How Wireless Charging Works: Induction, Qi and Qi2

15 min read · Updated: October 23, 2026

In wireless charging, a coil in the charging pad makes a "changing magnetic field." That field makes current flow in a coil inside your phone. This effect is called electromagnetic induction. No metal contacts are needed, but the two coils must be close together and lined up. The downsides are that it uses more electricity than a cable and tends to get warm.

What you'll learn

Why setting a phone down is enough to charge it (electromagnetic induction), and how this is like and unlike a transformer
Which parts the electricity passes through, from the charging pad to the phone's battery
Why position and distance matter, and how Qi2 lines things up with magnets
How the pad spots coins and stops (foreign object detection), and why charging is slower and warmer than with a cable
Common myths about radio-wave safety and battery damage

Prerequisites: a rough idea of what voltage and current mean. Knowing the difference between AC and DC makes this easier to follow ("AC vs DC Explained"). No equations are used.

In everyday life

Put your phone on a charging pad, and it starts charging with no cable plugged in. The cases of wireless earbuds and many smartwatches work the same way. You just set them down or snap them on, and the battery fills up.

Another everyday example is the electric toothbrush. The Philips Sonicare, for example, has a coil in the charging base and another in the handle (teardown article, EE World). The handle has no metal socket, so it is easy to use in a wet bathroom.

A 2016 document from Saitama University also says this technology is essential for charging electric shavers, electric toothbrushes and cordless phone handsets. In this article, we focus on phones and look step by step at how charging by just setting a device down works.

How it works 1: A changing magnetic field makes current in a separate coil

A part made of wire wound around and around is called a coil. When current flows through a coil, it creates a field of force around it. Like a magnet, this field pulls on iron and moves a compass needle. It is called a magnetic field.

In 1831, Michael Faraday in Britain wound two coils on an iron ring for an experiment. When he connected or disconnected a battery to one coil, current flowed in the other coil only at that moment. When he left the battery connected, nothing happened (Royal Institution).

In other words, a voltage appears in a nearby coil when the magnetic field "changes." This effect is called electromagnetic induction. The key point is that change is needed. So the charging pad sends its coil a current that switches direction about 90,000 to 200,000 times a second. This keeps the magnetic field changing all the time (the basic Qi method; WPC).

A device that uses the same principle to change voltage is the transformer. Why change is needed, and how this relates to AC, is explained in "AC vs DC Explained."

An analogy

Think of wireless charging as a transformer with its iron core removed. Its two coils are split up: one goes in the pad, and one in the phone. What is similar is one thing only: a changing magnetic field carries energy between two coils that are not joined by wires.

Where the analogy breaks down: In a transformer, the iron core gives the magnetic field a path, so almost all of one coil's field passes through the other. In wireless charging, there is a gap and a phone case between the coils. Part of the field spreads out and never reaches the other coil. A rough measure of how much passes through is called the coupling factor. It is almost 1 in a transformer and 0.5 or less in Qi (WPC, the group that sets the standard; simplified here). That is why wireless charging needs things a transformer does not: alignment, power adjustment and a "conversation" between the two sides.

How it works 2: The path of electricity from the wall outlet to the battery

On the way from the wall outlet to the battery, electricity changes form several times. First, the AC adapter turns the outlet's AC into DC. Inside the pad, a chip for sending power (the transmitter IC) switches that DC on and off rapidly. This turns it into a current that changes direction quickly, which it sends to the coil. A circuit that turns DC into AC like this is called an inverter ("What Is an Inverter?").

On the phone side, a current that changes direction appears in the receiver coil. A chip for receiving power (the receiver IC) first makes it flow in one direction only. This is called rectification, explained in "What Is a Diode?" Some receiver chips use transistor switches instead of diodes to line up the direction, which cuts the loss that turns into heat. This is called synchronous rectification.

Once it is DC again, the electricity goes to the charger chip. From here on, it is the same as charging by cable. The chip sets the current and voltage to suit the battery and charges it in a fixed sequence ("How Lithium-Ion Batteries Work").

The path of electricity from the wall outlet to the battery. The dashed boxes are the charging pad and the phone, which are not joined by any wire. A cable connects the AC adapter and the pad. Left out: communication from the receiver IC to the transmitter IC, temperature sensors, and the protection IC that watches the battery. In some models, the receiver IC and charger IC are combined in one chip.

