One problem is that all the wanted ones have more cells than the
not-yet-completely-dead packs.
I've tried rebuilds on multiple occasions.
Problem is that if one cell is dead, the others aren't far behind.
I have computer controlled power supply and loads that I've used to
try to match used cells. Best you can hope for is matched crappy
performance. Most often encountered symptom is at least one cell
that works just fine after you charge it, but self discharge takes
it to zero in a few days.
Problem with tool batteries is reverse charge.
You're drilling along and the drill starts to slow down.
Well, you've only got two more holes to drill, so you do it.
Now, you have at least one reversed cell. And it gets worse
every time you do it. When a cell is damaged it's capacity is
lower, so it's the one that gets reversed again next time.
Tool batteries are overcharged, overheated, over discharged, vented,
left unused for years. By the time you need em, they've self discharged
and some cells are likely shorted, which means the others get
overcharged and vent.
You find that a new battery cost about as much as a new tool set.
Then the tool sits in the basement until you decide to clean up
and sell it at a garage sale or give it to Goodwill.
I've bought a lot of used tool batteries. They are almost never good,
And new cells cost about as much as a new battery.
Much of this can be avoided by paying attention and taking care of
your batteries. But 99.9% of the people don't know, don't care.
That's why they go bad and you find a dead cordless tool at virtually
every garage sale. People who take care of their tools don't sell
em for 25-cents at their garage sale.