Did some real Applachian Engineering tonight.

I had a 18V Ryobi battery that wouldn't take a charge anymore. Multi-meter says like 0.5V across the battery (like on the array of cells contacts, not the tool contacts). Lopped the end off of a "12V" DC supply that read 15V on a multi-meter, touched wires to the contacts for ~30s at a time with a break in between until I got the battery to read ~8V, and now the standard charger is happy. Hopefully it doesn't burn my house down.