The battery pack is one of the most delicate and expensive elements of an electric car. The aluminium case guarantees lightness and shock resistance, combined with the high thermal conduction needed for the battery temperature management system
The electric car seems poised to be the future of the global automotive industry. Actually, there are still several obstacles which slow down the complete success of this technology. The main obstacles to this development are essentially related to the costs of battery production and subsequent battery replacement, charging times for fully electric cars, charging costs which may be compared to the costs of LPG, disposal of batteries at the end of the cycle and finally the fact that most of the electricity produced still comes from fossil fuels. All these critical points may, however, be partly overcome, and the support of the European Community and that of local governments is very important in this respect, allowing, thanks to incentives, a rapid increase in sales of cars with “clean” engines. The negative aspects take second place to the need to contain CO2 emissions into our atmosphere.
Among all the materials used in the manufacture of electric cars, aluminium has for several years now increased its penetration because, apart from the lightening factor, it ensures reduced assembly costs, a guarantee of complete end-of-life recycling, durability and resistance to atmospheric agents and corrosion, easy assembly, lower maintenance costs and more. The sales forecasts for electric cars, conventionally subdivided into BEV (Battery Electric Vehicle), that is, exclusively electrically powered vehicles, and HEV (Hybrid Electric Vehicle), that is, hybrid cars, indicate a significant upward trend. Beyond the unpredictable stop in 2020 due to the Covid-19 pandemic, electric cars are expected to account for more than 30% of the total market share in 2030.
At present, the content of the various aluminium products used in the different car segments worldwide is summarised in figure 2, taken from a DuckerFrontier study published at the end of 2019. The data, processed by analysing a large number of models currently in production, show the average values of aluminium content per car subdivided into four main categories of use: rolling, extrusions, casting and forging. As can be seen, foundry castings (especially engine components and alloy wheel rims) are the most widely used format, with about 116 kg per car and a total of almost 2,000 kt produced in the EU in 2019. The development of electric cars, which use a greater quantity of light metal, will make it possible to increase the average use of aluminium per car, as shown in the forecast in Figure 4.
From these predictive data the increase in the use of extrusions and laminates can be immediately seen, with substantial stability for forgings. Foundry castings should reduce their percentage share, precisely because of the introduction of electric motors, which use less metal than traditional fuel-driven engine blocks. Let us therefore take a closer look at the batteries and their housing.
Aluminium in electric cars
Battery cases for electric vehicles (also called battery frames, housings or battery packs) have a rather simple purpose: to hold and protect the battery modules. Of course, they are available in various shapes and sizes and can be easily adapted to the different characteristics of battery modules.
Just like other car parts, battery cases provide a field for a range of materials that compete for supremacy: aluminium, high-strength steel, carbon fibre and magnesium. While many housings are currently made of steel (or a combination of aluminium and steel), in the long run aluminium seems to have the upper hand, mainly due to its light weight and favourable technical characteristics. Car manufacturers expect the cost of the battery system, which now accounts for between 30 and 50% of the total cost of an electric car, to fall in the future.
The properties which battery containers must possess
Thermal conductivity is of paramount importance for battery casings, because they must provide thermal transfer capacity (as in the braking system of a car) to keep the battery cool or keep it warm when it is cold. Therefore, the construction structure can significantly affect the performance and life cycle of battery modules, as variations in temperature from module to module in a battery pack could lead to a reduction in performance. At the same time, the material must have sufficient mechanical and impact resistance to protect the battery modules from damage, as well as other parts of a vehicle.
Hybrid electric vehicles (HEVs) are generally more demanding than battery electric vehicles because they generate more heat, which is why they must have a thermal management system. The thermal management system must be compact and light, easy to install in the vehicle, reliable and economical.
To control the temperature of a battery pack in the optimum temperature range, the heat from the modules must be reduced during the hot season and increased during the cold season. Thermal management and control of battery packs can be achieved through air or liquid systems for insulation or thermal storage with active or passive approaches or a combination of both. The combination of these requirements makes aluminium the ideal material for this use, due to its well-known capability as a fast thermal energy conductor.
