39 Battery sealing | Focus Structural adhesives Structural adhesives can be used to seal battery packs. These have higher levels of shear strength to avoid any weak spots in the structure of the pack, with high levels of corrosion and hygrothermal resistance from the movement of both heat and moisture. These rubber-based materials are one component that is applied in the body shop during the assembly of the e-mobility platform. Fracture toughness is the crucial property for crash-resistant structural adhesives. The latest versions allow the bonded structure to absorb a high degree of impact energy. They use a core of rubber with pre-dispersed nanoparticles in the secondary phase to increase the fracture toughness of the epoxy polymers. The cavitation and debonding of rubber particles make the structure of the material more prone to shear-yielding instead of cracking. The resulting ability to resist the growth of cracks increases the adhesive’s load-bearing capacity, which allows impact energy to be absorbed more effectively, thus protecting the battery cells in the event of a crash. Pressure relief Having a totally hermetically sealed housing is not always advantageous for a battery pack. These larger housings can be prone to pressure differences caused by changes in ambient temperature or air pressure. As this can pose a risk to the mechanical stability of the pack, to prevent deformation or other damage, it can be protected with a pressure-relief system. Minor pressure variations or a sudden increase in pressure due to cell defects can jeopardise the integrity of the battery pack. This can happen at the start of thermal runaway, and the extra pressure can rupture the pack, rendering any thermal prevention materials irrelevant. The pressure release valve can prevent this and protect the housing. A pressure relief valve protects against mechanical damage that may occur in the event of pressure differences between the interior of the housing and the outside environment. They also seal against external liquids and dirt. The elastomer membrane is designed to open under pressure to release the gas, closing again to provide a secure seal against the ingress of dust and water to protect the interior of the pack. But, as battery packs become progressively more powerful and create more heat, the vent valve plays a crucial role in maintaining safety. In case of overpressure or excessive gas build-up within the battery, the vent valve allows the release of gases to prevent hazardous situations. Testing ensures the valve operates correctly, allowing for proper heat dissipation and to prevent overheating. Testing for leaks Leak testing is vital because it helps to ensure the safety and reliability of electric vehicle (EV) batteries. Leaks can allow moisture, dust or other contaminants to enter the battery and cause problems, from reducing performance by corrosion to increasing the risk of a thermal event. Dust and other contaminants can block the battery’s cooling system, leading to overheating and a shorter battery life. If moisture creates a short circuit, it could render a vehicle inoperable. For these reasons, it is important to perform leak testing on EV battery packs to ensure they are safe and reliable. This can be done using a variety of methods, from pressure testing to vacuum testing. With a pressure test, the battery pack is pressurised with a gas, and any leaks are detected by the loss of pressure. With a vacuum test, the pack is placed in a vacuum and any leaks are detected by the inflow of air. A helium leak check pumps helium inside the battery case and uses sensors on the outside to detect any leak points. This is an important part of the quality control process for battery packs for all kinds of e-mobility platforms, from EVs to construction equipment and the EV take-off and landing (eVTOL) aircraft that are now being certified as airworthy. By ensuring packs are leak-free, EV battery engineers can help prevent thermal events, reduce the risk of batteries failing and extend their lifespan. E-Mobility Engineering | May/June 2024 The CLiPHVent for relieving pressure in a sealed battery pack (Image courtesy of Parker Hannifin)
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