Why Your 10,000mAh Power Bank Won’t Charge Your Smartphone Twice

Manufacturers of power banks list capacity in milliampere-hours (mAh), but this figure does not reflect the actual amount of energy the device can deliver to your smartphone or laptop. The reason is that mAh is a relative value dependent on the battery’s nominal voltage, which is typically 3.7–3.85V for lithium-ion batteries. However, charging devices requires higher voltages: 5V for USB, 9V for fast charging, or 20V for laptops.

To understand a power bank’s real capacity, you need to convert mAh to watt-hours (Wh) by multiplying the mAh value by the nominal voltage. For example, a 10,000mAh battery with a 3.7V voltage has a capacity of 37Wh. However, when converting voltage to 5V or higher, some energy is lost due to inefficiencies in circuits and heat. Typical losses range from 10–30%, depending on component quality and charging conditions.

Additional losses occur due to battery and cable resistance, especially at high currents. For instance, charging a 100W laptop with a cheap cable can result in several percentage points of loss. Wireless charging is even less efficient: even modern standards like Qi2 lose up to 20% of energy due to coil heating and air resistance. As a result, the actual output of a power bank rarely exceeds 70–90% of the claimed capacity.

To avoid disappointment, experts recommend choosing power banks with a capacity buffer. For example, to fully charge a smartphone with a 5,000mAh battery, it’s better to use a 6,700–7,000mAh power bank. For extended trips, models with a capacity of 70Wh or higher are optimal, allowing you to charge multiple devices without significant power loss.