18650 battery Power bank - what capacity can it provide?

[ArminVanBuuren]

I found this wonderful devide:

**broken link removed**

Works on just 1 18650 battery. If I buy an 18650 Panasonic 3400 mAh, can I more than twice charge my 1500 mAh phone battery? Or the capacity is actually lower (dropping at certain cut-off discharge voltage)?

Thanks a lot.

 

[kam1787]

lower because of loss in the boost circuit, charger, and cut-off voltage

 

[ArminVanBuuren]

Can you predict a theoretical possible capacity it can supply? Is it at least 2000 mAh?