Why a 10,000 power bank does not charge a 5,000 phone twice

Power bank cells run at 3.7 V and a USB port at 5 V or more. Converting the voltage, and the heat, take about a third of the energy. That is why a 10,000 power bank gives a phone about 6,000 to 6,500 in practice.

We measured the real capacity of every power bank under a constant load and printed it beside the number on the box. If you see a 20,000 power bank weighing 200 g somewhere, doubt the number.