CO2-saving battery casings for electric cars

More charging power, more range, more climate-friendly – in the joint project COOLBat, researchers from the Fraunhofer Institute for Machine Tools and Forming Technology IWU are working with partners to develop the next generation of battery casings for electric vehicles. The aim is to make the central component of the electric car lighter and to reduce carbon dioxide emissions by 15 percent during its production. The project partners want to achieve this goal by combining individual systems, integrating more functions into a smaller space, using new heat-conducting materials and bio-based flame-retardant coatings.

The aim of the partners from industry and research in the project for CO2-saving lightweight construction solutions on the demonstrator Next-Generation Battery Housing Demonstrator, or COOLBat for short. This is because current battery housings, with structures for load distribution and temperature regulation, frames, lids and base plates, still offer potential for optimisation in terms of CO2-saving solutions. In the project, 15 partners are conducting interdisciplinary research into innovative lightweight construction principles for mass savings, lightweight materials and production processes that will help to make the production of battery system housings more environmentally friendly and improve their performance. The partners are pursuing a broad 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 is coordinating the project, which is funded by the German Federal Ministry for Economic Affairs and Climate Protection (BMWiK) as part of the Technology Transfer Programme for Lightweight Construction (TTP-LB) and supervised by Project Management Jülich (PTJ).

The principle is simple: the lighter the housing, the greater the range of the electric car, 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 space with fewer interfaces, weight can be reduced and at the same time a 15 percent reduction in CO2 emissions 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 designing the housing cover in a fibre composite, we were able to reduce the mass by more than 60 percent compared to the steel reference.’

Cooling and load-bearing capacity integrated into one component

The researchers see another opportunity for weight reduction in combining individual systems in the housing that previously performed thermal and mechanical tasks separately. For example, temperature control channels are integrated directly into supporting structures such as crossbeams – cast at the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM.

In addition, the function of the cooling unit is combined with that of the underride guard in a single component, the base plate. An aluminium foam integrated into the base plate ensures energy absorption in the event of stone chipping and accidents. It absorbs a large part of the energy generated during an impact. In combination with a phase change material (PCM), a type of wax that can store and release large amounts of heat and cold energy, the aluminium foam also reduces the energy required to cool the electric battery. The base plate was developed by the Fraunhofer IWU and the company FES/AES and manufactured, including the foam, at the Fraunhofer IWU.

The battery cells are protected in this way from mechanical loads and at the same time from overheating. A fluid flows through the channels and regulates the temperature of the cells not only from below but also from the side. This reduces the electrical consumption for cooling the cells, and cooling elements can be dispensed with elsewhere in the car. ‘We are focusing on function-integrated structures. We are integrating tasks that were previously the responsibility of different modules within the battery into a single component – in this case, into the floor assembly – thus saving space and interfaces,’ explains Schmerler. “In the future, the floor panels will protect against overheating and prevent damage to the battery core in the event of an accident.” The Mercedes EQS battery serves as a reference and technology platform for the researchers.

New heat conducting mats replace thermal paste

The quality of heat dissipation from batteries to 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 thermal paste. The project aims to replace the heavy, non-sustainable thermal paste with environmentally friendly heat conducting materials. To this end, the Fraunhofer Institute for Surface Engineering and Thin Films IST uses a plasma process to metallise open-pored, reusable foams, which are inserted in the form of 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 added safety. Applied to the underside of the housing cover, it prevents the spread of fire that may originate from battery cells located beneath it. One component of the coating is the bio-based material lignin, which is a substitute for petroleum-based materials and is non-flammable.

Design for reuse

The previous steel housing cover has been replaced by a new fibre composite cover structure made of carbon and resin – so-called towpregs – which not only led to a significant mass reduction but also to the reusability of the cover. The system, consisting of the cover, frame and base plates, was designed in such a way that it can be non-destructively separated and disassembled down to the component level. ‘We are pursuing the idea of a circular economy and material reduction through lightweight construction and reusable materials, which in turn results in a smaller carbon footprint and lower costs when repairs are needed,’ says the engineer.

Transfer to other industries in mind

The diverse project results should later be transferred to other applications and industries where large batteries are used – for example in trains, aircraft and boats. The cooling systems could be transferred to food and medical transport, and the fire protection solutions to buildings.

Partners in the COOLBat project are Auto-Entwicklungsring Sachsen FES/AES, INVENT GmbH, Compositence, iPoint-systems GmbH, TIGRES GmbH, LXP Group GmbH, Basdorf, Lampe & Pertner GmbH, MID Solutions GmbH, Synthopol Chemie Dr. rer. pol. Koch. GmbH & Co. KG, TRIMET Aluminium SE, Mercedes-Benz AG, the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM, the Fraunhofer Institute for Surface Engineering and Thin Films IST and the Fraunhofer Institute for Wood Research, Wilhelm-Klauditz Institute WKI.