Dr Joachim Döhner, vice president of Kuka’s battery business unit and chairman of the board for VDMA Battery Production, argues that laser welding is the most effective way of making electrical contacts in batteries
Within only a few years, electromobility has evolved from a futuristic idea and an exotic concept for idealists to being a technological revolution that is massively changing the automotive industry.
While the overall implications for the supply and distribution of electrical energy, total CO2 footprint, raw materials required for battery cell production, and aptness for typical everyday mobility use cases are still the subject of passionate discussions, one thing is certain: electromobility is happening today, and it is here to stay.
In Chinese mega-cities, battery-powered electrical buses, taxis and scooters are no longer special, but have quickly become standard. Even for the traditional combustion-based automotive suppliers, hybrid concepts leveraging batteries and electrical drives are the only remaining possibility to significantly reduce the consumption of combustion engines.
The battery of a hybrid or full electric vehicle is the most critical component in many dimensions. For example its cost determines the economic viability of the vehicle, its capacity defines the range of the vehicle – and thus its viability for specific use cases – and its reliability and lifetime are critical for acceptance by customers.
Today, a typical electric vehicle battery offers a capacity of 50-100kWh, corresponding to a range of 400-500km. It also operates in a high voltage regime of typically 400-800V and can supply or absorb up to several hundreds of kilowatts of power. An electric vehicle battery system is assembled by connecting many individual lithium-ion battery cells, each yielding some 3.6V and having a capacity of – depending on the cell technology used – several tens to several hundreds of watt-hours.
Making electrical contacts in these cells, between each other and/or to the current collector structures, is one of the key steps in the assembly process, and it must cope with several challenges to produce a high quality and reliable battery system:
- To supply the high voltage levels used to minimise currents, typically 1-200 cells are connected in series. A chain is only as solid as its weakest link, so one bad joint can compromise the power output of the complete string, making a robust and reliable welding process a must.
- During charging and discharging, a battery system may be exposed to power levels of up to 500kW or even more. Depending on battery design, the current in each string of cells can thus reach several hundred amps. To avoid energy loss – and thus unwanted heating of the cells – the contact resistance of each electrical joint must therefore be extremely small.
- With a trend towards higher battery capacities, a battery system may be comprised of several hundreds or even thousands of individual cells. Contacting each positive and negative pole by one or even two welded connections implies a massive number of welding points – in the extreme case several tens of thousands – requiring a fast and cost-efficient welding process.
- For certain battery designs, it is even necessary to perform intermetallic electrical joints, for example between the copper and aluminium electrodes of pouch cells.
- Much like humans, battery cells have a ‘feel-good’ temperature range: if too cold, performance is hampered; if too hot, they can get damaged or their life expectation is reduced. Therefore the welding process must minimise heat influx to the cell.
- Battery systems in vehicles are subject to mechanical stress resulting from thermal cycles, mechanical vibrations or shocks. Consequently, the welding process must provide a mechanically solid bond.
- From a production perspective, the process technology used must of course be cost efficient – both regarding investment and operational cost – provide a reliably high availability and, of course, must be safe to use.
Comparing available welding technologies, laser welding is the clear winner in the competition for ‘best battery contact welding process’. It is functionally superior to other processes used to contact battery cells. For example, resistance welding (gap welding) is a well-established and economical process, but while having an acceptable heat impact only supports small contact areas and is thus not suitable for high-power applications. Electro-magnetic pulse (EMP) welding, on the other hand, supports large contact areas and also intermetallic joints, but the large forces involved are a problem for the mechanical integrity of cells. Lastly, ultrasonic welding is sensitive to surface quality and limited in surface contact area. Alternative joining technologies, such as clamping or gluing, are generally of interest and are the subject of research, but are so far not established as a reliable process.
Meanwhile, laser technology is still constantly improving and also allows specifically challenging joining tasks to be accomplished, such as the contacting of thin electrodes by welding through thicker layers of the same or different material.
The two most fundamental key advantages of laser welding over other technologies are:
- Power density: the simple trick to minimising heat ingress into the welded object is to be so fast that welding is accomplished before the energy can dissipate into the surrounding material. With a very well localised average power density in the order of MW/cm2 on the metal surface, lasers can melt and weld material extremely quickly, unlike any other of the technology candidates mentioned above.
- Remote effectiveness: in high-volume applications, the physical positioning of the work piece towards the welding tool is typically on the critical path, because the acceleration and deceleration of significant masses consumes time and thus leads to reduced utilisation of costly process equipment and increased cycle times. Laser technology, however, allows the necessary relocation cycles to be minimised, as extremely fast-moving mirrors – scanners – can be used to redirect the laser beam from one spot to the next.
Within the wide spectrum of available laser technologies, more recently single-mode fibre lasers have become increasingly popular for battery contact welding applications. These lasers are characterised by an extraordinary beam quality and consequently extremely small focal areas – down to a diameter of about 20µm – and very narrow beam opening angles, and could therefore be considered the ‘scalpel’ in the laser family. They allow even better control of the weld line width and depth – and thus implicitly heat ingress/power consumption and joint quality. Figures 1 and 2 show the type of joints that can be realised using such lasers. Furthermore, they also enable the scaling of the joint cross section for power applications by modulating the weld line – also known as ‘wobbling’ – until the required contact transition resistance is achieved.
Figure 1: Cross-section of a laser weld, 1.2mm aluminium onto 0.2mm copper. (Image: Kuka)
Figure 2: Cross-section of a laser weld, 1.6mm aluminium onto 0.4mm aluminium. (Image: Kuka)
Laser welding technology has already become indispensable for battery assembly and thus electromobility. Also, it is not uncommon that the emergence of improved joining technologies leads to new product designs leveraging these possibilities for better performance – the same is to be expected in the regime of e-mobility as well.
Technology, however, will not stop here: research and development is focusing on, for example, further improving laser sources and accessing additional wavelengths, which will enable the optimisation of laser applications for use on specific materials or alloys with specific light absorption characteristics.
Of course, the selection of the right welding technology is only one important aspect; equally important is the process implementation, its integration, and its automation using specialised and customised fixtures. Therefore, a close cooperation between laser equipment suppliers, research institutions as well as experienced integrators in the field of battery assembly – such as Kuka – is key for setting up a successful volume production for quality battery systems, enabling electromobility to become a key contributor to greener and cleaner mobility concepts.
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