What Is Spot Welding and How Does It Work?

What spot welding means

Spot welding joins two or more pieces of metal at small, round spots instead of along a seam. The name comes from the result: each weld is a small fused spot, often called a nugget, between the layers.

It belongs to a family called resistance welding. The heat does not come from a flame or an arc. It comes from electric current pushing through the metal where the pieces touch, which is why the process is so fast and so clean.

You will see it in two very different settings. Factories use large machines to join sheet metal, such as car body panels and appliance housings. Hobbyists and repair shops use small battery spot welders to attach nickel strips to cells when they build or fix battery packs.

How a spot welder works, step by step

The sequence is the same whether the machine fills a factory floor or fits in one hand. Only the size of the current and the force change.

• Overlap the metal. The pieces to be joined are placed on top of each other, such as a nickel strip lying on a cell terminal.
• Press the electrodes on. Two electrodes, or the two tips of a welding pen, squeeze the layers so they make firm contact.
• Send a short pulse of current. The machine releases a brief, high current that flows in through one electrode, across the joint, and out through the other.
• Let resistance make heat. The current meets the most resistance where the two pieces touch, so that is where the metal heats and melts into a small pool.
• Hold, then release. The pressure stays on for a moment while the molten spot cools and solidifies, forming the weld nugget. Then the electrodes lift away.

Why the heat forms exactly where you need it

The key idea is that heat depends on current, resistance and time. A higher current or a longer pulse makes more heat. The contact point between two pieces has more resistance than the solid metal around it, so the joint heats up first while the rest of the metal stays comparatively cool.

The electrodes are usually made of copper or a copper alloy. Copper carries current very well and moves heat away from the surface, so the electrode tips do not melt onto the work. The heat is concentrated in the middle, where the layers meet.

Pressure matters as much as current. Too little force and the contact is poor, which can cause sparks and spatter. Too much force and the electrodes can squeeze out the resistance that creates the heat. Good welds come from balancing current, pulse time and pressure for the thickness and type of metal.

Industrial spot welding vs battery spot welding

The principle is shared, but the tools are built for different jobs. If you are shopping for a welder to build or repair battery packs, you want the small kind. Our best spot welders page compares battery tab welders, not car body equipment.

Spot welding battery packs: the specs that matter

Battery pack builders spot weld because soldering heats a cell for too long. A weld pulse lasts a very short time, so less heat travels into the cell.

When you compare machines, look at peak power, the number of power levels, the thickest strip it can weld, and how it is powered. More power levels let you match the pulse to thin or thick strips more precisely. Here is how a few welders on our comparison page line up, using their published specifications.

Choosing between cordless and mains models

Cordless welders such as the UK3 and SE-402 run from a built-in battery, so you can use them anywhere. The UK3 lists nickel, pure nickel and steel up to 0.3mm, and the SE-402 lists nickel-plated sheets under 0.3mm and pure nickel under 0.15mm.

Mains capacitor welders such as the P30C and UF20B store energy in capacitors and need to be connected to wall power. They suit a fixed bench. The UF20B listing specifically says to keep it plugged in while you work, because its capacitors store little energy.

Trigger mode changes how you work. In auto mode the weld fires when the pen touches the strip. A foot pedal frees both hands, so you can hold the cell steady and place the pen carefully. For a full side-by-side view, see our spot welder comparison.

Safety and good habits

Spot welding is quick, but it involves high current, molten metal and energy-dense cells. Take a few precautions every time.

Wear safety glasses, because sparks and spatter can fly off the weld. Keep your work area clear of anything flammable, and never weld on a damaged or swollen lithium cell. Do not let metal tools or loose strips bridge the terminals of a cell, since a short circuit can heat a cell quickly. Unplug mains welders and let them cool before you change tips or open anything up. Follow the maker's manual for charging and thermal limits.

To get consistent welds, follow these habits:

• Start on a low power level and raise it a step at a time.
• Make a test weld on scrap strip first, then pull on it. A good weld should tear the metal rather than peel off cleanly.
• Keep the strip pressed flat against the terminal and keep the pen tips clean.
• Avoid welding copper or aluminum unless your welder is specifically rated for it, as both are hard to weld.

Frequently asked questions

Does spot welding use filler metal?

No. The weld forms from the base metals themselves melting together at the contact point, so no filler rod, flux or shielding gas is required.

Can I spot weld without a special machine?

Not safely. The process needs a controlled, very short, high current pulse and steady pressure, which is what a purpose-built spot welder provides. Improvised setups can overheat cells or cause fires.

Is spot welding the same as soldering?

No. Soldering melts a separate filler metal onto the joint and heats the parts for longer. Spot welding fuses the base metals themselves in a brief pulse, which puts less heat into a battery cell.

What materials can be spot welded?

Many sheet metals can, including steel, stainless steel and nickel. Highly conductive metals such as copper and aluminum are harder, because they let current and heat spread away from the joint.