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Inside Novelis’ state-of-art Gen-II battery enclosure, from the top: Aluminum top cover; advanced cell-to-pack battery system (green); s701 and s650 roll-formed AL profiles; simple modular extruded frame enclosure; structurally integrated cooling plate (blue), fire resistant AL bottom plate. (Novelis) 

Battle for the box

EV battery enclosures are a hotbed of subsystem design, materials innovation, and vehicle integration.

Whether you call them packs, boxes, or trays, the structures that envelop and protect EV battery cells and their supporting electrical and thermal-management hardware are among the industry’s top subsystem priorities. Optimizing the battery pack involves a host of manufacturing and material choices, mass and package tradeoffs, safety provisions, and structural design/engineering challenges, OEM and supplier experts told SAE Media.

“Do you want the battery pack bolted into the vehicle or integrated into the body structure?” asked Darren Womack, senior department manager, body and structures, at Magna’s global R&D group. Hot stamping, cold stamping, roll-forming, hydroforming, casting and steel, aluminum, composites and thermoplastics — are all raising “lively discussions” in pack development, he noted at a recent meeting of analysts.

OEMs clearly want to eliminate redundant structure to optimize package space and reduce mass and complexity, Womack noted. Integrating the EV battery into its surrounding platform involves various configurations. Cell-to-pack design, currently in production at BYD, does away with the intermediate module stage, putting the cells directly into the pack. Cell-to-chassis technology integrates the battery cell with the vehicle chassis, electric drive and thermal management. All battery components are housed in the vehicle body-in-white, eliminating the separate pack.

In such setups, under investigation by Tesla and others, the chassis pan and vehicle side structure double as the battery’s bottom plate and sides. In this marriage, impact integrity, accurate pack assembly, and robust sealing are paramount.

“Each OEM is going to want a ‘playbook’— a menu of options based on their criteria including cell form factor, battery size, and vehicle,” explained Mario Greco, director of strategy and marketing, Global Automotive, at aluminum specialist Novelis. “No single solution for battery enclosures is going to fit everybody.”

For example, a high-volume solution may be a stamped, single-piece enclosure. It might include an integrated cooling structure because the vehicle architecture lends itself to a cell-to-chassis design versus a cell-to-pack, he said. “I think we’ll see a convergence between the ‘skateboard’ and next-generation unibody architectures,” Greco noted, “because the OEM incumbents that are stepping into EVs have all-new unibody architectures that are designed to serve them for quite a while.”

Wide-open field for design
Despite the significant mass burden of liquid-cooled lithium battery packs, EV mass reduction improvements are still possible, according to Gregor Klement, global chief engineer, Battery Trays, at Magna. “Looking into the future, we see more and more integration of the battery into the vehicle body,” he said, with light metals and composites both playing a role. “Magna R&D is working on cell-to-chassis solutions, and we see customers looking in a similar direction. By eliminating redundant structural parts, we see opportunity for good improvement in weight and cost.” But he reckons battery weight will likely never be completely offset.

With vehicle electrification still in its nascent phase, much EV subsystems development is on the critical path, noted Jeremy Loveday, program engineering manager on Cadillac’s superluxury 2024 Celestiq. In this scenario, Tier 1s are offering both near- and long-range solutions for OEM evalution. DuPont’s 3-in-1 battery-box concept unveiled in late 2022 is a new example of modular design that consolidates cell cooling, electrical interconnection, and structural components. Its housing is made of the company’s Zytel HTN, a nylon-based polyamide capable of resisting high temperatures.

According to Frank Billotto, battery materials business-development leader in DuPont’s Mobility and Materials group, the concept provides semi-direct cooling (cells are cooled through their tabs) and easy assembly via electrical interconnections. He said the design enables batteries with greater energy density, improving vehicle range and package efficiency.

The “battle for the box” has kicked off a new wave of creativity among engineers and materials scientists. Roughly 80% of current EVs have an aluminum battery enclosure, but engineers are quick to note that the field is wide open for alternatives, based on vehicle type, duty cycles, volumes, and cost.

