I recently received an external USB battery as a promotional gift (see image below). While I always thought of these as a superfluous gimmick, I realized that these devices could be quite useful as mobile power source for various projects. After all, dealing with lithium ion batteries in your own projects can be dangerous and you need additional circuitry to ensure charging and voltage conversion.
External USB batteries can be charged with a normal micro-b USB charger and are supposed to output stabilized 5V at above 1A. And they come fully integrated at a price point where it is difficult to get even the battery alone. See Aliexpress for example and many others. Since there is little reason to trust hardware at this price point, I decided to tear the device down to see whether all the necessary parts are there.
The device has a USB micro-b socket which is used as 5V input for charging, and a normal USB-A socket as 5V output. The output power can be turned off and on by a toggle button. There are LEDs to indicate active power out (blue) and charging (red) states.
The pictures above show the innards of the device. Most space is taken up by an ICR18650 LiIon battery, which are relatively common devices with 2600mAH. In addition, there is a tiny tiny PCB.
For more detail: Tear down of a cheap external USB battery