I've never killed a battery using it to weld with. I'm not sure on a total available time, I'm sure it depends on the size of rod you would be using. But like I said, I've never killed any batteries on trailside repairs while wheeling. The last thing I had to fix was a Isuzu rodeo's broken steering linkage. The guy broke this rig waaaaay down in the bottom of some god-forsaken hole in the Bohemia Mtn's of oregon about 20 miles from a paved road. I think I pumped about 4 full rods of 6011 into the break and just for good measure I welded a coule bolts across the break. He wheeled the piss out of the rig and then drove it home 150 miles after the weekend (for the record I did tell him not to wheel it hard (but he did) and I did state in front of a big group of witnesses that this was "for emergency repair only and you can't hold me liable" blah blah blah...) I'm not a huge fan of doing emergency work on other people I don't know's rigs (liability issues), but hey, when you gotta, you gotta....good samaratin laws and all that...

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any way, I found this on some aussie website:
"Most DC welders work at around 40 volts, and a general purpose electrode of 3.25mm will need about 120 amps for good penetration of weld. 36 volts is available from three automotive batteries connected in series. The batteries we use in 4WDs are more than capable of supplying the amperage for an ample amount of time for a repair weld; after all, they supply winches at up to 400 amps, and starters on diesels at 300 plus!"
The winch is the big killer. I killed an optima red top from over-winching without the engine being on once. When I was actively into 4-wheeling we had lots of rigs in our group that ran dual battery systems in their rigs to deal with winch loads and such. We never really went wheeling alone so there was almost always 3 batteries laying around. Even if we did manage to kill a battery, the chance of killing all three with welding is negligible. Just to be safe we would also try and park a rig where it could be push started if we had to.