Why Lightweight Materials Matter in Electric Vehicle Battery Pack Design
Improving Battery Pack Energy Density through Composite Materials
The automotive industry has undergone a significant transformation, with a notable shift of focus from traditional high-performance engines to the competitive realm of e-battery systems for cutting-edge electric vehicle (EV) technology. This change has led to a dynamic landscape where innovation in electric propulsion systems plays a central role, and renowned car manufacturers now emphasize the expertise of e-battery systems.
In this new era of automotive competitiveness, battery pack energy density, measured in watt-hours per kilogram (Wh/Kg), is the new metric for defining powertrain performance. Leading EV companies recognize the importance of a highly integrated structural housing for the battery enclosure, moving beyond a mere containment box to contribute to the overall vehicle’s performance. Original equipment manufacturers (OEMs) are competing in designing and manufacturing battery packs with integrated battery cells to achieve the highest pack energy density. Various components, such as battery cells, relay boxes, connectors, modules, management systems, and packaging solutions, contribute to achieving optimal energy pack densities. This article explores how metal-to-plastic conversion for these components using engineered polymers and polymer composite materials plays a crucial role in enhancing battery pack energy density due to their excellent mechanical properties at low weight.
The Battery Enclosure – More Than a Box
Today’s battery enclosures have evolved not only to house the vehicle’s battery system securely but also to seamlessly integrate with the vehicle’s structure, enhancing both safety and performance.
One of the main functions of the enclosure is to safeguard the battery from external impacts, such as road debris or accidents. This helps prevent damage that could compromise the battery’s integrity, thereby ensuring the continual operation of the vehicle and reducing the risk of unforeseen mechanical issues.
The battery enclosure also provides crucial fire protection. Batteries, particularly lithium-ion batteries commonly used in EVs, can pose a fire risk if damaged or malfunctioning. The enclosure is designed to contain any potential fire within the battery system and prevent it from spreading to other parts of the vehicle, thereby protecting both the passengers and the vehicle itself.
Additionally, the battery enclosure is instrumental in shielding the vehicle from electromagnetic interference (EMI). This interference can come from numerous sources, such as power lines, other vehicles, and even the vehicle’s electrical systems. By creating a barrier against EMI, the enclosure helps to prevent disruptions or malfunctions in the vehicle’s electrical and electronic systems, thus contributing to reliable and smooth operation.
The battery enclosure comprises a cover, inner support structure, cooling system, and the enclosure base (Figure 1). When designing these components to satisfy a range of unique mechanical performance requirements for these energy containment devices, automotive designers should consider the following:
- Cover
- Minimize vibration
- Support of external loads
- Reduce risk of thermal runaway
- Support Structure
- Secure battery modules to the enclosure base
- Satisfying dynamic loading
- Enclosure Base
- Support battery weight and g-loads
- Survive crash shock loads
- Provide vehicle torsional stiffness
- Provide underbody impact protection from multiple impact conditions
- EMI Shielding to protect sensitive electronics
- Fire Protection to protect occupants from a thermal runaway event
Figure 1
As the battery enclosure has evolved to be an integrated structural component of vehicles, the mechanical performance requirements have increased. Engineered polymers and polymer composite materials are uniquely well-suited for meeting the design needs while providing increased mechanical performance as well as superior specific strength (MPA / g/cm3), as seen below in Figure 2.
Figure 2
Continuous Fiber Reinforced Thermoplastics
Continuous fiber reinforced thermoplastics are composite sheet materials that have many benefits for designing enclosures to achieve high battery pack energy densities. The materials are used to design large enclosure parts typically made from aluminum sheets (Figure 3). With dimensions of approximately 6 ‘X 3’ (1.8m x .9m), cost-effective glass-reinforced composites can offer up to 40% weight savings over aluminum, which can reduce enclosure weight by nearly 78 pounds (35.4 kg). However, these savings are strictly based on material density. By leveraging the advantages of composite materials from design conception, engineers can develop highly energy-dense battery packs.
