Check out my newest 18650 salvage video:    • Revive dead 18650 batteries (29) from...  
Today I will show you how to revive a dead 18650 Li-ion cell that's being refused by the chargers. This method will work with any Li-ion battery, not just the 18650 cells. Here's my follow-up video:   • Follow up video: Will it blow up? Rev...   if you wonder how much current (Amps) goes through the wire.
Note: this is not a permanent way to charge an 18650 cell. The point of this method is just to bring up the voltage of the dead cell to a point that is enough for a real 18650 charger to accept it and charge it properly. When a Li-ion battery does not have high enough voltage, the charger would not charge it. I think the reason is safety because if the voltage is too low, it might take a risk of charging a NiMH or NiCad cell which are only 1.2 volts each. And if you put a NiMH or NiCad cell (at 1.2 Volts) in a Li-ion charger that charges up to 4.2 volts, all hell will break loose.
Also: don't overdischarge a Li-ion cells down to below 3V especially below the threshold recognized by the charger. Otherwise, it will not live long. The "Goldilocks" zone for a long and healthy Li-ion cell life is between 30% and 90% charge. Too much charge or discharge would be bad for the cell.