The "conversation" between the sending and receiving sides

The pad and the phone exchange signals the whole time they are charging. The receiver chip sends signals such as "stronger," "weaker" and "full, so stop." It does this by slightly changing how it takes in power. That change shows up as ripples in the current in the pad's coil.

Thanks to these signals, the pad sends only as much as is needed. With no phone on it, the pad stops sending and waits. In early versions of Qi, signals went one way only, from the receiving side to the sending side. From version 1.2, the two sides could talk both ways (WPC).

How it works 3: Why position and distance matter

The magnetic field from the pad's coil is strongest right above the coil. It weakens sharply as you move away or slide to the side. When less field passes through the phone's coil, the voltage it produces is smaller too.

To make up the shortfall, the pad sends more current. Then more electricity turns into heat in the coils and circuits. At a 2016 exhibition, TDK explained a weak point of the earlier method. If the centers of the two coils are 10 mm or more apart, efficiency drops sharply (MONOist). Efficiency here means the share of sent electricity that arrives.

A thick case or a ring stuck on the back adds distance, with the same result. This is why shifting the phone a little can make charging slow down or stop.

A side view of the coils, as if cut in half. The small circles are cross sections of the wound wire: the top four are the phone's coil, and the bottom four are the pad's coil. The curves are the magnetic field from the pad's coil. It passes up through the loop in the direction of the arrow and returns around the outside. On the left, the centers line up, and the field passes well through the loop of the phone's coil. On the right, the coil is shifted sideways, and less field passes through the loop (some passes the opposite way and cancels out). Left out: the number of turns, and the sheet of magnet-attracted material that keeps the field from spreading toward the phone's battery. Distances and sizes are not to scale.

How it works 4: Resonance makes up for a loose link

Pair a coil with a part that stores electricity for a short time, a capacitor. The result is a circuit that responds strongly at only one frequency, meaning the number of times per second the direction switches. This effect is called resonance. It is the same principle that lets a radio pick out a single station (resonant circuit).

In wireless charging, the coils in the pad and in the phone are also paired with capacitors. When they resonate near the sending frequency, power is easier to send even if the link between the coils is loose (STMicroelectronics). The pad adjusts how much it sends by changing the frequency or voltage little by little.

With stronger use of resonance, power can reach farther. In 2007, a research group at MIT in the US sent 60 watts over more than 2 m. The efficiency was about 40% (Science). But the farther the distance, the lower the efficiency. The method widely used in phones keeps the coils close together.

How it works 5: The Qi standard, and Qi2 with magnetic alignment

Pads and phones from different companies can charge each other because they follow shared rules. Those rules come from an industry group founded in 2008, the WPC (Wireless Power Consortium). Many companies, such as Apple and Samsung, are members.

The WPC's standard is called Qi (pronounced "chee"). The first version came out in July 2010 and could send 5 watts. Version 1.2 in 2015 added a level that can send up to 15 watts (WPC).

In October 2020, Apple launched MagSafe with the iPhone 12. Magnets are arranged in a ring around the charging coil, so the charger snaps into exactly the right position (Apple). At launch, the MagSafe charger delivered up to 15 watts.

In January 2023, the WPC announced a new standard based on MagSafe, called Qi2. Apple provided the technology, and magnetic alignment is at the core of the standard. Certification means testing products against the standard and giving a mark to those that pass. It began in November 2023, with an initial limit of 15 watts. In July 2025, "Qi2 25W" started, raising the limit to 25 watts (WPC).

The actual speed, however, depends on the pair of pad and phone. For example, Apple says that on many Qi-certified pads that are not MagSafe or Qi2, an iPhone charges at up to 7.5 watts.

How it works 6: "Foreign object detection" watches for coins and cards

Electromagnetic induction makes current flow not only in coils but in any nearby metal. If a coin or key gets between the pad and the phone, current swirls around inside the metal and heats it up. This is nearly the same principle an induction cooktop uses to heat a pan (Saitama University).

So the Qi standard requires the pad to notice metal and stop sending when there is a risk. This is called foreign object detection. The basic idea is to compare the power the pad sends with the power the phone reports as received. If the gap is large, the pad decides the electricity is turning into heat somewhere (WPC).

