Stamped battery contacts and tabs

What a battery contact has to do

A battery contact performs two functions that are usually separated in other parts. It carries the pack current, and it applies a spring force to the cell terminal or to the mating blade. The force keeps the interface stable; the section keeps the temperature rise acceptable. Neither can be designed in isolation.

In a consumer pack the contact may carry a few amps. In a power tool, a storage rack or a vehicle pack it may carry tens or hundreds. The geometry changes with the current, but the principle does not: the contact must hold force through vibration, thermal expansion and repeated mating without taking a permanent set.

Stamping suits the part because the spring and the conductor can be one piece of metal. A progressive die forms the contact, coins the interface, punches the locating features and separates the part. The alternative is a machined or a screw-machined contact, which is heavier and usually more expensive at volume.

Spring force and contact resistance

Two surfaces never touch across their whole apparent area. They touch at a set of small asperities, and the current passes through those spots. Increasing the normal force deforms more asperities and increases the real contact area, which lowers resistance. That is why a contact that has lost force shows a rising resistance before it shows any visible damage.

The design target is a force that is high enough to keep resistance stable and low enough to keep insertion and mating forces acceptable. The window is narrow on small contacts. A leaf spring with a long effective length gives a softer rate and a wider deflection range; a short, stiff spring gives more force in a smaller space but less tolerance to dimensional variation.

Stress is the limit. A spring that is deflected to a high fraction of its yield strength will relax over time, especially at elevated temperature. A practical starting point is to keep the working stress below about half the yield strength of the material at the maximum expected temperature, then confirm with stress relaxation data for the alloy and temper.

Fretting is the other failure mode. Small relative movement under vibration wears through the plating and exposes base metal, and the oxide that forms raises resistance. A higher force reduces movement, and a plating that resists wear, such as gold on a signal contact, reduces the damage.

Material and temper for a contact spring

Copper alloys dominate because they combine conductivity with enough strength to act as a spring. Pure copper is too soft for a contact that must hold force. Brass is cheap and forms well but relaxes at temperature. Phosphor bronze is a common middle ground. Beryllium copper and copper nickel silicon give higher strength and better relaxation resistance at a higher cost.

The temper sets the yield strength and the formability. A harder temper gives more force but cracks at tight bends. The bend radius on the drawing should be checked against the temper, not against a generic rule. A radius that works in annealed copper may fail in a hard temper.

Thickness is the third variable. Force rises quickly with thickness, roughly with the cube of the thickness for a simple cantilever, so a small change in gauge has a large effect on the mating force. If the strip thickness tolerance is wide, the force spread will be wide too. Ask for the thickness tolerance, not just the nominal gauge.

• Brass: low cost, good formability, moderate conductivity, limited relaxation resistance.
• Phosphor bronze: good spring and fatigue, widely used for pack contacts.
• Beryllium copper: high strength and good conductivity, higher cost, good relaxation resistance.
• Copper nickel silicon: lead-free, good strength, a common substitute where regulations require it.
• Nickel-plated steel: low cost and stiff, used in some consumer holders where current is low.

Plating for the interface and the weld

The plating on a battery contact has to do two different jobs. On the contact interface it controls resistance and wear. On the tab or the weld area it has to be compatible with the joining process, which may mean no plating at all in the weld zone.

Tabs and welded joints

A battery tab is a flat conductor that joins a cell to a busbar, a protection board or another cell. The tab is usually stamped from nickel, nickel-plated steel, copper or aluminium. The stamping sets the tab width, the fuse neck and the weld land.

Welding is the critical operation. Resistance welding, laser welding and ultrasonic welding each need a clean surface, a defined contact area and a process window. Plating can help or hurt: a nickel layer is usually weldable, a thick tin layer can flow or spatter, and a contaminated surface will produce a weak joint that passes a pull test but fails in service.

The stamping should provide a flat weld land with no burr on the contact side and enough clearance for the electrode. If the tab is bent close to the weld, the electrode may not sit flat, and the joint will be inconsistent. Put the weld land on the drawing as a controlled area, not as a note.

A fuse neck is a deliberate weak point. Its width and thickness set the current at which it opens, and the surrounding geometry must not heat-sink it so much that it never operates. If the pack relies on the tab as a fuse, the neck belongs in the design review, not in a late change.

Current, heat and thermal cycling

A contact that runs warm will relax faster, oxidise faster and lose force sooner. The temperature rise depends on the cross-section, the contact resistance, the ambient temperature and the cooling path. In a sealed pack there may be very little cooling, so the contact has to carry the current with a margin.

Thermal cycling is the test that separates a good design from a good prototype. As the pack heats and cools, the contact and its housing expand at different rates. The spring deflection changes, the force changes and the interface moves. A contact that is marginally stable at room temperature can walk out of its window over a few hundred cycles.

The design response is to give the spring enough deflection range to absorb the movement without going solid or losing force, and to avoid clamping the contact in a way that converts thermal expansion into stress. A slot rather than a round hole, or a slightly oversized locating feature, can remove a constraint that the design does not need.

Tolerances and inspection

The critical calls on a stamped battery contact are the contact height, the spring deflection at the mating position, the force at that deflection, the interface area, the tab width and the weld land flatness. The overall blank size is secondary.

Force is a functional dimension and should be specified as a range at a stated deflection, not as a spring rate alone. A rate without a fitted length does not tell the supplier what the part will do in the assembly. The drawing should also state the mating condition, because the same spring gives a different force at a different height.

Inspection should include the contact height, the force at the stated deflection, the plating thickness and a weld cross-section or peel test for the tab joint. For a pack contact, a contact resistance check at a defined current is more useful than a visual check, because the failure mode is electrical and often invisible.

Validation before the pack is released

A sensible validation sequence starts with the first article, then moves to the assembly. Measure the force at the fitted height, measure the resistance across the interface at the working current, then run a thermal cycle and repeat both. Add vibration and a mating cycle test if the contact is exposed to the user.

The results should be recorded against the drawing, not against a general specification. If the force falls outside the range after cycling, the design needs a change: a longer spring, a lower stress, a different temper or a plating that resists fretting. Increasing the force alone can make the mating force unacceptable, so the fix is usually geometric.

Keep the material certificate, the plating certificate and the weld parameters with the first article. A pack contact is a safety-related part in many products, and the evidence that it was made to the drawing is as important as the part itself.