The 9 v. battery I was thinking about was that thinking about is the common little tran. battery. I know a "D" battery is good for 1/4 amp for 4 hours , and it has a lot more stuff inside it . Many years ago I had a rechargeable soldering iron , I think that battery was 3 volt . ni-cad as I rember . A guy could hook up some car batteries and make a cheep d.c. welder . You could wire 10 togher to get 120 v.d.c. and power a soldering iron , or get one of thoes liq. propane ones . desert rat
I do not see a use for a USB soldering iron. I normally avoid chinese and japanese crap, but I do have a nice Hako hot air station. It has a decent(not great) soldering iron, and the hot air has come in very handy. I also have a Kado soldering iron on the side. Those two together, along with an inspection microscope, optivisors, etc, I have not found a project I can not tackle.