Several years ago, I was facing a similar situation. Professional tools that I hung onto after I retired, so the cost of updating was far beyond what I was willing to spend. I sat on the problem for a while until Harbor Freight had a sale on spares for a discontinued model. I bought a half dozen batteries and 3 or 4 chargers.
Still sitting on a couple of brand new batteries, never been opened. The original tools were DeWalt, 18 volts. I took the chargers apart and mounted the connectors on the bottom of the tools and plug the batteries right up. The tools are out of balance but quite usable. The charger for the original batteries works just fine, all it needed was an original battery connector for the new batteries.
The HF batteries didn't have the lasting power of the originals, but with a multiport charger and "lighter" hobby duty cycle, they do well enough. That was a one time shot, granted. But such an approach might be attempted with modern tooling.
The HF batteries were about $6, I don't recall exactly. But the DeWalt batteries were $150 for a pair. All told I spent less for the multiple packs than the cost for one of the originals. I had a number of esoteric tools for professional use that I wouldn't replace for hobby use. The HF batteries powered them fine. The NiCad batteries have been superceded these days. The stunt might well work again for newer tools.
Another stunt that might work is to gut one of the batteries and insert a piece of lamp cord about 10 or 20 feet long. A couple of cheap battery clips to hook up to a car battery or charger. For 18 volt tools, they run a little slow. But still usable in a bind.
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