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Battery Housings Flexibly Adaptable Lightweight Design

| Editor: Alexander Stark

Handtmann has developed a three-module system for high-voltage battery housings that can be easily adapted to the architecture of electric vehicles: Profile composite, profile-cast composite and die cast composite.

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In the event of a crash, the lateral honeycomb structure of the die-cast composite reacts with defined deformation and high energy absorption.
In the event of a crash, the lateral honeycomb structure of the die-cast composite reacts with defined deformation and high energy absorption.
(Source: Handtmann)

Housings based on the new modular system from Handtmann can save up to 100 kg in weight, depending on the model. The dimensions cover the usual housing sizes all the way up to 4 m2-units. If required, a battery housing can accommodate a payload of 500 kg to 700 kg of battery cells.

The three modules of the system are characterized by different material groups used for the battery housing. The frame for the first model, the profile composite, consists of extruded profiles which are mechanically welded. Extruded profiles are also used for the battery body of the second type, i.e. the hybrid design. In addition, there are highly integrated functional elements made of high-pressure die cast aluminum. They are welded to the supporting aluminum profiles produced by the extruder. The entire housing frame of the third type consists of a single die cast aluminum component. The bottom of the battery housing is made of rolled aluminum sheet, with or without additional cooling channels, as a standard for all three models.

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The Profile Composite

The extruded profiles for beams and side pod as well as the rolled sheet for the floor are characterized by their high strength. In the automotive industry, these materials are used because of their yield strength values of Rp0.2 >= 290 MPa and an elongation at break of A50 > 10 %, for example for crash boxes that must have a defined deformable structure.

The extruded or rolled material is virtually predestined for the battery housing of the future. Plain models can be produced rationally, the system costs are low, machine-made welding processes are a proven routine. However, the costs for such a design increase exponentially with the complexity of the assemblies, such as those required in the front or rear area of the housings for cable feed-throughs, cooling lines and the definition of fixing points for connecting the bodywork. To avoid the costs for assembly in large series from spiraling out of control, it is worth taking a look at the integrated hybrid design.

The Profile-Cast Composite

Only a few connection points are required for the hybrid design, which combines extruded profiles and die-cast aluminum for the housing frame. Cast parts are only used where an angled geometry and high functional density are required, e.g. for fastening points, feedthroughs for coolants and high-voltage lines. The extruded profiles in the frame result in closed side pods that feature a high moment of inertia and good rigidity. The adjustment of the overall length for extruded and rolled profiles allows easy scalability of the battery housing to different layouts and battery formats. Scalable battery sizes for different vehicle sizes and designs can therefore be implemented just as easily as with pure profile composites.

The Die Casting Compound

The high-pressure casting process allows to produce the entire circumferential frame of the battery housing together with all the complexly shaped functional parts in one piece. This architecture is firmly welded to the aluminum base plate. An advantage of the system is the possibility of die casting the side structure for the defined force absorption with a collapsible zone, for example with topologically optimized honeycomb elements.

For the payload of a set of battery cells weighing 500 kg, a cast housing weighing about 100 kg is sufficient for this type of design - provided that all welding and connection elements have been carefully designed. In a direct comparison, this results in a weight advantage of about 30 % compared to a conventional sheet steel construction, whereby the crash-proof design and the payload are retained as comparable values.

Only the dimensions set logical limits for the cast design. The closing forces for the mold would amount to astronomical dimensions for overly large housings with an area of several square meters, thereby exceeding the limits of what is technically feasible. For more compact housings, such as those needed for the energy storage of hybrid drives (HV) or plug-in drives (PiH) in the format of a normal suitcase, die cast aluminum is the ideal manufacturing process.

Each Design Approach Has Advantages

The obvious idea was to produce a combination of casting and extrusion structure for larger construction units, thus adding the advantages of both layout approaches. The combination of extruded beams (Rp0.2 >= 350 MPa, A50 > 10 %) with rolled sheet metal bottom for the housing results in an optimized welded construction, which offers significant advantages for lightweight design.

After extensive trials, Handtmann found that the most favorable solution was to opt for homogeneous designs. Such a prototype can be produced by machine welding with a high degree of automation. Extruded aluminum profiles function as load-bearing structures. If the front and rear housing areas can be designed in simple geometries, a pure profile structure offers distinct advantages: The housing can weigh as little as around 50 kg. In this case, too, the entire construction is designed for a payload of around 500 kg for the battery cells. Further improvements are also possible through targeted optimization of this system. However, this design approach for the battery housing requires a few special features. However, a high level of production know-how is required to integrate all connections for cooling, charging and traction current as well as the complete thermal management in a tight space.

Welded constructions soon reach their limits. In many cases, the design should be supplemented with cast structures in the area of the functional nodes - this leads straight to the hybrid model based on Handtmann's modular system.

Same Functional Scope

All three models cover the same functional scope. In all the solutions presented, the aggregates and lines for charging and traction current, thermal management and system control are structurally integrated. In order to be able to directly compare both the assembly effort and the system costs, the various elements were defined in advance:

  • Installation of the Battery Modules
  • Liquid cooling with connections for the coolant supply
  • Integration of the body connection points
  • Circumferential frame with Deformable Structure
  • HV plug connections and cable bushings
  • Bumper buffers and underride protection

Crash Safety of All Models Guaranteed

The so-called crash test at 200 kN as per standard GB/T 31467.3-2015 is indicative of a realistic load. The test simulates the resistance of the housing to a pile impact under defined conditions. The test must not cause any deformation in the housing in order to prevent short circuits of the cells.

Extensive tests at Handtmann have shown no failure in the load-bearing structure and no deformation of the inner geometry. The test showed that the cell compartments did not exhibit any plastic deformation. On the contrary, there are considerable safety reserves available. While the test requires about 200 kN force absorption, there is a potential buffer of about 30 %.

Weight Reduction of up to 50 %

With Handtmann's modular system for battery housings, it is easier than ever to adapt to a number of design specifications required for the current series of electrically powered vehicle. This includes the defined underride protection on the bottom as well as the central connections for the coolant, optionally for the variants with internal or external cooling. With all three designs, housing weights of 50 kg to 100 kg can be realized, depending on the design, which allow a weight reduction of 30 % to 50 % compared to sheet steel construction. The limits of the modular aluminum system have not yet been when it comes to dimensions.

This article was first published by konstruktionspraxis

Original by Dorothee Quitter / Translation by Alexander Stark

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