The principle of power bank is simple

The principle of mobile power supply is simple, in the case of finding an external power supply in advance for the built-in battery charge, that is, input electrical energy, and in the form of chemical energy pre-storage, when needed, that is, the battery provides energy and generates electrical energy, to a voltage converter (DC-DC converter) to reach the required voltage, by the output terminal (usually USB interface) output to supply the required equipment to provide power for charging or other purposes.