Lightweight construction Functionally integrated lightweight construction in electromobility saves CO2

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Fraunhofer-Institut für Werkzeugmaschinen und Umformtechnik IWU

Wuxi InfiMotion Technology Co., Ltd.

The aim of the industry and research partners in the project CO2-saving lightweight solutions for the next generation battery housing demonstrator, or Cool Bat for short, is to manufacture housings for battery systems in electric cars in an environmentally friendly way and thus contribute to an improved CO2 balance of the vehicles. The goal is to make the central component of the electric car lighter and save 15 percent carbon dioxide during its production.

In the project, 15 partners are conducting interdisciplinary research on innovative lightweight design principles for mass reduction, lightweight materials, and production processes that are intended to help produce battery system housings in an environmentally friendly way with better functional properties. The partners pursue a broad-based approach that focuses on aspects such as recyclability and reparability, resource and energy efficiency, safety, and fire protection at the design and material level. The Fraunhofer IWU in Chemnitz coordinates the project, which is funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) as part of the Technology Transfer Program Lightweighting (TTP-LB) and supervised by Projektträger Jülich (PTJ).

By implementing the housing cover in fiber composite construction, we were able to reduce the mass by more than 60 percent compared to the steel reference.

Rico Schmerler

The principle is simple: the lighter the housings, the more the range of electric cars increases, as power consumption decreases. "The energy density of today's battery systems, to which battery housings contribute significantly, can still be significantly increased. By integrating new lightweight construction methods and more functions in a smaller installation space with fewer interfaces, weight can be reduced and at the same time a 15 percent reduction in CO2 can be achieved," says Rico Schmerler, project manager and scientist in the Battery Systems Department at Fraunhofer IWU. "By reducing the mass, we increase the energy density and thus the range with the same number of battery cells. By implementing the housing cover in a fiber-reinforced composite design, we were able to reduce the mass by more than 60 percent compared to the steel reference."

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Cooling and load-bearing capacity integrated into one component

Another possibility for weight reduction, the researchers see, is in the combination of individual systems in the housing, which had previously separately handled thermal and mechanical tasks. For example, thermal channels are directly integrated into load-bearing structures such as crossbeams—manufactured by casting technology at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM. Additionally, the function of the cooling unit is combined with that of the underride protection in one component, the floor plate.

For energy absorption during stone impacts and accidents, an aluminum foam embedded in the floor plate is used. It absorbs a large portion of the energy generated upon impact. In conjunction with a phase change material (PCM), a type of wax that can store and release a lot of heat and cold energy, the aluminum foam further reduces the energy effort for cooling the electric battery. The floor plate was developed by Fraunhofer IWU and the company FES/AES and manufactured including the foam at Fraunhofer IWU. This way, the battery cells are protected against mechanical loads as well as overheating.

We rely on functionally integrated structures. Tasks for which various modules within the battery were previously responsible are integrated into one component—in this case, the floor assembly—thus saving installation space and interfaces.

Rico Schmerler

In this process, a fluid flows through the channels, and tempers the cells not only from below but also from the sides. This reduces the electrical consumption for cooling the cells, and cooling elements elsewhere in the car can be dispensed with. "We are focusing on function-integrated structures. Tasks that were previously the responsibility of different modules within the battery are integrated into one component—in this case, the underbody—thereby saving installation space and interfaces," explains Schmerler. "The floor plates will protect against overheating in the future and prevent damage to the battery core in accidents." The reference and technology carrier for the researchers is the Mercedes-EQS battery.

New thermal conductive mats replace pastes

The quality of heat dissipation from batteries towards the outer housing has a significant impact on the performance and lifespan of an electric vehicle. Typically, the battery module is thermally connected using conductive pastes. In the project, the heavy, non-sustainable pastes are to be replaced by environmentally friendly thermal conductive materials. For this purpose, the Fraunhofer Institute for Surface Engineering and Thin Films IST is metallizing open-pored, reusable foams using a plasma process, which are then inserted as mats into the spaces between the battery and the housing.

Improved fire protection through bio-based flame retardant coatings

A new fire protection coating, developed by the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI, provides increased safety. Applied to the underside of the housing cover, it prevents the spread of fire that can originate from the battery cells below. A component of the coating is the bio-based material lignin, which substitutes petroleum-based materials and is non-flammable.

Design for Reuse: Housing cover made of steel replaced by fiber composite cover structure

The previous steel housing cover was replaced by a new fiber composite cover structure made from carbon and resin—known as towpregs—which not only resulted in a significant reduction in mass but also allowed for the cover's reusability. The system of cover, frame, and floor plates was designed to be separable and disassemblable down to the component level without destruction. "We are following the idea of a circular economy and material reduction through lightweight construction and reusable materials, which in turn results in a lower CO2 footprint and lower costs in case of repairs," says the engineer.

The diverse project results are intended to be transferred to other applications and industries later on, where large batteries are used—such as in trains, airplanes, and boats. The cooling systems could be transferred to food and medical transport, and the fire protection solutions to buildings.