I did really need a spot welder though ... limiting resistor to the battery while also holding a multimeter in place, or connecting a large variable-voltage power supply with current limiting ...

The spot welder [Erwin] used is the work of a user by the name of [aulakiria] (link is Korean, machine translation here) and is designed to be portable and powered by batteries commonly used for RC.

This rechargeable battery-powered welder provides up to 180 amps of welding power (True DC Power). Quiet with no exhaust fumes, making it suited for indoor use Lightweight, portable and compact at ...

The Hobart Trek 180 is a portable MIG welding that operates off its self-contained battery or 115V power and welds 24-gauge up to 1/4-in. mild steel in a single pass and recharges off an ...