Perhaps a silly question, but why don't portable power banks (when wired) use the 3.7V that both the battery and device it's charging already have? Why the need to convert to 5V (or higher), necessarily adding nontrivial losses?
Surely it's not only because that's what the USB standard says?
@jhpratt you would need much thicker cables and contacts than a normal USB cable provides or little of thar 3.7V would actually reach the device with any significant current. A lot of the power would be lost heating the cable and contacts. 5V is still a quite low voltage for power delivery.
And, of course, you need more than 3.7V to charge a 3.7V cell.
Jacob Pratt
Jajcuś