A common frustration among portable battery buyers is charging arithmetic that fails in practice. A buyer purchases a 20,000 mAh power bank, expects it to fully charge a 5,000 mAh smartphone four times, and finds the battery pack completely drained after barely 2.8 charges.
This gap between advertised and delivered power isn’t necessarily counterfeit labeling. It’s the direct result of basic electrical physics: nominal cell voltage, conversion stepping, and thermal losses during power transmission.
1. Nominal Voltage vs. USB Output Voltage
The headline capacity printed on a battery bank represents the rating of its internal lithium-ion or lithium-polymer cells, not the energy delivered at its USB ports:
- Internal Cell Voltage: Lithium pouch cells and 21700 cylindrical cells operate at a nominal voltage of 3.7V (charging to 4.2V peak).
- The USB Standard: Standard legacy USB outputs deliver power at 5.0V, while modern USB Power Delivery (USB-PD) profiles step voltages up to 9V, 15V, 20V, or 28V (EPR).
To calculate true stored energy, always convert to Watt-hours (Wh):
$$\text{Energy (Wh)} = \frac{\text{Capacity (mAh)} \times \text{Nominal Cell Voltage (3.7V)}}{1000}$$
A standard 20,000 mAh power bank contains:
$$\frac{20000 \times 3.7}{1000} = \mathbf{74.0\text{ Watt-hours}}$$
2. Conversion Losses: The 15–25% Thermal Tax
A power bank cannot directly feed 3.7V cell power into a phone’s charging circuit:
- Step-Up (Boost) Conversion: The battery bank’s internal circuit board must boost 3.7V up to 5V or 9V/15V PD levels. Switching boost converters generate heat and draw operating power, operating at roughly 85–92% efficiency.
- Step-Down (Buck) Conversion inside the Device: The smartphone takes that incoming 9V/5V current and steps it back down to its own battery’s ~3.85V chemistry, introducing another small thermal conversion loss.
- Cable Resistance: Thin or long copper traces within charging cables dissipate measurable energy as heat over fast-charging sessions.
3. True Usable Capacity Matrix
| Advertised Cell Capacity | Nominal Stored Energy | Typical Usable Output (85% Eff.) | Equivalent 5V USB Output | Real Full Charges (5,000 mAh Phone) |
|---|---|---|---|---|
| 5,000 mAh | 18.5 Wh | ~15.7 Wh | ~3,140 mAh | ~0.8 Charges |
| 10,000 mAh | 37.0 Wh | ~31.4 Wh | ~6,290 mAh | ~1.6 Charges |
| 20,000 mAh | 74.0 Wh | ~62.9 Wh | ~12,580 mAh | ~3.1 Charges |
| 26,800 mAh (Flight Cap) | 99.1 Wh | ~84.2 Wh | ~16,840 mAh | ~4.2 Charges |
Airline Travel Limits
Aviation security agencies (TSA, EASA) regulate spare lithium-ion batteries by Watt-hours (Wh), not mAh. The standard universal threshold is 100 Wh. A 26,800 mAh pack sits at 99.1 Wh, making it the largest battery you can bring aboard commercial flights without explicit airline permission.
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- Off-Grid Solar Charging: Check out our guide to Foldable Solar Arrays.