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 ...

That need is what led [Erwin Ried] to purchase a small, battery-powered spot welder from a maker in Korea ... and is designed to be portable and powered by batteries commonly used for RC.

powered by a Honda iGX800 V-Twin EFI gas engine, includes a 40 CFM rotary screw air compressor, an 8 kW generator, a 250 amp welder, a 300 amp battery booster, and a 12/24 V battery charger.