RE: I can spot weld A123 cells!!!
1. The light is for the inline resistance that is needed for the audio capacitors to charge from a discharged state. They include a resister with the caps, but i thought the light was cooler.
2. I actually ended up using a 6-15v Pyramid bench @ 15V. Caps would pull about 5 amps tops (I think the headlamp acted as a current limiter). I powered the SCR from the same source. I also think a slightly higher voltage, maybe around 16-20v would do a bit better, but the car audio caps yelled at me for exceeding 16v, so the 15v PSU was perfect.
I actually had to give one of the caps back to my buddy who let me borrow it. I didn't get very good welds on the positive terminal until I hooked all four caps up for a total of 21F. I think the caps are over rated though, higher quality ones might do the job with less.
I also successfully soldered several cells that had lost the thin nickle plat on the positive terminal. It kind of sucked though and took a 260W iron (my 140W didn't cut it) and a combination of zinc solder and flux on the aluminum cell and lead solder on the tab; heated and dropped together.