“I think we will see more lightweight-steel enclosures in the future, mainly on smaller, shorter-range vehicles,” offered Dr. Andreas Asfeth, technical director, North American Automotive at Constellium, a specialist in aluminum extrusions and sheet. He acknowledged steel’s “strong cost-competitiveness” and said the ferrous metal’s significant weight penalty versus aluminum isn’t a huge issue with small-vehicle batteries.

But in larger, long-range vehicles, “the battery represents the value of the vehicle. The larger the battery, the more aluminum makes sense for battery packs,” Asfeth asserted. Bucking that trend is GM’s 9000-lb. (4082-kg) Hummer EV, which uses a multi-material battery enclosure. Tesla also has reduced the amount of aluminum in the battery enclosure for the Model 3 and Model Y compared to what was used in its S and X models. And public statements made by the company regarding the structural battery pack expected to come from Tesla’s Berlin plant indicate the upper and lower covers are steel.

Aluminum rules – for now
Aluminum battery enclosures typically deliver a weight savings of 40% compared to an equivalent steel design. According to Asfeth, the alloys best suited for battery enclosures are the 6000-series Al-Si-Mg-Cu family — alloys that are also highly compatible with end-of-life recycling, he said. The current state-of-the-art solution for bottom plates is high-strength 6111 alloy in peak-aged temper, which reduces weight by 30% compared to the benchmark 5754 O-temper alloy, he said.

Looking intensely at vehicle impact requirements, Constellium is developing a “cost competitive” 4xxx-series alloy boasting 80-GPa E-modulus and 350-MPa yield stress, according to Asfeth. The alloy offers potential for a 40% weight reduction. He added that the 4xxx-series gauges and widths will be similar to 6000 series alloy and is compatible with conventional cold forming.

The company also has a 7075 T6 alloy in development. Asfeth told a Center for Automotive Research webinar audience that the material offers 500-MPa yield stress and 70-GPa E-modulus. Potential applications include battery-pack bottom plates where impact resistance is key. However, the new alloy requires special manufacturing processes the added cost of which might offset the 10% weight savings benefit. Such are the tradeoffs in battery-box and EV development.

Aluminum’s workhorse 6xxx-series alloy is used in two different advanced extruded alloys that underpin a recent Constellium dual-frame enclosure prototype. The inner frame (a second buttress to protect the cells in an impact) is in strength-optimized 6000 (HSA6 family). The outer reinforcement, designed as a crumple zone, is a ductile 6000 alloy, HCA6 family. Pack design could shift, however, if the industry moves to solid-state lithium batteries, Asfeth noted. “We may see some load-bearing function in the solid-state battery cells themselves and therefore less structural demand on the enclosure,” he said.

Molded packs’ promise
Suppliers of composites and plastics are undeterred by aluminum’s current dominance in EV battery enclosures. They’re developing new formulations and processes aimed at matching or exceeding the performance and cost-competitiveness of the light metal.

“Current battery packs use a lot of metal that is not optimized. They were designed using existing materials and technologies,” asserted Dhanendra Nagwanshi, global automotive leader, EV Batteries and Electricals, at thermoplastic giant SABIC. He argues that compared to aluminum, new-generation thermoplastics offer mass savings of 30 to 50% depending on application. They also offer equal or better impact performance, lower cost through simplified assembly, and less environmental impact than aluminum, he claimed. Engineers’ interest in thermoplastic EV battery trays began with GM’s 1990 Impact concept car. The EV-1 production car that followed used a tray made of glass-filled polypropylene (PP).

SABIC’s latest innovation aims directly at one of aluminum’s weaknesses — its very high thermal conductivity. Aluminum begins to melt at approximately 630 deg. C (1166 deg. F); temperatures generated by an internal thermal runaway can lead to battery fires as hot as 1100-deg. C (2012-deg. F). Aluminum also burns outright at high temperature, as evidenced in 1975 when the U.S. Navy missile cruiser USS Belknap collided with the aircraft carrier USS John F. Kennedy. The resulting fire effectively melted much of Belknap’s aluminum-intensive superstructure.

Nagwanshi noted SABIC’s development of new Stamax FR long-glass-fiber PP with “unique” flame-retardant properties. “When the material comes in contact with fire, it chars. The char then becomes an insulating layer,” he said. The company’s testing based on the UL2596 standard for battery enclosure materials demonstrated the plastic can withstand 1000-deg. C (1,832 deg. F) for 30 minutes—“a temperature threshold where aluminum would be perforating,” he noted. Global Technical Regulation No. 20 on EV safety (GTR 20), which aims to protect occupants during thermal runaway for at least five minutes, has been adopted in China and is coming to Europe and elsewhere.

