How Power Banks Work: How Many Charges From 10,000 mAh?

In everyday life

When your phone runs low while you are out, you may take a power bank from your bag and plug it in. The box shows a big number such as "10,000 mAh" or "20,000 mAh."

Say your phone has a 4,000 mAh battery, so you expect 2.5 charges. In practice, you often get fewer than two. The power bank is not broken, and the label is not a lie. The number on the box and the amount that reaches your phone are counted at different places.

Power bank fires and changes to airline rules are often in the news, too. This article starts with how a power bank works inside. Then it covers how to read the numbers and how to use one safely.

How it works 1: What's inside

A power bank has two main parts inside. Opening one is dangerous, so we look at a diagram instead. One part is the battery that stores electricity. The other is a board of circuits (the circuit board) that moves electricity in and out. Several chips with different jobs work on this board.

Let's look at each part's job, starting from the bottom of the diagram.

How it works 2: The "cell" that stores electricity

Each single battery unit that stores electricity is called a cell. Power bank cells are lithium-ion batteries, the same kind used in phones. One cell gives about 3.6–3.7 V, and the value on the label differs by product. Products with a large capacity connect several cells together.

Some boxes say "lithium polymer battery." This is not a different kind of battery. It is one way of making a lithium-ion battery. The name usually means that the liquid inside is made into a gel and packed in a thin pouch. From the user's point of view, the two behave almost the same (Battery University). Because these cells can be made thin and flat, they suit slim products.

How it works 3: Making 5 V from 3.7 V

Here there is a problem. USB power is set at 5 V to begin with ("What Is USB?"). But a cell gives only about 3.7 V. On its own, that is not enough voltage to pass to a phone.

So a circuit that raises the voltage is used. A transistor switch turns the current on and off hundreds of thousands of times a second or more. While it is on, energy is stored in a part called a coil, which is wound wire. The moment the switch turns off, the stored energy is added on top of the battery's voltage and sent out. To keep things simple, we leave out parts such as the one that smooths the output voltage.

A circuit that makes a higher voltage than its input in this way is called a boost converter. Some chips that charge the battery (charger ICs) can also boost. One example is the BQ25895 from Texas Instruments. The idea of shaping electricity with fast on/off switching is explained in "What Is a Power Semiconductor?"

Boosting is very efficient, but its loss is not zero. The switch, the coil and the wiring each have a little resistance, so they give off heat when current flows ("What Is a Resistor?"). The cell also has a small resistance inside, which gives off heat too. These losses are why a power bank gets warm when you use it.

How it works 4: The PD controller agrees on the voltage

Products that support fast charging can put out 9 V and other voltages, not just 5 V. The power bank and the connected device agree on which voltage to use. They talk through a wire in the cable. The rules for this conversation are called USB Power Delivery (USB PD).

A chip called the PD controller handles the conversation. Once the voltage is settled, it tells the circuit that makes the voltage what the target value is. Right after you plug in, until the agreement is reached, only 5 V comes out ("What Is USB?").

Some products use a circuit that can make voltages both higher and lower than the cell's. This is called a buck-boost converter. It helps when the output voltage range is wide, or when the power bank is charged at many different voltages.

Even with a single USB-C port, a power bank can be both the "receiving side" and the "sending side." Plug it into a wall charger, and it takes in power and charges its cells. Plug a phone into it, and it sends power out.

How it works 5: A watchdog chip, and the charge-level display

Right next to the cell sits a watchdog chip called the protection IC. It looks for problems, such as a voltage that is too high or too low, or a current that is too large. If it finds one, it turns off a switch in the path of electricity. The idea is the same as the protection IC in a phone's battery pack.

When you press the button, the number of lit LEDs gives a rough idea of how much charge is left. The remaining charge is estimated by counting the current that flows in and out, or by measuring the voltage while the battery rests (Texas Instruments). Some products use a dedicated chip for this, called a fuel gauge. In others, the charging chip makes a simple estimate.

Even if you store it unused, the charge slowly drops. The cell itself loses a little electricity. The watchdog circuits also keep using power without a break. The current a circuit uses just by waiting like this is called quiescent current.

How many phone charges is "10,000 mAh"?

mAh (milliampere-hours) is an amount of electric charge: current multiplied by time. For example, 1,000 mAh can supply a current of 1,000 mA for one hour. A power bank's "10,000 mAh" is counted at the cell's voltage, about 3.7 V. But it sends power to the phone at a higher voltage, such as 5 V.

The energy that electricity carries depends on "voltage × amount of charge." The unit for energy itself is the Wh (watt-hour). To get Wh, multiply mAh by the voltage and divide by 1,000 ("milli" means one thousandth). For the same energy, a higher voltage gives a smaller number in mAh.

Let's work it out. Steps ① to ③ follow the example on Anker's support page. Steps ④ and ⑤ are assumptions made by this article. The numbers are only an example, and efficiency varies with the product and how you use it.

The drop at ② is only because the way of counting changed. No energy was lost there. The energy that really turned into heat is the part lost at ③ and ④. The "rated capacity" printed in small type on the box is usually the amount counted at the 5 V output, as in ③ (Anker).

