I made the first "serious" welding test on 3 18650 cells tonight.

The blue part is a piece of heavy gauge copper bar in a thermo sleeve. It serves as a weight to control the welding pressure. Each arm can move independently (to some extent...).

Close up on the welding electrodes

The electrodes are made of machined copper bar. The electrodes are perfectly perpendicular to the surface to be welded. The gap between the electrode is 1.6 mm.