Normally MIG welding requires a big fancy machine, sometimes also with a large tank of pressurized gas. Instructables user TimAnderson managed to hack together a hand-held wirefeed MIG welding gun and a couple car batteries for a more portable effect. He writes:

Here’s my new wire-feed (MIG) welder.
All of it.
They sell these things as accessories for commercial MIG units, but they can also work great on 24 volts from two car batteries.
I have it loaded with .030″ flux-core welding wire.
It cost me less than $100 on ebay.
The “Ready Welder” is a similar commercial product.
WARNING: The tip is electrically on all the time!