The “gas tank” is about to get a lot more sophisticated. Electric vehicle (EV) battery trays are highly-engineered components that barely existed a few years ago but play a wide role in an EV’s safety, rigidity and weight – and therefore efficiency. They also need to be watertight. The enclosures typically reside within an EV’s wheelbase and often serve double duty as the vehicle’s underbelly, housing the complement of batteries, control and monitoring electronics along with cooling and high-voltage circuits.

Constellium, the Netherlands-based global Tier-1 supplier and aluminum specialist, recently invited SAE to speak with the project lead on its first OEM battery tray, a 2.5 x 1.4 m (8.2 ft. x 4.6 ft.), 70-odd-kg (154-lb.) enclosure featuring cast, extruded and sheet aluminum that will house a 100 kWh battery pack for a soon-to-be-announced EV. Well known for its aluminum expertise in engineered extrusions for crash-management systems, as well as aluminum structural components for high-volume vehicles such as the Ford F-150, the project is Constellium’s first fully engineered battery enclosure.

“We do currently supply certain components for battery enclosures,” Alex Graf, director of development for automotive structures – electric vehicles, said during an interview at Constellium’s offices in Livonia, Michigan. “But we have a full battery enclosure coming into production in the next six months with an OEM and the design was done by a team collocated at the OEM. I would say our task was to seriously and severely modify their existing design to make it manufacturable and come into target for pricing and performance.”

Lightweighting, for volume
Constellium’s long expertise with aluminum should see it involved in more EV projects, as the substance’s strength-to-mass and stiffness-to-mass ratios factor well into the EV's efficiency equation. “Now that we have exhausted most of the bumper reinforcements being in aluminum, we are going to more-complex structures that can save weight without having to go to a full-aluminum body,” Graf explained. “Things that can be purchased outside at a significant mass reduction without having to retool a body shop. These are bolt-on components. The figures for growth are quite staggering and I think the industry may be extrusion supply-constrained if we are not careful.”

An EV’s battery enclosure is a prime, bolt-on lightweighting opportunity. “In general, aluminum boxes will have a 30-to-40% advantage to a steel box in terms of weight. When you say 30 to 40% of the weight of a fender, you say 30% of three to five kilos is one kilo to a kilo and a half,” Graf said. “When you look at a battery enclosure, it's 70 to 80 kilos, and 30 to 40% makes a big difference. So battery enclosures are one chunk of a bolt-on structure that can be outsourced from the OEM to suppliers like us.”

Mass reduction not only improves EV range and performance, it is also becoming part of the regulatory landscape. “We are monitoring the Chinese regulatory system. They have an incentive for range, but there is [also] an incentive for kilowatt hours per kilometer. That is basically a button to push the technology, because you can just fill the trunk with batteries and you're going to get the range. The kilowatt-hours-per-kilometer component rewards the efficiency of the overall system, and mass plays a big role.”

If EVs are to become volume products, ease of manufacture of their components will be crucial. “If you look at an aluminum fuel tank on a truck, they don't make very complicated tanks for those TIG-welded [components]” Graf offered. “It's either a cylinder, or at the most it's going to be a rectangle with very rounded corners. They want easy welds to ensure leak-free components. In a corner, you have to design the joint to make it easy to weld. That was our task.”

Safety angles
“You get a significant contribution of the battery pack to the global stiffness of the car,” Graf said of a well-engineered battery enclosure. “They're 120 millimeters tall and an expensive commodity compared to other materials, so you want to put every gram of it to work every moment.” Beyond positioning batteries, another key functional aspect is safety.

“One function we need to take into account is intrusion, managing of damage from the bottom, which is purely penetration, a ballistic-type of approach,” Graf said. “The other is side crash, which is a combination of what does the car structure do versus what is the battery box contribution to the overall crash deformation? The goal is to limit the damage to the battery box… not to cut the cooling line, not to cut the high voltage line and not, of course, intrude on a module itself.”

The keen engineering comes into play via managing and absorbing crash energy while still protecting the battery box and the vehicle’s occupants. “You can make the thing where we're going to put out a block of aluminum seven centimeters wide, three, four centimeters tall, solid and it will stop the side pole, but it will break your neck because of accelerations,” Graf explained.

Aluminum expertise
Graf, whose education includes degrees in mechanical engineering and a PhD in materials, has worked for the Dutch concern for more than 20 years and described his role on the OEM battery tray project as a “traffic director,” with overview of the 3-year-plus project and directing or pulling resources from where they were most needed. He noted Constellium’s extensive experience in aluminum as one of the real advantages for their OEM clients.

“It helps on the alloy selection. It helps on the shape selection. Because shape means how easy or complicated it will be to push an extrusion,” Graf said. “How easy or complicated it will be to push means how tight or wide your manufacturing tolerances of that extrusion are going to be. There is always a balance between how complex you want to make a component to integrate the most functions, versus will you get someone to make it consistently the way you want it?

“You wouldn't dream of designing a car fender in extrusions,” Graf continued. “On the other side, I wouldn't dream of designing the transverse members of a battery enclosure in sheet. You want to extrude them, because you get closed sections with multi-chamber performance in one push, with full thickness optimization. That is where our expertise comes in.”

Wait, waterproof?
“Most design engineers come from the structural world, so things need to be strong, durable and stiff. But then when you put the things together, they leak,” Graf explained, noting that EV battery enclosures, “have to be IP66 [rated], which is basically submersing the enclosure under one meter of water for 30 minutes, and it needs to take on no more than 3 cc of water. Working with extrusions, it’s not easy to come up with a leak-proof design.”

“We had to develop testing techniques for the leak-proofness of fasteners, of rivets, of rivnuts. Anything that has to do with leak-proofness, we manufacture,” Graf said. “Even though it would be much easier to have a supplier put in that fastener – because it would save one station and two or three guys in our plant – we want to do it because we want to control all those things that contribute. We will be providing fully leak-proof battery enclosures.”

A nice engineering problem
Graf noted that the world of EVs and the creation of a new components such as battery trays was an opportunity and new challenge to apply expertise. “We got to a point that we could engineer a crash-management system in under two weeks,” Graf said. “The OEM gives us the design package – location of the rails, barrier heights – and two weeks after, we have a design. Weight, cost, everything.”

“On a battery enclosure, you really have to work at engineering to solve all the little problems, because not all the boxes are the same,” Graf said. “Some OEMs want to route the cooling through the center, a problem because it cuts in half all those nice crossmembers that we are putting there for strength and rigidity. Some OEMs want to put the coolant through the sides. Better for stiffness, but now we have a protection problem because on side crash, we can pinch those hoses.”

“So the concept is how do you design something that can take the impact, manage the energy, manage the acceleration, and protect the box. Everything at the same time, so it's quite exciting. We needed a lot of CAE, a lot of material design, a lot of local simulations and a lot of testing to optimize material performance. This is one of the most advanced pieces of engineering that I’ve ever worked on for the company,” Graf said, smiling. “It's a very nice engineering problem.”