Yeah, 4.2 volts and 0.7 ohms works out to 6amps (25.2 watts). That comes close for 22gauge, but remember its a tradeoff between wire size and flexibility as well. The thicker wire gets, the more difficult it is to work with. The nature of mods/pv's is high-current demand for short periods, 22awg is probably ok unless you go any lower on atty resistance. The 18-watt soldering iron might have a tough time with soldering heavy gauge wire to large terminals or big pieces of metal. I use one of the weller 60-watt temperature controlled soldering stations. The one on the homedepot site is $100... an extra investment up front, but worth it in the long run if you plan on doing a lot of that type of work. Its also temperature-variable. For lighter stuff the handheld one is probably ok, but you definitely want one thats temperature controlled. In most weller irons, this is done in the actual tip itself unless its temperature-variable. Ive always used 700F tips for everything from smd rework to audio/video repair.
Also note that the 6A figures for a worst-case scenario... The battery will never be at 4.2volts for very long and also the battery isnt a perfect 0 ohms it has its own internal resistance which further limits total circuit current.
