The Misleading mAh Figure
When shopping for a new power bank, the mAh figure is a metric that we all look at. It’s a relative measure of the internal lithium battery’s capacity based on its nominal operating voltage. This can lead to confusion, especially when comparing it to a device’s battery capacity.
The Actual Capacity Calculation
To determine the actual capacity of a power bank, we multiply the mAh rating by the cell’s nominal voltage to get the total average power output per hour in watt-hours (Wh). This is why the mAh figure doesn’t always represent the amount of charge that will end up in your phone. The mAh figure is a relative measure, not an absolute one.
Voltage Differences and Their Impact
The voltage difference between the power bank and the device you’re trying to charge can significantly affect the actual capacity. For instance, a 20,000mAh, 3.7V battery might seem to go a lot further on a low-voltage smartphone than on a high-voltage laptop. However, this doesn’t mean the power bank is lying; it’s just that the voltage difference affects the actual capacity.
Efficiency and Losses
The efficiency of power delivery is not 100%. Voltage conversion is not a lossless process. DC-to-DC converter chips are typically 85% to 98% efficient, depending on factors such as chip and inductor quality, load voltages, and the currents being converted. This means that some energy is lost during the conversion process, leading to inefficiencies.
Wireless Charging and Energy Losses
Wireless charging is even more energy-wasting. Energy is lost both in the coils and across the air, which is a natural insulator. Even the latest Qi2 and MagSafe accessories could result in a conversion loss of 20% or more for the added convenience of no wires.
Over-Spec Your Power Bank
To avoid disappointment, it’s best to over-spec your power bank. A 5,000mAh battery pack just can’t charge an equivalent-sized smartphone back to full. Adding up converter losses, cable losses, and the phone-side charge IC, even a good setup rarely beats 85–90% end-to-end, and cheaper models could result in efficiencies of just 70% or below. As a very rough ballpark, I’d target 75% extra capacity to avoid disappointment.
Conclusion
Understanding the mAh figure and the actual capacity of a power bank is crucial. By over-specifying your power bank, you can ensure that it provides the full charge you expect. This helps avoid the disappointment of a power bank that doesn’t deliver the expected capacity. Remember, the actual capacity is determined by the voltage difference and efficiency losses, so it’s important to do the math to ensure your device gets the full charge it needs.
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