For reasons inexplicable (the battery pack was sitting directly above where I was welding, it may have cooked and melted), one of my Milwaukee 18V batteries went to the great charger in the sky.
I took it apart, found the battery numbers, and ordered a set off Amazon for $35. Much better than the $100 for a new pack locally.
I clipped the (spot welded?) connections, removed the batteries, tinned the connections and battery ends, and soldered it together (yeah, I forgot to take a pic of the soldering).
Seems to work so far!