I fixed a spring hanger on a jeep in the mountains by taking all the batteries out of the trucks and stringing them together with jumper cables and a starter wire or two. I was lucky I left the welding rods in the back of my jeep but I didn't have a face shield so I just closed my eyes as soon as the arc started and went by sound. It was a terrible weld but it held up good enough to get it home so it could be fixed properly.
I think we used 4 batteries and had to jump start one of the trucks after putting everything back together. That was 15 years ago so I am not real sure
One thing that wasn't mentioned was an alternator makes an excellent high frequency DC welder. Just take out the voltage regulator and run 12V to the field and you get about 70-90V out of the alternator that can weld pretty much anything by varying the idle speed. Just watch the duty cycle so you don't overheat the diodes in the alternator.