[MrElectronicman]

I built a spot welder to use on battery tags using an old microwave transformer.
I removed the high voltage winding and using 5 mm welding cable wound 2 turns on the transformer, the O/C voltage was 2.6 VAC. The current running though a 3 mm bolt for a short circuit, measured more than 1000 amps on my clamp-on-Ammeter, perhaps not surprising as the Microwave was rated at 1200 Watts. Needless to say the bolt only lasted a few seconds as it became white hot and melted.

Since the experiment appeared to be much too aggressive for a simple spot welder, I abandoned it for the time being, but I have some 2.5 mm welding cable I might try one day