What is in this article?:
- Selecting, Applying the Right Technology for Welding Batteries
- Choices for Battery Manufacturing
- Welding Tabs to Terminals, and Buss Bars
- Battery Pack Manufacturing
A guide to selecting the optimal approach — laser, micro-TIG, and resistance welding — for fabricating batteries, battery packs
- Materials joining requirements
- High-speed seam and plug sealing
- More/less heat-input control
- Proximity considerations
- Production volume, consumer demand
Welding Tabs to Terminals, and Buss Bars
From a welding perspective, the important aspects of tab welding are the thickness and material of both the tab and the terminal. Resistance welding is extremely well suited to welding nickel tab material up to 0.015-inch thickness, and nickel or steel clad copper tab material to around 0.012-inch thickness to a wide variety of terminal materials.
Due to a different welding mechanism, laser welding is able to weld both thin and thick tab materials, with a capability of welding copper or aluminum tab material above and beyond 0.04-inch thickness. Avoiding penetration of the can and overheating the battery are important aspects of tab to terminal welding.
Welding tabs or terminal connections to buss bars generally does not require as much penetration of heat-input control as the tab-to-terminal welding requires. The materials, material thickness and combination of materials determine the best welding technique.
Resistance welding — Resistance welding has been, and continues to be, the most cost-effective method for joining tabs on a wide range of battery types and sizes, using both DC inverter closed loop and capacitor discharge power supplies. With fast rise times, closed-loop feedback control, polarity switching, and options for displacement and force sensing, the process can be finely tuned and monitored to ensure both high quality and yield.
For nickel tab thicknesses up to 0.0070-inch, the tab can be welded without modification. Beyond this thickness, and to prevent electrical shunting and excessive electrode wear, a slot and projections are placed in the tab as part of the stamping process. The projections act not only as energy concentrators for the weld, but also greatly increase electrode lifetimes. Figure 2 shows several examples of the wide range of resistance tab welding applications.
Micro-TIG welding — Micro-TIG offers excellent welding of copper, and so presents a good solution for buss bar welding that would require a brazing material for resistance welding or a large power laser welder. Both butt, fillet and lap welds are possible up to and beyond thickness of 0.02” thick copper are routinely welded. When welding copper using micro-TIG it is extremely important to use a pulsation function that creates the weld without porosity, as show in Figure 3.
Laser welding — For tab and buss bar joining, laser welding offers a high degree of flexibility, welding both thin and thick tab materials, and materials such as copper, aluminum, steel and nickel as well as dissimilar material combinations. Two example welds are shown in Figure 4.
When welding a tab to a terminal, the general rule of thumb is that the tab should be thinner than the can terminal thickness. As the can thickness decreases the tab usually must be 50 percent of the can thickness for a safe processing window that provides the weld strength and conductivity whilst not penetrating the can.
As laser welding has no limitation on the proximity of the welds, the laser can place any pattern of weld spots on the tab according to strength requirements. It is worth noting that, in nearly all cases, if the joint’s weld strength 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.
As shown in Figure 5, the placement of the weld spots on the tab is completely flexible, and can be tuned to the strength requirements of the pack or tab. For example, peel strength is often used as a metric for weld quality. Therefore the welds can be positioned accordingly. The peel strength of (a) is 15 lbs. and (b) is 60 lbs.
The time needed to add additional weld spots is very short; sufficient tab strength can be achieved with very little impact on cycle time.
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 of weld spots shown in (c) provides the solution.