A 4,000mAh power bank is generally a compact reserve rather than a guaranteed full phone recharge. At the example 3.7V nominal cell voltage it stores 14.8Wh. The table deliberately tests 70% and 85% usable-energy scenarios, giving about 10.4–12.6Wh; these are transparent sensitivity inputs, not measured averages for every product.
A 4,000mAh Power Bank Stores About 14.8Wh
4,000mAh × 3.7V ÷ 1,000 = 14.8Wh
This uses a common nominal cell voltage for illustration. The exact product label may use another nominal voltage, so its printed Wh value is the better reference. The power bank must convert cell voltage to the USB output, and the phone converts that input again while operating. Not all 14.8Wh reaches the phone battery.
The 70% and 85% values are deliberately chosen planning scenarios. Multiplying 14.8Wh by those inputs gives 10.4Wh and 12.6Wh. For a specific model, replace the scenarios with declared rated output energy or measured delivered Wh.
Estimate the Phone-Charge Equivalent
| Phone battery example | Approximate battery energy at 3.85V | Planning result from 10.4–12.6Wh |
|---|---|---|
| 3,300mAh | 12.7Wh | About 0.8–1.0 equivalent |
| 4,500mAh | 17.3Wh | About 0.6–0.7 equivalent |
| 5,000mAh | 19.3Wh | About 0.5–0.7 equivalent |
A “full-charge equivalent” is an energy comparison, not a guarantee that a phone starting from empty will reach the same displayed percentage. Screen use, wireless radios, temperature and charging taper affect the result.
Wireless output normally adds another conversion step, so the same pack may deliver less energy to the phone than through a good cable. Testing while the phone is actively used will also produce a lower net percentage gain because part of the input powers the phone instead of charging its battery.
When Is 4,000mAh Enough?
- Useful: emergency calls, navigation, tickets or a commute top-up.
- Possible: one near-full recharge for a small phone under favorable conditions.
- Usually insufficient: a full day of heavy camera, gaming or hotspot use.
- Not ideal: charging two phones, a tablet or a laptop.
If you need a compact pack with more margin, compare the 5,000mAh power bank guide. For custom capacities and voltages, use the mAh-to-Wh calculator.
Check More Than Capacity
Verify the USB output profile, recharge input, connector type, dimensions, weight and exact Wh label. Built-in cables reduce loose accessories but create a fixed wear point. Stop using a pack that is swollen, leaking, smoking, hissing or rapidly heating. A low-capacity product is still a lithium battery and requires the same safe handling as a larger pack.
For a fair product comparison, start both tests at the same battery percentage, use the same cable and phone, keep the screen off, and record the power bank’s starting and ending state. Without a defined method, one anecdotal recharge is not a reliable capacity claim.