Buying a 10,000mAh power bank and finding it only charges your flagship phone one and a half times is a common disappointment. The manufacturer is not lying, but the explanation lies in technical factors that reduce the effective capacity. The rated capacity printed on the box refers to the internal cell, not the energy that actually reaches your device. To understand the real performance, we need to examine three key aspects: the difference between battery capacity and usable capacity, conversion losses, and voltage.
The Difference Between Battery Capacity and Usable Capacity
Power banks use lithium-ion cells with a nominal voltage of 3.7 volts. However, to charge a smartphone, the output must be at 5 volts (USB standard). The boost circuit converts the voltage from 3.7V to 5V, and this conversion is not 100% efficient. Moreover, the milliamp-hour (mAh) rating is calculated at 3.7V, but when delivered at 5V, the available current drops. A simple calculation shows that 10,000mAh at 3.7V equals 37 watt-hours (Wh). At 5V, those same 37Wh become about 7,400mAh, before considering any losses. So the effective capacity is lower on paper.
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Conversion Losses and Heat
During voltage conversion, a portion of the energy is dissipated as heat. Good-quality circuits achieve 85-90% efficiency, while poor ones can drop to 70%. Assuming 85% efficiency, from the theoretical 7,400mAh we get roughly 6,290mAh usable. Add cable and connector losses, which can subtract another 5-10%, and a 10,000mAh power bank typically delivers between 5,500 and 6,500mAh actual charge. That is enough for one and a half charges of a modern 5,000mAh flagship.
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Voltage and Watt-hours the True Comparison
To properly compare power banks and phone batteries, it is better to use watt-hours instead of mAh. An iPhone 16 Pro Max has a battery of about 18Wh, while a Samsung Galaxy S26 Ultra (discussed recently during its Prime Day price drop) sits around 20Wh. A 10,000mAh power bank stores about 37Wh, but after losses about 30Wh reach the phone, enough for one and a half charges. Fast charging increases thermal losses further. To maximize efficiency, use high-quality cables and USB-IF certified power banks.
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Another factor is the voltage of newer power bank cells, which sometimes reach 3.85V for higher energy density. In such cases, the real capacity is slightly higher, but the difference is marginal. For a reliable estimate, always look for the watt-hour specification on the package. If missing, multiply the mAh by 3.7 and divide by 1000. For more background, see the Wikipedia page on power banks.
Interestingly, device evolution shapes expectations. With iPadOS 27 dropping support for older iPads, users seek larger power banks. Understanding real capacities helps make smarter purchases.
Source: https://www.androidauthority.com/why-your-power-bank-feels-small-3675437
