Battery Tab Welding: Welding Battery Tabs, Connecting Plates and Terminals

A battery pack is assembled from cells that are joined to each other and to the outside world through tabs, connecting plates and terminals. Those joints carry the full current of the pack, they are made from dissimilar metals in many cases, and they have to survive vibration and thermal cycling for the life of the vehicle or the tool. Welding is the usual method, and the choice of process determines both the resistance of the joint and its durability.

The three connections are different in nature. A tab is a thin foil emerging from a pouch or a prismatic cell, a connecting plate is a thicker busbar that carries current between cells or modules, and a terminal is the interface to the external circuit. Each demands a different welding process, and getting the process wrong produces a joint that measures acceptably at build and fails in service.

The Materials and Why They Matter

Aluminium and copper are the two materials that appear in almost every pack. Aluminium is light and inexpensive and is used for pouch tabs and for busbars where weight matters. Copper conducts better and is used for terminals and for high current links. Both form an oxide layer that has to be broken before a weld can form, and aluminium oxide is particularly tenacious and reforms within seconds of cleaning.

The combination is where the difficulty lies. Aluminium and copper do not form a stable alloy at the interface, and the intermetallic compounds that grow there are brittle and electrically resistive. A joint between them that is made hot and held at temperature grows a thick intermetallic layer and becomes fragile; one that is made quickly with a solid state process limits the growth and remains ductile. The process choice therefore follows from the material pair as much as from the thickness.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/Rigid-Flex-PCB-1.jpg" alt="Ultrasonic welding tool joining battery tabs to a busbar” />

Ultrasonic Welding

Ultrasonic welding joins metals by friction rather than by melting. A sonotrode presses the two parts together and vibrates at twenty kilohertz or more, and the relative motion at the interface breaks the oxide and creates a solid state bond. Because the temperature stays below the melting point, no intermetallic layer grows and the joint retains the conductivity of the parent metals.

It is the standard process for aluminium to aluminium and for aluminium to copper in battery packs. The parameters are the amplitude, the clamping force, the weld time and the tool geometry, and they interact in a way that makes the process window narrow. Insufficient energy leaves an unbonded area, while too much energy work hardens the material and can crack a thin tab. The tool also wears and has to be dressed or replaced on a schedule derived from the weld quality tests.

Resistance Welding

Resistance welding passes a current through the joint and relies on the contact resistance to generate heat. It is fast, inexpensive and well suited to thin tabs and to nickel strips on cylindrical cells, where the geometry allows the electrodes to press directly on the parts. The weld forms at the interface between the two metals, and the parameters are current, force, time and electrode geometry.

The limitation is that the heat is generated where the resistance is highest, which is not always where the weld is wanted. On a thick busbar the current spreads and the heat is distributed, so the process becomes inefficient. On a cell with a low internal resistance, the current may prefer to flow through the cell rather than through the joint, which wastes energy and heats the cell. That is why resistance welding is used for thin connections and ultrasonic or laser welding for thick ones.

Laser Welding

Laser welding offers a narrow heat affected zone and a good depth to width ratio, which suits busbars and terminal connections where the parts are thick and the joint has to carry a high current. The beam can be positioned accurately, so the weld can be placed away from the cell body and away from the heat sensitive separator, which is a significant advantage in a pack where the cell cannot be heated.

The process is sensitive to the surface condition and to the fit between the parts. Aluminium reflects the wavelength used by most fibre lasers, so a high power density is needed, and the resulting keyhole can produce porosity if the parameters are not controlled. Copper is even more reflective at those wavelengths, which is why laser welding of copper has become practical only with shorter wavelength sources or with very high power density. Where the joint is aluminium to copper, a laser produces a melted interface and an intermetallic layer, so the energy per unit length has to be limited to keep that layer thin.

Joint Design and Current Path

The electrical design of the joint matters as much as the welding process. The weld area determines the contact resistance, and a joint with a small welded area has a higher resistance and generates more heat under load. A wide lap joint with several weld spots, or a continuous seam, distributes the current and reduces the local heating. Where the joint is a lap of a thin tab onto a thick plate, the geometry should avoid a step that concentrates current at one edge.

Thermal expansion is the second design consideration. Aluminium expands more than copper, so a joint between them moves when the pack changes temperature, and a rigid welded connection transfers that movement into the tab or into the cell. A designed bend or a flexible link between the joint and the cell allows the movement to be absorbed without loading the weld. This is the same reasoning that governs any current path design, applied to a joint that also has to be mechanically robust.

Inspection and Qualification

Inspection of a welded joint is difficult because the weld is internal. A peel test on a sample shows the weld area and the failure mode, and the fracture surface indicates whether the bond is complete. Cross sections show the interface and any intermetallic layer. For production, the resistance of the joint is measured, either directly with a four wire method or indirectly through the pack level measurement, and a rise over time is the indicator of a degrading joint.

Qualification should include thermal cycling, vibration at the frequencies the pack will see and a current cycling test that reproduces the load profile of the application. The joint temperature under load is the most useful measurement, because a joint that is degrading will run hotter for the same current. Recording that temperature, and the resistance, before and after the tests, gives a quantitative measure of the joint quality that a visual inspection cannot provide. Where the pack is safety critical, the quality control records for the welding process should be retained for the life of the product.

Fixturing and Process Monitoring

The fixture determines whether the weld is repeatable. It has to locate the tab and the plate relative to each other, apply the clamping force without crushing a thin foil, and support the cell so that the force is not transmitted into the separator. On a pouch cell the tab is flexible and easy to mis-position, so the fixture usually includes a comb or a nest that presents the tab at a defined height and angle.

Monitoring the process is the second half of the control. On an ultrasonic welder, the displacement of the sonotrode over time is the most informative signal, because it shows how much the material deformed during the weld. Too little displacement indicates insufficient energy, and too much indicates that the material is being over-worked. Recording that curve for each weld, and comparing it with a reference from a joint that has been verified destructively, gives a per unit check that a resistance measurement alone cannot provide.

Additional Considerations for This Build

Practical attention to battery tab welding pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating battery tab welding explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to busbar joint pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating busbar joint explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why not solder a battery tab? Soldering heats the cell, and the heat can damage the separator and the electrolyte. The joint also has a low melting point and a higher resistance than a welded joint.

Is ultrasonic welding suitable for copper to copper? It is possible but harder, because copper is softer and tends to deform rather than to bond. The tool and the parameters have to be developed specifically for the material pair.

How is the quality of a weld verified on every unit? By measuring the joint resistance and by monitoring the process parameters. A destructive test on a sample confirms that the parameters still produce a sound weld.