Battery Welding: Using Lasers for Battery Lug Welding Applications

This article will narrow the discussion to laser welding of tags to the terminals of a battery pack.

From a welding perspective, the most important aspect of lug welding is the thickness and material of the lugs and terminals. Conductivity is the name of the game, so battery cells are usually made of aluminum or copper, sometimes plated with nickel or tin. Depending on the physical dimensions of the finished cell, the terminals can be cold rolled steel, aluminum or copper.

The most common battery types are cylindrical lithium-ion cells around 18650 size (18 mm x 65 mm), large prismatic cells, and lithium polymer soft pack cells. Each battery type has different soldering requirements.

Cylindrical Batteries

The key to welding cylindrical battery types is the negative terminal weld, where the cell is welded directly to the can, rather than a separate platform on the positive side. The weld on the negative terminal must not penetrate the can thickness, which is typically about 0.015 inch (0.3 mm). The thickness of the can determines the thickness of the label - as a rule of thumb, the label should be 50-60% of the can. Cylindrical cell can materials are usually nickel-plated steel, with nickel or tin-plated copper for the pole lug material. Nickel plating is preferred over tin because it is more stable; tin has a very low boiling point, which can lead to solder seam porosity and excessive spatter.

Large Prismatic Cells

These high-capacity cells require thick joints to ensure sufficient current-carrying cross-section to provide pack output. However, the tab joints only need to handle the capacity of a single cell. Therefore, thinning or "embossing" the thick joint material to achieve lap welds or creating through holes for fillet welds can greatly reduce the size of the required welds. This, in turn, reduces the heat input to the can, which is always a problem when welding thicker tabs.

For lap weld geometries, reducing the lug thickness to a thickness of 0.01-0.02 inches ensures sufficient weld area to increase strength and capacity while keeping the temperature low enough during the welding process to avoid cell damage. The material of choice for terminals and lugs is typically aluminum - recommended lug materials are 1080 and 1100. avoid using aluminum alloy 6061, which will crack when soldered. If this material is specified and cannot be changed, use 4047 preforms as a third material, which will introduce a large amount of silicon into the weld, thus preventing cracking of the weld.

Lithium Polymer Cells

These pouch-type batteries are thin and rectangular in shape and have really gained traction in consumer electronics. The terminals on these cells consist of thin layers of copper and aluminum foil that are laser welded to the copper and aluminum sheets, respectively. Because of the need to weld through a bunch of foil, this welding has traditionally been done using ultrasonic technology, however, fiber laser welders are now used to improve the quality and strength of the welds. The key to success in this application using a fiber laser is to ensure (a) that the foils are in close contact, and (b) that a pulsed laser is used to avoid overheating.