Almost two years ago I decided to build spot welder for batteries. After researching my options I choose construction by Albert van Dalen

https://www.avdweb.nl/popular/spotwelder/spot-welder-controller-building-kit

It is microwave transformer based, controlled by arduino. It switches primary side with two big-ass thyristors. It also does peak detection of sine wave, to turn on welding exactly when phase voltage is highest. It uses double pulse method. Read his article. I think this is best free construction you can build.

I bought Sunkko welding tweezers. But those are crap. I dissembled them and created two independent swing arms. Arms are long enough for very big battery. Pushing force is adjusted by hand. There is a pedal input to keep both hands for welding.

I measured current trough secondary side with Rogowski probe. It was not during welding, but output was connected firmly together. Over 1200A, nice 🙂

And here is some examples of batteries made with this one:

And this one is done with my spot welder by friend of mine whom I borrowed it.