33 Cell-to-chassis | Tech focus E-Mobility Engineering | May/June 2025 The CTP approach needs half the thermal interface materials (one rather than two), half the number of interfaces (two rather than four) and no module housing. These features greatly lower the thermal resistance of the stack, which reduces the load on the cooling plate and enables the use of gap fillers with lower conductivity. However, this does require more stringent environmental and mechanical performance requirements because a module housing is no longer available to protect the batteries. Numerous OEMs are now requiring thermally conductive gap fillers capable of maintaining strong, flexible bonds between polyethylene terephthalate (PET)-wrapped battery cells and aluminium-based cooling plates after 1000 hours (six weeks) of aging at 85 C and 85% relative humidity (RH). The PET is used primarily to provide individual prismatic cells with an additional layer of electrical insulation to prevent electric arcing. Thermally conductive urethane adhesives in a two-part form provide a good balance of strength, ductility and formulation versatility, but their use must consider the adhesive performance on PET and aluminium substrates with environmental ageing in mind. CTP designs require the adhesives adopted to maintain excellent adhesion under more stringent environmental Gap fillers, with the aid of a liquidcooled plate, help regulate the temperature of the modules to ensure safe and efficient performance. The upper gap filler fills in the large spaces or gaps between the lower sides of individual batteries and the gaps between the bottom of the batteries and the inside wall of the module housing, serving to firmly adhere the batteries in place while providing a continuous, thermally conductive pathway for heat dispersion. Such a gap filler, referred to here as a cell-to-module (CTM) gap filler, is often based on compounds such as urethanes that offer strong adhesion coupled with good flexibility that help absorb stresses. The lower gap filler plugs large spaces between the cell modules and the large cooling plate for the entire battery pack. This module-to-pack (MTP) gap filler also serves to conduct heat between adjacent interfaces. However, unlike the CTM gap filler, it is designed to adhere lightly to the cooling plate surfaces. The low bonding strength enables easy removal of discrete modules for serviceability reasons. Typically, MTP gap fillers are based on very compliant chemical backbones such as silicone or soft urethanes. The inactive portions of the module such as the housing, terminal plates, side plates, internal connectors and controls all add weight, occupy precious volume and ultimately translate to compromised pack energy density. Moreover, the many discrete parts increase the complexity of the design, manufacturing and supply chain logistics. Moving to the CTP approach increases volume utilisation from 15% to 50%, depending upon battery cell design, and reduces the number of parts by up to 40%. This not only greatly improves pack energy density, but also gives manufacturers the option to use less expensive, lower energy density cells given the extra space. Gap fillers help regulate the temperature of the modules to ensure safe and efficient performance Different cell formats need different materials (Image courtesy of HB Fuller)
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