Why a 10,000 mAh power bank does not give two full charges

A 10,000 mAh power bank and a phone with a 4,500 mAh battery: the quick sum says more than two charges. In practice you usually get about one and a half, sometimes less. This is not a product fault: it is the physics of energy conversion. This calculator estimates the real number of charges, the charges from the level at which you actually start charging, the capacity actually delivered and the energy in watt-hours, the figure that matters for taking the power bank on a plane.

It works for phones, earbuds, tablets, cameras and any device charged over USB.

How the calculation works

You enter the capacity of the power bank and of the device battery in mAh, the charging efficiency and the start and end charge levels.

• Capacity delivered: power bank capacity × efficiency.
• Equivalent full charges: capacity delivered divided by the device battery.
• Real charges: capacity delivered divided by the share of battery you actually recharge, for example from 20 to 100%.
• Nominal energy: mAh × 3.7 V / 1000, the conventional voltage of a lithium cell.

The calculation compares mAh with mAh, assuming similar nominal voltages, as is the case for most phones and power banks, which use 3.6–3.85 V lithium cells.

Where the energy goes

The mAh printed on a power bank refer to its internal cells, at about 3.7 volts. To charge the phone, the power bank steps the voltage up to 5 volts or more for the USB output; the cable dissipates some energy; the phone steps the voltage back down to its own battery level, and some turns into heat. Each stage loses something, and the typical result is an overall efficiency between 60 and 80%.

The gap between an efficient power bank and a poor one is worth almost a third of a charge. Short, good-quality cables, moderate temperatures and not using the phone while charging help you stay at the top of the range.

Rated capacity and real charges

The table compares the "naive" mAh ratio with real charges at 70% efficiency, for a 4,500 mAh phone.

A small 5,000 mAh power bank, often sold as "one full charge", in practice does not fill a modern phone from zero. For a reliable full charge you need at least 1.5 times the mAh of the phone's battery.

Charges from your real level

Hardly anyone charges a phone from zero. Plugging in at 20% and unplugging at 100% uses only 80% of the battery per charge, so you get more charges than the "full" ones. The calculator shows both figures: equivalent charges, useful for comparing power banks, and charges from the level you enter, closer to everyday use.

Power banks on planes: the 100 Wh limit

Airlines generally follow the International Air Transport Association (IATA) guidance on lithium batteries. Power banks must always go in carry-on baggage, never in the hold, and the limit is expressed in watt-hours, not mAh.

Rules can change and differ between airlines and countries: some also ban using or charging power banks during the flight. Before travelling, always check your airline's instructions.

The four bands of the verdict

• Less than one charge: a lifeline to get through the day.
• One to two charges: the typical reserve for a day out.
• Two to four: a weekend or a trip without sockets.
• Above four: camping, trekking or several devices, where weight matters too.

How to choose the right power bank

Start from how many charges you really need, multiply by your device battery's mAh and divide by a cautious efficiency of 65–70%. Consider weight: power banks weigh on average around 18–25 grams per 1,000 mAh, so a 20,000 mAh model often weighs between 350 and 500 grams. If you also charge a laptop, check the output power in watts as well as the capacity.

Limits of the calculator

The model uses a constant efficiency and assumes similar nominal voltages for the power bank's and the device's cells. It does not account for battery ageing, which reduces real capacity over the years, or for the device's consumption while it is on and charging. Air travel rules are indicative: those of the airline you fly with are the ones that apply.

Frequently asked questions

How many charges does a 10,000 mAh power bank give?

For a 4,500 mAh phone at a typical 70% efficiency, about 1.5 full charges. Charging from 20 to 100%, about 1.9.

Why don't I get the charges the mAh ratio suggests?

Because some energy is lost: the power bank steps the voltage up from 3.7 to 5 volts or more, the cable dissipates energy and the phone converts it again, producing heat. Overall efficiency is usually between 60 and 80%.

What efficiency should I enter?

If you do not know it, use 65–70%, a cautious value for most power banks. Some manufacturers state the actual output capacity, which gives a better estimate.

How are a power bank's watt-hours calculated?

mAh × voltage in volts / 1000. At the conventional 3.7 V, a 10,000 mAh power bank is 37 Wh.

Can I take a power bank on a plane?

Generally yes, in carry-on baggage and never in the hold. Up to 100 Wh is allowed; 100 to 160 Wh normally needs airline approval; above 160 Wh is normally not allowed. Always check your airline's rules.

How many mAh is 100 Wh?

At the conventional 3.7 V, about 27,000 mAh. Many 'travel' power banks stop at 20,000–26,800 mAh precisely to stay below this limit.

Does a 5,000 mAh power bank fully charge a phone?

Usually not for modern phones with 4,000–5,000 mAh batteries: with conversion losses it reaches about 70–80% of a charge from zero.

Does using the phone while charging change the result?

Yes: part of the energy powers the phone instead of charging its battery, so the number of charges falls.

Does a power bank lose capacity over time?

Yes, like all lithium batteries: after a few hundred cycles the real capacity drops. An old power bank gives fewer charges than a new one.

Does it work for tablets and laptops?

Yes for the number of charges, entering the device's battery capacity. For laptops, also check the power bank's output power in watts.

Why does the calculation use mAh rather than Wh for the device?

Because mAh is the easiest figure to find. The comparison holds when the cells have similar voltages, as with lithium phones and power banks.

Is the result precise?

It is an estimate: real efficiency depends on the model, the cable, the temperature and the age of the batteries.