The controversy over whether combustion engines, fuel cells or electric vehicles are the better choice was steered after much discussions and perhaps also through political decisions derived from them – at least for the time being. And when market-leading OEMs clearly choose a battery strategy, this gives the market an extreme dynamic.
It is exciting to see how electromobility is constantly evolving in almost every detail. Hundreds of thousands of engineers and scientists worldwide are working to optimise the systems. The battery is the detail of electromobility which is crucial for the ecological and economic suitability of this type of drive. And due to its range and ease of charging, the cost-intensive battery becomes the key decision-making criterion for buyers for or against electric vehicles.
Crash safety and fire protection are two of the most important requirements that every manufacturer, regardless of design strategies, must meet. The joining technology has the task of durably connecting the battery components, even if they are made of very different materials. The challenges for joining technology are assessed below using three application examples from the fastening technology specialist EJOT.
A large number of joining processes are used in battery systems, detachable and non-detachable. The first application shows the possibilities of detachable screw connections in the area of the battery box cover. A variant of the EJOT FDS® screw is used here, a flow-drilling fastening element for detachable high-strength sheet metal connections which usually does not require any component preparation, such as pre-drilling or punching. FDS® screws have proven themselves millions of times in body-in-white construction. Due to the increased thread engagement in the formed draught, a high-strength screw joint without any undesirable swarfs is created.
The special feature of this battery cover screw connection is the pre-assembled embossed aluminium sealing washer, which ensures an under-head seal at the cover. The sealing function is given by the surface of the aluminium washer. In this application, pre-drilling of the clamping part is indispensable. Thanks to the embossed assembly of washer and screw, the components are rotatable and captive. Protection of the battery interior from moisture and dirt is ensured and the screws can be loosened for maintenance purposes without any problems. An additional sealing of the screw head is not necessary. An important criterion for the OEMs, the global availability of the joining technology, is guaranteed by the manufacturer and its licensees.
The second example from the field of battery systems is the EJOWELD® process, friction welding with a friction element. This process offers the possibility to join composite material with robot-controlled friction welding equipment without any pre-treatment. The battery frame box has a base which, in this example, is made of aluminium for weight reasons. In a multi-stage installation process, the aluminium is penetrated, the surface of the steel material is activated, the friction element as well as the base sheet are plasticised and finally the welding is formed. There are no intermetallic phases and a tight joint, since the lower joining material is not penetrated. And since the production of the entire battery system is ultimately measured by its CO2 footprint, this technology offers significant advantages over the alternative method of resistance spot welding due to its low energy balance.
In the battery sector, housings made of extruded aluminium and shells made of cast aluminium are common in the market. Direct fastening in light metals is particularly suitable here. With the ALtracs® Plus, EJOT has a self-tapping screw in its product portfolio which was developed for maximum strength values in light alloys and is thus perfectly suitable for this application. This innovative fastening element can be directly fastened into cast holes and hence saves time and costs. With the ALtracs® Plus, attachments such as plugs or battery box elements are fastened to the cast structure. Further applications, such as the dynamically reliable fastening of the battery covers or the battery packs, have already been implemented or could be realised.
In summary, it can be said that almost all connection and joining technologies can be found in current battery systems. The special requirements on battery technology regarding fire protection, crash safety, watertightness or high-voltage technology, with its strict safety regulations, demand the highest level of assembly reliability and performance from the joining technology. Due to the short development times of new battery generations, the manufacturers must be able to quickly test new, robust joining technologies and the corresponding assembly systems for the relevant material combinations and to make them ready for series production worldwide.
Author: Heinrich Georg Homrighausen
Head of Business Development Industry