The YouTube videos I watched seemed to most often use e6011 on the negative side, so that was what I tried. Something else might have worked better, but that was all I had. I was hoping someone on here would tell me specifically what rod and voltage to use, but all I got from you guys was "you'll shoot your eye out!" type posts.
Ok, that was hyperbole, and Goatkiller did tell me it worked, but I would have needed to spend $850 to get all the stuff he said I needed. The whole idea was to see if I could weld with stuff that I already had - jumper cables and a couple of batteries.

This is on a 5x8 utility trailer, and one of the couplings that holds the gate up had broken off. The other side was still holding it, so the part wasn't critical. Thanks for all the ideas.