Figure 3
The battery enclosure’s components have varying functional and structural requirements. Certain areas may need higher functionality, while others may require higher structural strength. Continuous fiber reinforced thermoplastic composite materials can have their properties customized based on fiber type, fiber content, fiber orientation, and resin type to ensure efficient material use, contributing to weight reduction and cost-effectiveness. Additionally, thermoplastic composites can be combined with other functional materials, such as conductive veils and metalized materials. This combination can take place before the composites are formed into EV components. This enables simplified handling and molding of these specialized materials and also allows properties such as EMI shielding and fire protection.
Metallic materials such as aluminum offer vehicle integration challenges requiring cuts and welds to fit into the design space. For example, the battery pack sits under the chassis and is directly exposed to the road surface. The enclosure base must conform to complex geometries for vehicle integration. Unlike metals, composite materials can be molded to these geometries while maintaining uniform material properties. Furthermore, the enclosure is exposed to harsh road conditions, including moisture, salts, and chemicals, which can cause metallic protective coatings to dent and corrode. Composite materials, on the other hand, are inherently more impact and corrosion-resistant than metal and can prevent damage and degradation of the enclosure. Finally, designers need to consider other material properties such as coefficient of thermal expansion (CTE) and thermal mass. Composite materials have a lower CTE than aluminum and a lower thermal conductivity, providing a more stable temperature environment for the housed batteries.
Traditional thermoset composites are a great choice for battery enclosures, but their long cycle times have prevented widespread adoption, especially in the automotive industry. However, thermoplastic composites are different because they can be heated and formed within minutes, which makes them suitable for high-volume automotive production. Additionally, the production processes for thermoplastic composites are very uniform, allowing for consistent part quality.
OEMs must also consider development costs, validation timelines, and tooling costs when deciding on the materials used in their products. Composite materials can be very useful during the prototyping phase, as they can be made quickly from soft tools, which can speed up development timelines. Lastly, the compression molding tools required for composites can be less expensive and easier to fabricate than the stamping dies and welding fixtures associated with metallic components.
Long Fiber Thermoplastics
Long Fiber Thermoplastics (LFTs) are engineered composite materials in pellet form specifically developed to be used in injection molding machines (Figure 4). These materials are a popular choice of designers to replace aluminum components due to their cost-effectiveness and ability to reduce weight. LFTs have high strength and stiffness, fatigue endurance, excellent performance at both high and low temperatures, and excellent chemical resistance. LFTs using glass reinforcement can offer approximately 40% weight savings on a material density basis, compared to aluminum. As mentioned, by considering these materials early in the design process, injection molded parts with complex geometries can be easily optimized for weight and performance.
Figure 4
Using LFTs for brackets and other components within the battery enclosure is beneficial because these materials can complement the composite enclosure. By working with materials that have similar coefficients of thermal expansion, designers can reduce unnecessary stress that may occur during the vehicle’s life. Depending on the joining method used, LFTs can be heat-staked or hot plate welded, potentially saving additional adhesive costs.
LFTs can also be a strategic material to reduce vehicle weight and cost further when used for ancillary components such as power electronic housings, cooling pump housings, cooler fans, and fan housings.
Switching from Metals to Plastics
Metal-to-plastic conversion offers many benefits, including reduced part weight, higher impact resistance, improved corrosion resistance, reduced secondary operations, and part consolidation. However, careful analysis is required at the design stage to achieve the full benefits available from engineered thermoplastic composite materials. Additionally, these materials can give manufacturers more design freedom when dealing with complex geometries. To fully benefit from thermoplastic composite materials, consider them as the material of choice from the start rather than merely as a one-for-one component replacement for cost and weight reduction in the future.
Figure 5
Avient’s in-house team of experienced industrial designers and project engineers use simulation technology (Figure 5) with conventional (isotropic) and advanced (anisotropic) modeling data from material characterizations, mold filling analysis, and finite element analysis (FEA). These virtual simulations can more accurately predict where an injection molded plastic part may crack or break when exposed to physical loading forces or a sudden impact, such as in a crash simulation. You can see the results and benefits of the predictive simulation technology in this case study. Additionally, the complex modeling software Avient uses can factor in temperature and humidity levels to further optimize part design and material selections, mitigate risk, improve performance, and reduce costs early in the development process.
Hank Crawford, Market Development Manager, Transportation at Avient Advanced Composites and PlastiComp
