How to Spot Weld 18650 Cells (and Why Not to Solder Them)

Join 18650 cells with a spot welder and pure nickel strip, 0.10 mm or 0.15 mm thick. Don't solder to the cell.

A soldering iron holds heat on the terminal long enough to damage the cell inside, even when the outside looks fine. A spot weld pulse lasts a few milliseconds. This works the same for sodium-ion, LFP and lithium-ion 18650s.

Five steps to a good weld

1. Use pure nickel, not nickel-plated steel. Plated steel has higher resistance and welds poorly.
2. Start at your welder's lowest setting. Too much energy can damage the cell, and the damage may not show until the pack underperforms.
3. Test on a spare cell first. Weld a strip, then tear it off with pliers. A good weld rips the nickel and leaves small spots of metal on the terminal. If the strip peels off clean, raise the power one step and try again. Stop at the first setting that holds.
4. Use firm, even probe pressure and make at least two spot pairs per terminal.
5. Check every weld. Pry under the strip edge with a fingernail. Nothing should lift.

Protect the positive end

The whole steel can is the negative terminal, right up to the edge of the positive cap. Put a fish paper insulator ring on every positive end before you weld, so a strip or a slipped probe can never bridge the cap to the can.

Measure every cell first

Check the voltage of each cell when it arrives. Cells from one batch ship at about the same partial charge, so they should read close together. Set aside any cell that reads far from the rest, and ask us before you build with it.