Real phone charges from any power bank, counted in watt-hours instead of label mAh, with the losses from 3.7 V cells to 5 V USB and the FAA limit for flying.
How many times will a 30000mAh power bank charge a 5,000 mAh phone?
Real phone charges from any power bank, counted in watt-hours instead of label mAh, with the losses from 3.7 V cells to 5 V USB and the FAA limit for flying.
The calculator below is set to a 30000mAh power bank, entered in mAh, with cells at 3.7 V, charging a 5,000 mAh phone whose battery sits at 3.85 V, the phone battery in mAh as well. Change the phone to your own model: the count scales with its capacity.
No efficiency is entered, so the headline box uses about 2/3, the average share of cell energy that measured power banks delivered to devices, and names the ceiling with no losses at all. The bank's mAh and the phone's mAh are counted at different voltages and the energy passes through converters on the way, which is why dividing one mAh figure by the other promises too many charges.
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How many charges a power bank really gives
Why doesn't a 20,000 mAh power bank charge a 5,000 mAh phone four times? Because the two numbers are counted at different voltages and the energy passes through two converters on the way. This calculator turns both batteries into watt-hours, applies the efficiency you enter (or the measured average of about 2/3), and gives 2.56 charges for that pair, plus the FAA rule for taking the bank on a plane.
Dividing the labels
- 20,000 ÷ 5,000 = 4 charges
- ignores that the phone cell sits at 3.85 V, the bank cells at 3.7 V
- ignores the step up to 5 V and back down
- says nothing about flying with it
Counting watt-hours
- 74 Wh in the bank, 19.25 Wh per phone charge
- 3.84 charges is the ceiling with no losses at all
- 2.56 at the measured average, 3.27 at 85%
- 74 Wh is under the 100 Wh carry-on limit
Six fields plus two optional ones
- Power bank capacity and unit - mAh from the label, or Wh if the label prints it (airline-friendly banks usually do).
- Power bank cell voltage - the voltage printed next to the mAh, nearly always 3.6 V or 3.7 V. Not needed for Wh.
- Device battery and unit - the phone, tablet or earbud case capacity in mAh or Wh.
- Device battery voltage - about 3.85 V for recent phones, 3.7 V for many other lithium-ion devices.
- Overall efficiency - optional. Empty uses the measured average of about 2/3 and also shows the no-loss ceiling.
- Device level when you plug in - optional. Topping up from 20% counts as 80% of a charge.
- Read the result - charges, watt-hours delivered and lost, the 5 V figure, the FAA tier and a table of common bank sizes.
Where the missing milliamp-hours go
Wikipedia's power bank article puts the first loss plainly: a 3.7 V rating read at a 5 V output is only 74% of the printed mAh, before any conversion loss. The same article cites measurements in which the average power bank moved about 2/3 of its cell energy into the devices it charged.
What the box promises and what the phone gets
Charges of a 5,000 mAh, 3.85 V phone from empty, all banks at 3.7 V. The middle column divides the labels; the last one is the calculator with the field left empty.
| Power bank | Energy | Label ÷ label | At about 2/3 |
|---|---|---|---|
| 10,000 mAh | 37 Wh | 2 | 1.28 |
| 20,000 mAh | 74 Wh | 4 | 2.56 |
| 26,800 mAh | 99.16 Wh | 5.36 | 3.43 |
| 30,000 mAh | 111 Wh | 6 | 3.84 |
| Gap between the columns | - | the label division overstates by 56% in every row | |
Flying with a power bank: the FAA tiers
The FAA treats power banks as spare lithium-ion batteries: carry-on bags only, never checked, and if a carry-on is checked at the gate the bank has to come out. The size tier depends on watt-hours, which is why the calculator reports it. The mAh column shows the same limits for 3.7 V cells.
| Energy | At 3.7 V | FAA rule |
|---|---|---|
| up to 100 Wh | up to 27,027 mAh | carry-on |
| 101-160 Wh | up to 43,243 mAh | airline approval, two at most |
| over 160 Wh | over 43,243 mAh | above the FAA limit for spares |
Airlines can set stricter limits on how many power banks a passenger carries, so check with the carrier as well.
Asked before a trip
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See also
Calculator verified by the LiczGrupa.pl team
Content, formulas and results have been reviewed for accuracy and relevance by our team of specialists.
Reviewed by: Natalia Skrzek