The other issue with the cheapo (er - expensive) online pre-built welders is that they are lower voltage. Usually only 3.7V parallel Li-ion or Li-Po are used.

Fortunately, they sell 12V standalone board alternatives:

These are actually quite nice, with more modes than the other one. They refer to power readings as "gear" modes in the description usually (this has 6).  And while it has one bulk capacitor, it's quite small.
What would happen if I hooked up a massive 1 Farad capacitor to this?  That would give me a lot more discharge current :)  Maybe I could use a AC power supply then?

I tried this at first with my old spot welder, but it didn't work. Even with massive 30A and 40A supplies, it simply wasn't enough current to do welding without tripping the supply safety limits.  Whoa.

I wondered, what can I use around the house?
And then it dawned on me: my old wheelchair batteries!  They no longer move my ass for half a dozen city blocks, but they still pack a punch!  Do they have enough juice for spot welding?

This would be amazing, since I have to replace them every two years anyway.  It would be so nice to find a use for them, instead of letting them go to waste.

Some testing proved that yes - they have more than enough power with the added capacitor!  So much in fact, I can't use it beyond "gear 2" or I will melt a hole in the nickel strip!

But nothing about this setup was safe. There is no case for the welder, no protective covers for the probes (which create a spot weld when touched), and no way to transport it.

A little 3D printing and wood working was in order :)