SABIC currently produces a flame-resistant PP battery pack cover used by Honda in the China market. The cover eliminates thermal blankets, reducing weight by 40% vs. a similar metal cover.

Next-generation thermoplastic battery pack and module prototypes are in development. Rhode Island-based Tri-Mack Plastics recently showed lightweight, high-strength enclosures made from just eight plies of unidirectional carbon-fiber reinforced thermoplastic composite (TPC) tape, one millimeter (.040-in.) thick. The company’s process engineering manager Ben Lamm noted that the material, combined with Tri-Mack’s manufacturing process, offers new opportunities in part geometry, parts consolidation and integrated EMI countermeasures.

New twists on proven resin families and compounds are also aimed at the battery box. Among SABIC’s projects is an all-plastic EV battery tray with integrated cooling channels and crash protection elements. It offers up to 12% mass reduction compared with an aluminum pack, Nagwanshi claimed. Integrated plastic-metal hybrid structures based on the Stamax FR long glass fiber PP are also in the works. Testing has demonstrated the structures meet drop-test requirements. Engineers are encouraged by its ability to absorb “significant” energy, as would be required in vehicle side-impact tests.

Two more characteristics make thermoplastics competitive with aluminum for EV battery boxes, Nagwanshi said. One is their anisotropic thermal conductivity — plastics’ ability to simultaneously conduct/dissipate heat in one direction, while providing insulation in other directions. Metals, he noted, conduct heat, requiring EVs to employ thermal blankets—such as the mica sheets used by Tesla.

And with thermoplastics, “geometries come free,” he said. “Injection molding makes it easy to create honeycomb structures to provide strength where needed in the part. When you combine the geometry with a high-performance glass-fiber-reinforced resin, the result is structural performance that’s comparable with aluminum. And the CO2 emitted from making the thermoplastic part are about 10-15 percent lower than those of an aluminum part,” Nagwanshi stated.

He considers thermoset plastics such as SMC to be a competitor in some regards but criticizes the material type as “not optimized” due to the need for a thermal blanket, as well as its secondary manufacturing operations. “And you cannot recycle thermosets,” he asserted.

Hugh Foran might disagree with his industry colleague. As director of New Business Development/New Markets at Teijin Automotive Technologies (supplier of the C8 Corvette’s thermoset exterior panels), he noted that SMC regrind has variety of second-use applications. For the EV market, Teijin has become a major supplier of thermoset battery box top covers.

“We’ve got over 30 of them in production for various EVs,” Foran told SAE Media. Top covers are typically a fire-retardant-loaded polymer or steel sheet, which resist internal fire for longer periods than aluminum and provide crash protection. But the Japan-based supplier is looking beyond just pack covers.

“Our R&D group has developed SMC in five different formulations for battery boxes,” Foran said. One is “simple vinylester and glass fiber,” Another SMC features the fire-retardant ATH (aluminium trihydrate). Teijin also has intumescent SMC — materials whose surface provides a thermal and physical barrier to the underlying substrate. When they sense flame or heat, intumescents essentially create a ‘scab’ to shield themselves. Teijin also has been molding some prototype battery-box parts in phenolic resins that resist heat to 1100 deg. C and providing them to customers for testing.

According to Foran, another little-publicized issue that EV makers grapple with is battery-pack leakage. This occurs no matter the structural material, he said. To address this problem, Teijin developed a tray design that does not have through-holes, which require caulking and sealing. Instead, the mounting holes are cored in. Composite/plastic pack structures also feature molded-in, rather than welded-on, mounting points. “And one OEM requested stress sensors co-molded into the material, enabling them to know, at the vehicle level, any impact that has occurred,” Foran said.

Will the EV battery enclosure of 2028 be significantly different than that of 2023? “With some customers, we see the change to structural batteries and cell-to-chassis starting in the next couple years,” Magna’s Klement noted. “Others are in the concept phase. It’s not far away anymore, but I’m not sure everybody will go in this direction.”

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