Welded contact connections between the individual battery cells, for example, have proven to be more reliable,sustainable and above all cost-effective than bolted contacts or the use of bimetallic busbars.
The boxes of the rigid battery geometries are also welded, because they have to be gas-tight up to a pressure of 40 bar. These types of welding connections must be entirely maintenance-free. Electric cars should soon be able to drive in all climate zones, amid great temperature fluctuations and on all of the world's roads, whether good or bad.
Since each individual cell of a battery pack delivers just a few volts of voltage at high current, they must be connected in series to achieve a sufficiently high module voltage. The terminal contacts made of aluminum and copper are connected for this purpose by preference by welding. This is technologically challenging, as the resulting weld seam is more brittle the more that the dissimilar metals mix, as is unavoidably the case in conventional laser welding. Brittle weld seams – without a doubt – are simply unacceptable given the advanced requirements of electromobility. Therefor we have our Wobble technological solution.
The increased application for lithium batteries in electric cars and many electronic devices means fiber laser welding is used in the product design. Components carrying electric current produced from copper or aluminum alloys join terminals using fiber laser welding to connect a series of cells in the battery.
Aluminum alloys, typically 3000 series, and pure copper are laser welded to create electrical contact to positive and negative battery terminals. The full range of materials and material combinations used in batteries that are candidates for the new fiber laser welding processes.
Overlap, butt and fillet-welded joints make the various connections within the battery. Laser welding of tab material to negative and positive terminals creates the pack’s electrical contact. The final cell-assembly welding step, seam sealing of the aluminum cans, creates a barrier for the internal electrolyte.
Because the battery is expected to operate reliably for 10 or more years, these laser welds are consistently high quality. The bottom line: With the correct fiber laser welding equipment and process, laser welding is proven to consistently produce high-quality welds in 3000 series aluminum alloys that have connections within dissimilar metal joints.
With the current strong interest in energy storage, QCW lasers look set to play their role beside CW lasers. Welding of battery tabs at high speed using single laser pulses from a QCW laser is now well established. Dissimilar metal joints between aluminum and steel and even copper and aluminum have now been developed.
There are two approaches to achieving sufficient electrical contact in battery connections from laser welding:
- A spinning beam technique (WOBBLE) to produce spiral or small-diameter concentric ring welds
- A number of high-pulse-energy, single-pulse laser welds, one beside the other, on each tab
Using combinations of dissimilar metals produces welds that might not be recognized as such by welding metallurgists, but clever design of the modules to limit mechanical loads on these joints appears to have solved some of these issues. But Continuious Wave Welding with a wobble function stay far superior over the pulsed QCW if we consider the mechanical loads capacity: a circle has alway more contact than differnt points.
Most manufacturers are looking for ways to save money and improve efficiency without negatively impacting quality. Manufacturing operations that are using resistance spot welding can reap significant productivity and quality benefits in some applications by converting to laser welding.
Implementing laser keyhole welding can result in a welding process that is three to four times faster compared to resistance spot welding, while also increasing weld quality and repeatability. However, laser welding may not be the right fit for every job. Keep several factors in mind when considering if laser welding is the best choice for your welding application.
From a welding perspective, the most important aspects of tab welding are the thickness and material of both the tab and the terminal. Conductivity is the name of the game, so battery tabs are generally made of aluminum or copper, sometimes plated with nickel or tin. Terminals may be cold rolled steel, aluminum, or copper, depending upon the physical size of the finished battery.
The most common battery types are cylindrical lithium ion cells around the 18650 size (18 mm x 65 mm), large prismatic cells, and lithium polymer pouch cells. Each cell type has a different set of welding requirements.
The key to welding the cylindrical cell type lies in the negative terminal weld, where the battery tab is welded directly to the can as opposed to the separate platform on the positive side. The weld on the negative terminal must not penetrate the can thickness which is typically around 0.3mm. The thickness of the can dictates how thick the tab can be – a rule of thumb is that the tab should be 50-60 % that of the can. Cylindrical battery can material is usually nickel-plated steel, and the tab material nickel or tin-coated copper. Nickel plating is preferred over tin because it is more stable; tin’s very low boiling point can lead to weld porosity and excessive spatter.
Large prismatic batteries
These high capacity cells need thick tabs to ensure a sufficient current carrying cross-section to deliver the pack output. However, the tab connection needs only to deal with the capacity of a single cell. Therefore, thinning or “coining” of the thick tab material to enable a lap weld or creating a through hole for a fillet weld greatly reduces the size of the weld needed. This in turn reduces heat input to the can, which is always a concern when welding thicker tabs.
For a lap weld geometry, reducing the tab thickness to a 0.25-0.5 mm thickness enables sufficient weld area for strength and capacity while keeping the temperature during the weld low enough to avoid battery damage. Material selection is generally aluminum for both terminal and tab – recommended tab materials are 1080 and 1100. Avoid aluminum alloy 6061, which cracks when welded. If this material is already specified and cannot be changed, use a 4047 pre-form as a third material which will introduce a large amount of silicon into the weld, which prevents weld cracking.
Lithium polymer batteries
These pouch type cells, which are thin with a rectangular footprint, are really gaining traction for consumer electronics. The terminals on these batteries are made up of thin layers of copper and aluminum foil which are laser welded to tab of copper and aluminum respectively. This weld is traditionally made using ultrasonic technology due to the need to weld through a stack of foil, however, fiber laser welders are now being used for increased weld quality and strength.
The key to success in welding polymer batteries with a fiber laser is making sure that the foils are in close contact and you’re using a pulsed laser or even better a wobbling laser to avoid overheating.
Resistance welding is suited to welding nickel tab material up to 0.4 mm thickness, and nickel or steel clad copper tab material to around 0.3 mm thickness to a wide variety of terminal materials.
Laser welding is able to weld both thin and thick tab materials, with a capability of welding copper based or bi-metal tab material above and beyond 1.5 mm thickness
Although able to weld both thin and thick tab materials, laser welding is particularly well suited to addressing the needs of high power battery welding. The tab material used in the development of high power cells must be able to accommodate the associated higher capacities and power levels. In order to provide effcient energy transfer, a tab thickness of minimum 0.3 mm or greater is required, as is the use of more conductive materials. For high power lithium ion cells, the terminal material for certain battery manufacturers is different. Therefore the need for bi-metal and smart terminal design solutions is required. Defining the optimal tab material may require some development work both on the welding and material costing. In these cases, the laser is an invaluable tool that offers outstanding welding performance and ﬂexibility.
As the laser Welding process is non contact and the beam is steered by motion of the head, the welding speed is determined by the tab materialsa nd thickness and the terminal material along with the selection of laser power. As an example a 0.4 mm thick nickel plated copper tab can be welded to an aluminum terminal in 10 milli Seconds
As laser welding has no limitation on the proximity of the welds, the laser can place any pattern of weld spots on the tab or with the wobbling technology the wobble laser can weeld any diameter of circle according to strength requirements. If the weld strength of the joint is achieved, conductivity follows.
For more conductive materials, the weld area required for strength can be as much as 10 times that required for conduction. Although peel strength remains an important weld test, vibration is also important. As vibration strength places an emphasis on having good weld strength in any direction, the circle weld of wobble welding provides the solution.