Hi chemelec,
I have a similar device made by " Maxtec" It contains a single lithium ion cell (Type 18650) and a single PCB which must contain the charging circuit and a step up converter. I thought that the 3.7 volts you measured may be just the cell voltage and as soon a some current was drawn from the output the step up converter would start. When I tested my unit it gave 5.0 volts with just the meter connected. I even tried removing the USB plug in case inserting that operated a switch to start the converter but its still gave 5.0 volts with jus the meter leads poked into the USB socket.
Here are pictures of the PCB from both sides.
The micro USB socket is for charging the unit and the standard USB is the output.
Hi Les, THANKS.
I Checked this one with a Load and Yes it does go up to 5.12 Volts.
So Obviously it does have a Switching Power supply in it. That was not what I was expecting.
I have a bunch of Similar switching supplies that I bought from China for $1.79 each and Free Shipping.
(but with an Adjustable Output from 3 to 32 volts, with a minimum of 3 volts input.)