I have heard of those devices you linked being used with good results, although I have never seen one.
I had a welding instructor who told us that he had made an emergency field repair by hooking up welding leads to batteries. That would make for some very crude DC welding. You would have to control your heat by varying your arc length. I have no way of knowing if he was full of BS but if DC current is all you need then it should be possible. If anyone ever tries it, make sure you are as far away from your batteries as practical.
I’m not sure why everyone is so crazy about using gas with your mig welders. I am required to weld on a regular basis for my employer and have all types of machines available. I always prefer the flux over anything else. I know it throws some splatter that you don’t get with the gas but big deal, for what we are doing just brush it off. I have a very inexpensive 110v unit for home with a gas setup and guess what? I don’t use the gas and keep it loaded with the flux core. I wrote about some of the disadvantages of using gas in that thread I linked to. I’m not saying that gas doesn’t have its place but it’s not worth the extra expense for home use if you ask me. Use the money you save and get an auto-darkening helmet. It’s one of the best things ever invented for a novice welder. Also buy good quality fresh wire, I speak from experience.
Good luck,
Laryn