Many people wonder: A 20,000mAh power bank can only charge their phone a few times before running out - is the capacity falsely advertised? Actually no, the issue lies in our understanding of the units.
mAh ≠ Real Energy
The mAh (milliampere-hour) marked on power banks is just "charge capacity", which doesn't represent actual energy. What truly determines energy is Wh (watt-hour), calculated by the formula:
Wh = (mAh ÷ 1000) × Voltage (cell platform voltage)
For a 20,000mAh power bank with cell voltage typically at 3.7V, the actual energy is:
20,000 ÷ 1000 × 3.7 = 74Wh
Where Does the Energy Go? Two Stages of Conversion Loss
1. Power Bank Circuit Loss (80~85% Efficiency)
Power banks need to boost 3.7V to 5V output, and circuit conversion loses approximately 18% of energy:
74Wh × 0.82 = 60.7Wh
Converting to 5V output capacity: 60.7Wh ÷ 5V ≈ 12,140mAh (this is the "rated capacity" on packaging)
2. Device Charging Loss (80~90% Efficiency)
Assume that phone/laptop charging circuits also lose about 15% of energy.
How Many Times Can It Actually Charge?
Taking common devices as examples (Note: Battery capacities vary greatly among different devices and models, check brand official websites for details):
• Phone (using 18.2Wh as example): 60.7Wh × 0.85 ÷ 18.5Wh ≈ 2.8 times
• Laptop (using 100Wh as example): 60.7Wh × 0.85 ÷ 100Wh ≈ 0.5 times
Conclusion: Power Banks Aren't "Stealing Power"
The fast power depletion of power banks is an inevitable result of physical laws, not false advertising. When shopping next time, focus on Wh (watt-hours) rather than mAh - it's the true indicator of energy. Remember: Energy conversion always involves loss. This is a law of physics. Power banks are honest; science doesn't lie.
