Cell capacity is measured at the cell's own voltage, typically 3.7 volts. A phone charges at 5 volts, so the power bank has to step the voltage up, and that conversion is not free. Even an efficient converter loses energy as heat, which is why a well-made 10000 mAh bank delivers roughly 6000 to 7000 mAh at 5 volts.

This is not a defect and it is not a supplier being dishonest. It is how the arithmetic works, and every power bank on the market is subject to it. What varies between products is conversion efficiency, which is a function of the circuit design rather than the cell.

For a seller the practical implication is in the listing. Stating cell capacity is standard practice and is what customers compare; stating it without context invites a review complaining that the bank does not charge a phone three times. Some markets now require the rated output capacity to be stated as well.

Ask for the cell capacity and understand that is what you are buying. If your market requires or expects a delivered figure, ask us and we will confirm what is stated for that specific model rather than applying a rule of thumb.