I run an older Mobi-Weld (www.mobi-arc.com). It's saved out butts several times on the trail. Mine is put in place of the alternator. In normal mode it's a 90 amp alt. In weld mode, the charging system is cut out, and it puts out about 135 amps for welding. It produces a very high frequency arc that strikes easily, and welds well even on dirty metal.
Mine has to be manually switched, I've heard the new models are automatic.