Hi
Using a bank of 2V cells I have used this method several times, tapped at 32 to 48 V the batteries were the exciting current for a 240V 9.5 KW diesel generator. It works especialy well on cast iron, you do need the correct rods for cast.
Anything below 30 volts and you will not get the penetration, most AC welders use 48 or 90 V open circuit. MIG welders use 24 to 48 V OC but they rely on rectified AC giving 100 pulses a sec (UK) TIG welders use various voltages but need AC if you want to weld aluminium, DC for Stainless and iron based metals.
If you have never welded before learn on a basic AC unit where the current will remain stable not reduce as the battery discharges, and welding a tower as a learning project which will have twisting and bending forces on it not to mention vibration, is a recipe for disaster..
Brian