I took a slightly different approach and put 2 diodes in series to drop the voltage from 5V to 3.8V and a Male USB-A plug on the end.
I tested on a single cell power bank and drew 900mA.
This way my friend can just go out and buy whatever power bank he wants.
I recommended a single cell, but I think he prefers a 4 cell as that's what the original battery was.
I can see his point of view. He commutes about 15Km and it's no fun running out of light before you get home with traffic about!
Then again I did 17Km on a single cell, but hey! Oh and I recharged at work, and I don't think he does.