Battery Charge Time & Power Bank Capacity Calculator
A 10,000 mAh power bank charges an iPhone 15 about 2 times (75% efficiency). Work out the real number of charges and your phone charging time.
Charging time and number of power bank recharges
Charging time is calculated in energy: capacity in Wh (mAh × 3.8 V ÷ 1,000) divided by the power actually delivered, i.e. 88% of the charger rating, reduced to 45% above 80% charge. A 5,000 mAh smartphone thus goes from 15 to 100% in 57 minutes on a 25 W USB-C charger. A 10,000 mAh power bank at 75% efficiency delivers 7,500 usable mAh, or 1.5 recharges.
How many charges does a power bank give?
| Power bank | iPhone 15 (3,349 mAh) | Galaxy S24 Ultra (5,000 mAh) |
|---|---|---|
| 5,000 mAh | 1.1 | 0.8 |
| 10,000 mAh | 2.2 | 1.5 |
| 20,000 mAh | 4.5 | 3.0 |
| 26,800 mAh | 6.0 | 4.0 |
Full charges at 75% efficiency, the calculator default (voltage conversion and heat losses).
Formulas
- Battery energy (Wh) = mAh × 3.8 ÷ 1,000.
- Usable power = charger power × 0.88. Over USB-A it is capped at 10 W before this efficiency; wireless (Qi) gives 70% of the charger power, capped at 15 W.
- Time = energy from 0 to 80% ÷ usable power + energy above 80% ÷ (usable power × 0.45).
- Recharges = power bank capacity × efficiency (65, 75 or 85%) ÷ device capacity.
Example
A 5,000 mAh (19 Wh) smartphone charged from 15 to 100% with a 25 W USB-C charger: 22 W usable. The 12.35 Wh up to 80% take 34 minutes and the remaining 3.8 Wh 23 minutes, 57 minutes in total. The same phone needs 32 minutes at 45 W and 2 h 15 min on a 15 W wireless charger. A 10,000 mAh power bank at 75% gives 7,500 mAh, or 1.5 full recharges.
Limits
The tool assumes the device accepts the charger's full power, which is not always the case; temperature and use while charging lengthen the time. Capacities of the preset models are indicative values.
On a plane
For a power bank's energy, use mAh × 3.7 ÷ 1,000 (its cell voltage): 20,000 mAh = 74 Wh. Power banks travel in the cabin only. The ICAO addendum of 27/03/2026 limits each passenger to two power banks, with no recharging on board; IATA asks for 100 Wh at most per power bank, as the 100 to 160 Wh band is removed from its regulations on 01/01/2027. Device batteries remain allowed up to 160 Wh with airline approval (EASA), and airlines may be stricter.
Sources and dates: IATA, Guidance Document for Passengers Travelling with Lithium Batteries (revised 31/03/2026); EASA, Safety Information Bulletin 2025-03 (power banks banned from checked baggage, issued 27/05/2025) (checked 14/10/2026)
Frequently Asked Questions
How many recharges does a 10,000 mAh power bank give?
At 75% efficiency it delivers about 7,500 mAh: 1.5 recharges of a 5,000 mAh phone, or 2.2 recharges of a 3,349 mAh iPhone. The losses come from converting 3.7 V to 5 or 9 V and then into the battery.
Why does charging slow down after 80%?
Lithium-ion batteries switch to constant-voltage charging near the end and the current drops. The tool models this by cutting power to 45% above 80%: on a 5,000 mAh phone at 25 W, the last 20% takes 23 minutes against 34 minutes from 15 to 80%.
Can I take a 20,000 mAh power bank on a plane?
Yes, in the cabin: 20,000 mAh × 3.7 V = 74 Wh, below the 100 Wh limit. The ICAO addendum of 27 March 2026 limits each passenger to two power banks, which IATA asks to keep under 100 Wh each; they are banned from checked baggage.
Is wireless charging slower?
Yes. The tool uses 70% of the wireless charger's power, capped at 15 W: a 5,000 mAh phone takes 2 h 15 min from 15 to 100% versus 57 minutes over 25 W USB-C.
Is this tool really free?
Yes, all tools on Kristools are 100% free and accessible without registration.
Is my data protected?
Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.