Battery capacity

mAh and Wh are not the same

mAh measure charge, but energy also depends on voltage: Wh = mAh × V ÷ 1000. A 10,000 mAh power bank (3.7 V cells) holds 37 Wh; a 60 Ah car battery at 12 V holds 720. That's why airport rules talk watt-hours, not mAh.

A power bank's real charges

Between the bank's cell (3.7 V), the USB output (5 V or more) and the phone's battery, energy is converted twice and partly lost as heat: 60-70% arrives. A 10,000 mAh bank on a 5,000 mAh phone gives ~1.3 charges, not 2: that's physics, not a defect.

The airplane rule

Spare lithium batteries (power banks included) travel in the cabin ONLY: up to 100 Wh (≈27,000 mAh) without approval (recent IATA rules still cap spares at ~20 each), 100-160 Wh only with airline approval and max two, over 160 Wh never. They're banned from the hold because a fire there couldn't be handled.

Note

Efficiency and runtime are declared estimates (real electronics vary); the airplane verdict summarises standard IATA/ICAO rules — your airline may be stricter, check before flying.