I used two laptop salvaged fully charged Samsung ICR18650-24E in this USB Emergency Charger.
The unit stopped power around 2.6v on both batteries. Is that too low?
Put it to charge, after a few hours, one battery was at 3.05 and the other one at 2.10v. After a few minute of rest it was at 2.29v.
Should I discard the battery?
If it stops at 2.6 volt it is fine to use with 2900/3100/3400 mAh batteries, but if you first measured the voltage after it stopped, the voltage was much lower before it stopped and it will be unhealthy for your batteries.
I don’t think I saw it mentioned, but you’ll want to use batteries with pretty much the same capacity and resistance. For salvage batteries, that means testing with a charger that will measure capacity or a lot of work doing it manually with a light, timer, and DMM.
Try the 3100s and please let us know how they work!
Use a proper charger to recharge the batteries (very slowly until they manage to get over 3V if they get there), if they are dead, they are dead and get rid of them safely.
Check for an under voltage cut off of your device with a variable output power source.