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The most important reason is the conversion factor. Because most of the voltage of the power bank battery is 3.7 V, and the power bank should increase the voltage to 5 V when charging a mobile phone, which makes the power bank conversion efficiency of only 3.75 / 5 = 74%, also includes the generated heat with increasing battery voltage, and various wire consumption, electricity and self-consumption. These points add up to 30% -40% of the power of the mobile power source. Thus, 10000 mAh power bank can really be used by us, it is about 6200 mAh.
An external battery usually has two standard indicators of capacity - battery capacity and nominal capacity. A portable external battery of 10,000 mAh, marked with an internal capacity of 10,000 mAh / 3.7 V, after conversion will show a rated power of 6500 mAh / 5 V.