Need help identifying what kind of Li-ion battery I have and what to do when they're below 2,5V.
These are the classic 18650-type OEM batteries, Li-ion chemistry, charge voltage should be 4.2 V. Their design capacity might be 2500 - 3000 mAh, depending how long ago the were made. 

If some are "undervoltaged", below 3.0 V (or even to zero), you can try to recover them by applying "pre-charge" current, 100 - 200 mA, until they get to 3.3V-something, then a normal charger will work. Be aware however that the battery capacity might be substantially reduced, down to 20 - 40%, or it could remain dead.