Minimum voltage of 18650
As already mentioned by 'Transistor' your battery will indeed be completely drained by the LED's, which is not a good thing. Since you took the battery out of a power bank that you got from Ebay, you don't have any information about it. So in advance, please excuse my 'simplistic' solution to your problem:
You could buy a ready made protection module from Ebay. Something like this or this for instance. They look like this:

So it fits the shape of your battery. As you can read in the specs on Ebay (which might or might not be applicable for your specific battery) the cut-off voltage of those modules is somewhere around 2.45V - 2.55V. That works for a battery that is rated around 3.7V.
EDIT:
To be able to charge your batteries correctly, you'll need to know their rated voltage. Please be careful as you charge them, as charging them in the wrong way may damage them, set them on fire or make them explode...
Please be safe, those batteries can be dangerous!
Hope this helps.