We’ve covered a variety of battery welding related topics in this blog space - not surprising, given the importance of batteries in today’s world. This post, however, will narrow the discussion down to laser welding the tabs to the terminals for a battery pack.
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 0.015-inch (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 0.01-0.02-inch 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 this application with a fiber laser is making sure that (a) the foils are in close contact and (b) you’re using a pulsed laser to avoid overheating.