It's criminal that two new batteries are about 10% less than buying a new drill driver combo with new charger and batteries. I've an ~ 7 year old 18v makita drill driver whose batteries are approaching worthlessness.
The cheaper aftermarket packs have wildly varying reviews and therefore untrustworthy. I've no desire to buy individual cells or a battery spot welder then still have to deal with makita's electrical neuterer, and the drill is showing some issues, so I am likely to get new
I've wired up 9.6 12v and 14.4v drills and ran them off a 12v battery, but this defeats the cordless. The 14.4v drill, one might think it would be weak on a 12v battery, but it is not. The 9.6v drill on a 12v battery works amazingly well.
I've some 24v hedge clippers that I have run on a small 12v AGM but they are weak and bought another 12v AGM for 24v and 10AWg to motor and this works well, but for the 10awg powercord and 25 Lb battery.
1.5v NiMH individual cells, when a charger refuses to charge them, I have hooked them to 12v for about 1/3 a second, and they get incredibly hot, and then once cooled slightly, the charger recognizes them and charges them. I'd not try this with Lithium cells.
I'm seriously irritated with the fact that perfectly good tools are designed for landfills, because new batteries for them are only slightly less expensive than new tools that come with two new batteries.