also externally. For this step, a precise and uninterrupted sealing application is vital. This can be done either on the cover or on the tray. Since the battery cannot be exposed to heat, materials such as 1C hot butyl, 2C polyurethane or 2C silicone is suitable, as they also do not require any oven curing. Another advantage of using hot butyl is that it is also removable for any required service works. However, regardless of the material, the company’s sealing solution ensures uniform adhesive distribution from beginning point till end of the bead with highest precision to ensure a tight seal.
6. Cover-to-tray joining: Serviceability calls for reverse joining
As the final step, the cover is mounted on the ‘housing.’ At this stage, the housing is only accessible from the outside and this needs to be considered while selecting the joining technology. This bond should also be detachable to facilitate maintenance and dismantling. Flow drill fastening technology meets these requirements perfectly. The screw is rotated at high speed and pressure to warm up the material. This allows the fastener to push through the material stack, cutting the thread in the process. This is an efficient and flexible joining technology for multi-material stacks. The process provides reliable mechanical bonding, is reversible and requires only one-sided access. No surface preparation is needed, as well. The metallic components are thus in a conductive bond and form a Faraday cage, which prevents electromagnetic interference.
It may sound hard to believe, but Atlas Copco is the only company to offer a variety of technologies and truly “The Total Solution.” It is all about creating value for clients across all fields of manufacturing. Customers prefer Atlas Copco’s solutions because of the company’s focus on ergonomics, as well as high-quality, leading edge software-based technologies. Atlas Copco builds things to last, and it called this “Sustainable Productivity.”