Power MIG welder on batteries
If you can, just stringing batteries together and using a spool gun such as a Ready Welder (TM) directly is a LOT more efficient and so allows you much more power for less money. However, depending on what you're welding, 24v may be the &quot;wrong&quot; voltage and you may need lower or higher - we found 36v (3x 12v batteries) was good for reasonably robust field-repairs on vehicles.
I don't recommend butchering your welder to achieve this unless it's a cheap one you don't care about.
You need to check how the internal parts (wire feed motor &amp; gas solenoid) are powered - if they accept 24vDC you may be able to do this if 24v will work for what you want to weld... but many components are AC, and powered from a separate stable supply (secondary winding on the transformer) as obviously when you're welding the &quot;main&quot; output / battery voltage will vary as the arc forms and put a lot of spikes etc. up the wire.
Also, do not wire DC to the wire coming out of the transformer - you'll toast the winding - you need to find the rectified side (after the diodes), which would be pretty close to the final output connected to the torch &amp; ground connections on the welder. Unfortunately these are most probably after the contactor so you have no on/off control over them.
Products like the Ready Welder are fairly popular among off-roaders, it's a long time since I looked at them, and spool guns are a common thing in industry. There are (or were) a lot of cheap copies around from China so one of those could be worth a shot if only to prove the concept.