Lowest voltage to bother recharging?

Newat this and I'm harvesting a few batteries. 18650s from power tool packs and laptops. What is the lowest voltage that will recharge??? I know low discharge is 3.0, but should I even try when the batts are in the 2.5 and up range??

If a battery drained naturally all the way down to 0.0V (e.g. was left on a shelf for years) then it would still be safe and viable to use, they should just be charged around 50mA until they reach 2-3V. You should note however the difference between 0V because it is empty, and 0V because it is shorted through - if the cell is shorted then its just dead.

I have been processing cells for a while now and I've had great success with cells showing greater than 2v and some success with 1v+. It's pretty hit and miss with cells that come out of packs below 1v, though I do have a few that have tested stable at 1500mah and better when they were showing nearly 0. Not entirely sure it's worth the effort of slowly coaxing them back to life though....

There are a bunch of youtube videos out there sharing some great (and some horrifying) information.

This guy knows what's what

If a cell is 0.0V truly, do a resistance test on it. If it reads infinity, toss it. If you still have good resistance, then you can probably charge it back up.

Any cell lower than 1V especially, should be charged at 50mA or lower if possible. If it's 1-2V, then 50-100mA would be acceptable. After 2.5V, it should be able to be charged at normal current.
daromer's videos are great an informative (DIY Tech & Repairs on YT) as the link above is to. Watch his videos as he explains a lot.

Also read the FAQ as it has a lot of basic questions answered.

If it's truly 0V (as in resting voltage after removing from the pack) it's almost definitely a leaker (self discharging cell) and not worth trying to revive. Any other cell would bounce back up to at least around a volt. I haven't had much luck with very low voltage cells though, they tend to be leakers/heaters.