Examples of design and construction techniques for battery containers
Good battery enclosure design uses aluminium extrusions to simplify the assembly process and fastening of the individual battery modules. Extrusions also allow better energy absorption in case of an accident, compared to other materials and processes. Large casings can affect the entire body architecture. In some models, the battery casings are as large as the entire car floor. This means that they must be carefully integrated into the car’s structure and must interact with the car body. Some of the leading manufacturers of aluminium for the automotive industry have developed their own groups of battery housings with thermal management systems, such as, among others, Constellium, Novelis and Hydro.
Constellium’s battery enclosures are manufactured from high-strength aluminium alloys and designed to withstand impact and at the same time cool the individual modules using innovative design and sophisticated joining techniques. For the lower and upper part of the enclosures, rolled products in alloys of the 3xxx, 5xxx and 6xxx families have been used, due to their good thermal conductivity and corrosion resistance, as well as their easy formability. A further advantage of the aluminium casing is its ability to create a natural electromagnetic shield, which helps to block interference with other electrical or electronic systems in the vehicle.
The enclosure structure produced by Constellium consists of extruded aluminium tubular elements. The cavity is filled with a phase change material, a substance which will absorb and release thermal energy during freezing or melting. It is a wax-like material which acts as a passive cooling system for a battery, whose properties allow the material to retain heat or “melt” to remove heat from the battery. By shielding the battery from extreme temperatures, the structure of the container helps to extend its service life.
The solution chosen by Novelis for the manufacture of the case involves the use of rolled recycled aluminium alloy from the Advanz™ series, a solution which also provides the necessary protection against impacts and the intrusion of road debris or stones. The alloy used has high formability and at the same time has good tensile strength and excellent energy absorption properties. The compartment is designed to accommodate 90 kWh battery modules, a size used in electric pick-ups or large SUVs; the design is also adaptable to batteries of different sizes. By using sheet metal instead of extrusions or foundry castings, the solution allows a good cost containment. Novelis has designed and supplied the recycled Advanz alloy, particularly for the Jaguar Land Rover I-PACE model for sale in the UK. The recycled aluminium supplied by Novelis, used in the car, is produced at the plants in Sierre, Switzerland, and Nachterstedt, Germany.
In partnership with Audi, Hydro Aluminium has developed a battery housing system for the Audi e-tron, the German manufacturer’s first fully electric model. The car uses a 95 kWh battery pack with a nominal capacity and voltage of 396 Volt consisting of 36 lithium-ion cell modules shaped as parallelepiped aluminium housings. 12 “pouch” cells are contained in each module, equipped with a flexible outer coating in aluminium-coated plastic material, able to guarantee a long life with higher power and energy density. Another task of aluminium in the new e-tron is to store the battery and protect it at the bottom by means of a special sheet metal. For the production of automotive profiles, Hydro used, among others, the new HS 400 AlMgSi high strength alloy, presented in detail in issue 1-2019 of A&L.
The future market for aluminium extrusions for electric cars
Bloomberg New Energy Finance estimates that the quantity of aluminium extrusions will reach 80 kg per electric vehicle in the coming years, much more with respect to the Ducker forecast, as shown in the graph in Figure 5, including the quantity required for battery casings. Considering the Chinese market, by 2040, the total amount of aluminium extrusions required for electric vehicles could exceed 3 million tonnes. For this reason, a large number of new extrusion presses will have to be installed worldwide to meet growing demand, and in parallel the heat treatment and finishing capacities required to provide automotive aluminium extrusions with the required quality and performance characteristics will have to be increased.
Competition on sustainable metals will continue between aluminium and steel, especially because the price difference between the two will decrease if aluminium housings are made from recycled aluminium. Extruded aluminium alloys are an optimal and sustainable choice for selection in battery housings, especially when properly combined with vehicle structures which previously reduced it in favour of cheaper steel. Aluminium consumption in electric vehicles is expected to accelerate in the coming years, also because Covid-19 has in fact accelerated the transition to a more environmentally friendly and sustainable economic recovery. Electric cars, with aluminium and its light weight, will allow the reduction of CO2 emissions in the air, and this will be a decisive factor to be taken into account for our “green” future.