A Novel Materials Approach to EV Battery-Box Design

CSP readies its new multi-material battery enclosures for 2021 production.

A cutaway of CSP’s battery box, which uses the supplier’s standard 50% vinyl ester grade of SMC. (CSP)

A battery enclosure that features a single-piece, metal-reinforced composite tray and one-piece composite cover is a step closer to an electric vehicle (EV) production application. “We’re currently in pre-production with our multi-material enclosures and anticipate production launch on a new vehicle in late 2021,” said Mike Siwajek, vice president of R&D for Continental Structural Plastics (CSP).

An offset view of the CSP battery enclosure covering tray with batteries shown inside the box. The image also shows the aluminum reinforcement. (CSP)

“Several leapfrog advancements are provided with our battery enclosure technology,” he said during a December online presentation from the supplier’s 47,500-ft2 Advanced Technologies Center in Auburn Hills, Michigan. CSP is North America’s largest manufacturer and molder of composite materials. The company has produced more than 30 different composite battery-box covers for EVs in China and North America, including the Chevrolet Spark EV.

The move from supplying battery box covers to fully assembled, multi-material battery enclosures is in full swing. CSP technical specialists are prototyping 1.5 x 2-meter trays and covers that are “about the size of almost every vehicle manufacturer’s battery box,” noted Hugh Foran, CSP’s executive director of new business development. The composite trays and covers are made from production-grade molds that are chrome-plated for wear durability. The material’s charge pattern is cut from a slither table, while a 4,000-ton press with leveling and vacuum capabilities completes the in-house production process.

CSP presently has four different composite material options for the cover and the tray. There are different material options for electromagnetic interference (EMI) and radio frequency interference (RFI) shielding systems. The company has pending patents for box assembly and fastening systems. CSP also has patents on its frame technology relating to crash performance.

An exploded-view graphic of CSP’s battery box concept. The CSP multi-material battery enclosure can be certified prior to shipment. (CSP)

“For crash worthiness, we’ve experimented with varying woven materials and types of glass in different percentages. And we’re using our in-house design capabilities to optimize the battery pack’s structural frame design,” Siwajek explained.

Most of the EV industry’s battery trays are made entirely of metal and can weigh more than 1,000 lb/454 kg including the batteries. CSP’s lightweight composite counterpart, reinforced with aluminum and steel, has advantages. “We can mold in attachments and sealing grooves and do unique design geometries to make it a leak-proof, sealed system,” Siwajek explained.

Unlike metal trays that require multiple welds and fasteners, the CSP one-piece composite tray has no through-holes. “We can incorporate molded-in cored holes,” Foran said, “which are not through holes, and we can use hi/low threaded fasteners to assemble attachments.”

In case of thermal runaway, high temperature resistance is a critical requirement for battery boxes. To recreate a thermal runaway event, CSP worked with an automaker and a third-party testing lab. Their novel thermal runaway test involved a single lithium-ion cell, housed inside a cylinder, rupturing after a short circuit. “That rupture focused the blast reaction upward and into a sheet molding compound (SMC) panel, but the composite panel retained its integrity and withstood the blast that produced a temperature of more than 1500 C,” Siwajek said.

CSP’s materials R&D team has formulated multiple proprietary materials to address thermal issues. Each of the material chemistry options including a phenolic system can be used with different fiber types or formats (including glass/carbon/blended, chopped and/or continuous), and can be formulated to meet volatile organic compound (VOC) requirements.