Some heavy gauge copper wiring (or sheet copper in my case), a starter solenoid, a microswitch, and two copper contacts (solid copper roofing nails).
I added a bit of polycarbonate and some rivets to keep everything in place, and soldered the nails in using a micro torch (not in that order).
The 150-200A of current delivered by the battery is just about right for the short, high current pulses needed to make a cell welder. You physically provide the short pulse width, with a tap to the microswitch.
Derived from this video I saw on YouTube. Dead simple.
I love it! Thanks Daren.
I have a bunch of 2nd hand BAE modules built out of A123 26650 cells that I need to cut apart and reconfigure but I hadn't started to read up about spot welders. This project makes it clear that I can tackle it myself without significant expense and probably without even going to the store! Thanks for sharing.
Dan, on the copper nails there are definitely some that are just copper plated so be sure to cut one up and test with a magnet to be sure.