* Benchtop power supply @15-16v (amps depend on cap charging rates... mine used about 5A max using a foglight for the charging control resistance)
* SCR (Thyristor) 220v/220a Single or Dual channels. You only need one unless you want to add a second set of capacitors and a reed switch for dual-pulse welding, but that's much more expensive I got a used dual channel one from an ebay surplus store for about $40, would have cost hundreds to buy one new
* Charge controlling resistance - not sure how to calculate for different cap set-ups, but mine ran as stated above using an automotive foglight @ about 5A max for ~40 second charges
* Foot peddle ON/off to activate SCR switch and weld (I used the same 15v power source for the switch, which is fine with such a heavy duty SCR.)
* Heavy 2gage braided copper car audio wiring between main cap lead, SCR and contact points
* Thick solid copper grounding wire, clamped to braided wires and filed to a point at the ends (you can fix these however you want, just make sure your hand isn't going to be in danger of becoming part of the circuit!
* ~21+ Farads total capacity - I used: one 10F, two 5F, and one 1F car audio capacitors all wired in parallel with 6g wire. You can find these cheap on ebay.
How you wire it up is up to you, this is just a rough list of what I used. Keep in mind there are many factors that will effect the quality of your weld.
* Clean all welding surfaces with a non-residue cleaner like high % alcohol. You want to optimize the metal to metal contact, so it must be free of oils and debri
* Keep the welding tips clean by regularly filing them to a rounded point. The size of this point will effect your weld, too large of a point won't weld fully, too small and it will likely blow the tip off before it welds the material.
* Play with voltage and capacity, using my values as a baseline. I had a feeling that if I went to a bit higher voltage I might have gotten better welds, but the circuitry of the audio capacitors limits it to about 16v and sets off the alarm (my PSU maxed at 15.5v anyways). The caps are rated to 24v, so if you could bypass the control circuit you could easily try out the 16-18v range if your PSU can do it.
* Make sure you apply adequate pressure to both contact points and that you are only welding within the center button area. A strike to the outer edge of the positive terminal can easily rupture the cell. It's not particularly dangerous, but fluids will leak out. I managed to seal a couple that I ruptured during wiring, by accidental arc across cells, with JBweld, only apparent affect on the cell was a minor reduction in capacity. According to the California Materials spec sheet for A123 cells, they do not contain any toxic or hazardous chemicals.
* Always wear eye protection, you'll be throwing sparks in your face for hours!
* Ventilation is recommended
[link=http://www.ledhacks.com/power/battery_tab_welder.htm]Here's the site that gave me the idea in the first place... however they hadn't been successful with A123's with their setup.
Here is the basic wiring diagram:
EDIT: More recently I was able to repair my RC packs using only 16F, you just have to make sure everything is flat and clean so you get good conduction.
[link=http://www.ledhacks.com/power/dual_pulse!!!.htm]If you'd like to give the dual-pulse welder... here's some more info (click go there)