Howdy Tim........The answer to your questions are "maybe" and "maybe".
You'll find its very difficult to maintain a good arc with a single 12 volt
battery (the voltage of which will drop a lot as you try to strike the
arc). My experience has been with using 2 x 12 volt group 8D batterys in a
24 volt setup..........in this case I kept the marine engines running with
their 80 amp alternators floating the group........used 6011 x 3/32 and I
believe we also tried 1/8 rods to put a couple of pipe bulkheads through a
3/8" deck. What you'll find is that your welding characteristics will
change a lot from when you first strike a new cold rod to nearing the
stub......which will end up red hot a lot quicker than using a more
regulated (and not pure DC) welding machine.

Whats important.............battery capacity (depends on whether you're
going to just tack or spot some sheet metal vs running multiple
passes)......battery voltage to start and maintain your arc (this is where
the batterys will be a lot different).....your ability to adjust rapidly to
the changing welding characteristic as the rod is used up......GOOD
CONNECTIONS to the battery (not booster clamps if at all possible).

Would I try running some cables out from under the hood of your modern
computerized car.........NOPE!



Note there are other possibilities here..........welding inverters etc /
suitcase welders etc............do some research here..

Warnings about ruining the batteries (increases as the size decreases or
your demands increase) are valid as well as the possibility of a hydrogen
explosion occuring from the gassing cells (if lead acid batteries are
used)......you don't want to be welding in the direct vicinity of those
batteries etc.


Hope this helps answer the questions.