Well, I just had to try it in the interest of science.

I successfully attached the coupling back onto the trailer and I don't think it is gonna come off again. I used a couple of 8 yr old deep cycle marine batteries to make 24 volts, and a set of heavy duty jumper cables. I set the batteries as far away from the trailer as the cables would allow and made sure they were attached tightly to the posts. I think the biggest risk would be if the connection was loose and it sparked; that could blow up the battery. Other than that, I don't think there is anything more dangerous than any other kind of welding.

But the bad news is that the arc was very unstable and I was never able to get it to run a solid bead the full length of the coupling. It would splatter and lose the connection, and then I'd have to strike a new arc and then I would quickly lose that one. So the result was quite ugly, but it's all melted together and I expect it to work. I guess a transformer would stabilize the arc?

I was really interested in knowing if this method of welding would work well enough for emergency use on the farm. With the set up I had, I would say no. But I suspect there is a combination of voltage and electrode that would be acceptable for welding plows and such if I could figure it out. I think it's too much trouble, so that is my contribution to science for today.

Irishguy, do you know the make and model of the welder you have? It would be worth it for me to buy something that would work on 120 volts if it isn't too expensive. My shed isn't wired for 220, and it doesn't make sense to spend $400 on a welder I might use once a year.