For the phone in ⑤, we used a 4,000 mAh, 3.7 V battery (about 15 Wh) as the example. Re-count the 27.3 Wh from ④ at the phone battery's 3.7 V, and you get about 7,400 mAh. It is a coincidence that this matches ②. It is about 74% of the original 10,000 mAh. That is fewer charges than the "2.5 times" you get by dividing mAh by mAh.

This share can drop further, depending on conditions. Anker Japan's FAQ says that about 30–45% is used up by circuit losses and voltage conversion. You also get fewer charges if the battery is old, if you use it somewhere cold, or if you use your phone while it charges.

Taking one on a plane: rules set in Wh

Airline rules are set in Wh, not mAh. Wh lets batteries with different voltages be compared fairly by their amount of energy. If a Wh value is printed on the product, use that. If not, multiply mAh by the cell voltage and divide by 1,000.

In Japan, the national rules for domestic flights and for international flights to and from Japan are as follows. They apply from April 24, 2026, and were current in October 2026, when this article was checked (Ministry of Land, Infrastructure, Transport and Tourism, Japan).

Keeping power banks out of overhead bins has been requested since July 8, 2025. In that announcement, the ministry mentioned a fire that burned an airliner at an airport in South Korea in January 2025. A power bank catching fire has been pointed out as a possible cause of that fire.

Japan's 2026 rules follow an international standard. The International Civil Aviation Organization (ICAO) urgently revised that standard on March 27, 2026 (ICAO).

As this shows, the rules have changed several times in a short period, and they may change again. Some airlines also set stricter rules of their own. Before you fly, always check the guidance of the airline you are using. If you are outside Japan, check your own country's rules and your airline's.

Why power banks sometimes catch fire

Inside a cell, the positive and negative electrodes face each other with a thin film between them. If the film tears and the two sides touch directly, a large current flows there. The stored energy then turns into heat in a short time. This shortcut inside the battery is called an internal short circuit.

There are three main triggers for dangerous heating. The first is outside damage, from dropping, stepping on or pressing hard on the battery. The second is tiny foreign particles mixed in during manufacturing, or poor-quality materials. The third is leaving it somewhere hot, or charging it beyond its limit (UL Research Institutes, a US safety research organization).

When heat speeds up the reactions inside, they give off more heat, and the temperature can keep rising. This vicious circle is called thermal runaway. The liquid inside burns easily, so smoke or flames may come out. How this works in detail is explained in "How Lithium-Ion Batteries Work."

In Japan, the National Institute of Technology and Evaluation (NITE) published a report on July 15, 2026. In the accident reports NITE received, products using lithium-ion batteries had 2,140 accidents in 2021–2025. By product, power banks had the most. By month, August had the most. The report also shows that many accidents happened during charging.

Choosing, using and disposing of them safely

NITE offers the "three Cs" as a motto for preventing accidents: choose wisely, use carefully, and dispose of properly. This section follows guidance from Japan's government and other public bodies.

Choosing: in Japan, check for the PSE mark and recalls

In Japan, power banks are covered by a law called the Electrical Appliances and Materials Safety Act. Since February 1, 2019, products without the round PSE mark cannot be sold (Ministry of Economy, Trade and Industry, Japan). Near the mark, the label also shows the name of the maker or importer that notified the government.

Even with the mark, a defect may be found later and the product recalled. For example, in October 2025, Anker Japan recalled power banks and other products. The reason was that foreign matter may have got into the cells at the contractor that makes them (Ministry of Economy, Trade and Industry, Japan). Japan's Consumer Affairs Agency advises people to check, even after buying. It points to its recall information site and to SAFE-Lite, NITE's search site.

Using: watch for heat, impact and anything unusual

• Do not use or leave it anywhere that gets hot, such as in a car in summer or in direct sunlight
• Do not drop it, press hard on it, or put it under something heavy
• Avoid charging it while you are out or asleep. Charge it where you can see it, with nothing flammable nearby (NITE)
• If it swells, gets unusually hot or smells strange, stop using and charging it at once, and contact the maker or the store
• Do not open, disassemble or repair it

Disposing: do not mix it with ordinary trash

Mixed in with ordinary trash, a power bank can be crushed in a garbage truck or at a treatment plant and catch fire. In Japan, the Ministry of the Environment says such fires have happened in many areas. When you throw one away, follow the rules of your local government.

Some makers take power banks back, and some stores, such as electronics stores, have collection boxes. Which products they accept, and whether they accept swollen ones, depends on the place. If you are unsure, ask your local government or the maker.

Common myths

By the numbers

The 160 Wh carry-on limit is a little over four times the 37 Wh cell in ①. Even a larger 20,000 mAh product is 74 Wh at 3.7 V, about half the limit.

Learn more

• Related articles: "How Lithium-Ion Batteries Work" (inside the cell, thermal runaway) / "What Is USB?" (the USB PD conversation) / "How AC Adapters and Chargers Work" (the charger at the wall outlet) / "What Is GaN (Gallium Nitride)?" (small chargers) / "How Wireless Charging Works" (some power banks charge a phone you just set on them)
• See the differences in a table: Buck vs boost converter / LDO vs switching regulator / Series vs parallel circuits (voltage and capacity when cells are connected)
• Try the numbers: DC-DC converter calculator (it has a "lithium-ion battery to 5 V" example; intermediate level)
• Next terms: boost converter, buck-boost converter, DC-DC converter, power MOSFET, quiescent current