Mike has a good point. I ended up doing the reactor thing later, on the advice of a neighbor. I had a 12" by 4" mild steel core with three wraps of #4 THW (copper) wire wrapped around it. If I had to do it again I would use #6 so I could get more wire wrapped around the core. As well the additional voltage-drop would be helpful. I also had a 200 Amp fuse in the line to protect the batteries from a prolonged dead short. My first welder was (3) 8V batteries in series. I had 32 volts open circuit and about 20-25 under load. To regulate the current I wrapped 8 feet of steel band strapping (used for securing lumber and shipping crates) around some porcelain insulators and used a car-jumper cable to choose different spots on the strapping. Worked like a charm. I went through quite a bit of strapping though. That's how I built my first turbine. In the case of 12-volt batteries you will need two in order to get at least 24v. I used 4 but you could get away with the equivalent of 3 (or 2 12v batteries.) A 24-volt charger to keep the batteries topped up between weld cycles would be a good idea.
Offered by Rob.
My only experience with his is seeing a welding kit in either the Northern or Harbor Freight catalogs, seems like a car alternator that had heavy windings that could either be powered by a power take off (pto) or by some form of engine or motor.
Offered by Mike.
The Lower the voltage and the higher the amperage - the better it works. e.g. a 6 volt battery is a better current source than a 12 volt battery. Commercial welders typically use .5 volt current. Hint: use two 6 volt batteries wired in series from the 12 volt source for charging. detach the charging cables from the batteries and attach the welding cables to the batteries in parallel when you want to weld. In common parlance, this is called "stick welding". You need welding cables, welding rod of the proper alloy for the material being welded, and of course, welders goggles and/or hood. Out in the country, I have seen local gentlemen doing emergency repairs using jumper cables attached to a 6v. tractor battery and a wire coat hanger as welding rod. This is not recommended. I include this anechdotal information to suggest how easy it is. If you do not have text books on welding technique, I have seen encyclopedia articles with pictures depicting how the bead of molten metal should be layered on the work piece.
Offered by John.