Ken Sterling wrote: (clip) The welder electrode holder would be
"positive" probably for this....(clip)
^^^^^^^^^^^^^^^^^
The OP said the welder is AC. This would mean that any rectification would
be for battery charging only. This is consistent with his statement that he
can weld, but can't charge batteries. Sounds like the rectifier or
something else in the charging circuit is bad.

To the OP: If you need DC as you say, you could set up some external diodes
to hook your welding cable to. You wouldn't have to open the unit up, so
would avoid draining the oil, and all that. The risk is that you may buy
diodes that you didn't really need, but it may be worth it to save all that
work and mess, and it will get the jopb done without any welding down-time.