A WPC document describes uncertified chargers tested by an independent lab. With metal in place, they got as hot as nearly 200 °F (about 93 °C). That is hot enough to cause burns. This is why it is worth choosing a charger with the Qi certification mark.

Cards need care too. Apple advises not to place anything between the phone and the charger. Doing so can damage the magnetic stripe on a credit card or the chip in an ID badge. If you use a wallet-style case with card slots, check what is inside before charging.

How it works 7: Why it is slower and warmer than a cable

In wireless charging, the electricity passes through extra steps on its way to the battery that cable charging does not have. These are the switching in the transmitter IC, the handoff from coil to coil, and the rectification in the receiver IC. At each step, part of the energy is lost as heat.

In one prototype example from a research review, sending 5 watts over 5 mm gave an efficiency of about 70% (Lu et al., 2016). In a 2024 measurement by iFixit, a repair information site, wireless charging also used more electricity than a cable for the same charge. If the position is off, even more is wasted (How it works 3).

Most of the lost electricity turns into heat. What's more, that heat appears right next to the phone's battery. Batteries do not handle heat well, so this heat is a real concern.

According to Apple, an iPhone gets slightly warm while charging. If it gets too warm, it may hold back charging above 80%. This protects battery life, and charging resumes once the temperature drops. This is one reason wireless charging can feel slow.

Other kinds of wireless power

Contactless transit cards and NFC

The contactless cards you tap at transit gates have no battery inside. The reader sends out a magnetic field that swings back and forth 13.56 million times a second (13.56 megahertz). That field makes current flow in a coil inside the card, and that electricity runs the chip. The card replies by changing how it takes in power, which comes back as ripples in the field (STMicroelectronics).

This kind of communication over a few centimeters is called NFC. Tap-to-pay on phones uses the same method. The power sent is only enough to run the chip, which is a completely different scale from charging a phone.

Wireless power for electric vehicles

For electric vehicles too, systems are being developed that send power from a coil on a parking space's floor to a coil under the car. SAE, a US automotive engineering group, issued its J2954 technical standard for passenger cars in 2020. It has three levels of power, and the top one is about 11 kilowatts (SAE). The 2024 revision also sets the maximum at 11 kVA (kilovolt-amperes, a unit for the size of AC power).

There are also experiments in charging while driving. In Japan, for example, the University of Tokyo and partners began a test in Kashiwa, Chiba Prefecture, in October 2023. Coils buried in the road send power to moving cars, in Japan's first such test on public roads. Neither of these is nearly as widely used as phone charging.

Common myths

Myth: With wireless charging, you don't need any cables at all

In fact, the pad itself is connected to the wall outlet by a cable. Only the link between the pad and the phone is wireless. Also, the phone has to stay correctly placed on the pad. That gives you less freedom than a cable, which lets you hold and use the phone while it charges. It is more accurate to think of it as charging with "no contacts" rather than "no wires."

Myth: The electromagnetic waves from a charging pad are dangerous to your body

In fact, unlike X-rays, the magnetic field used for charging does not have enough energy to knock electrons out of atoms. The known effects of strong magnetic fields on the body are nerve stimulation and a rise in body temperature. An international commission, ICNIRP, sets limits in its guidelines that prevent both, with a wide safety margin. Its 2010 guidelines cover changes up to 100,000 times a second (100 kilohertz), and its 2020 guidelines cover faster ones. Also, the magnetic field is strong only very close to the coil.

Medical devices such as pacemakers are a different matter, though. Apple advises keeping the phone more than 30 cm away from medical devices while charging wirelessly. If you use a medical device, check with your doctor or the device maker.

Myth: Wireless charging ruins the battery all by itself

In fact, heat and sitting at full charge for a long time are the main things that speed up battery aging. They matter more than the charging method itself ("How Lithium-Ion Batteries Work"). Wireless charging tends to make heat, so it can be at a disadvantage there. A few habits reduce heat: line the phone up with the coil, avoid charging on bedding that traps heat, and use a certified pad.

By the numbers

Number	What it is
1831	The year Faraday confirmed electromagnetic induction
2008 / 2010	WPC founded / first version of the Qi standard (5 W)
15 W	Limit of the top level of Qi version 1.2 (2015), of MagSafe at launch, and of Qi2 (2023)
25 W	Limit of Qi2 25W (from July 2025)
7.5 W	iPhone limit on many Qi-certified pads that are not MagSafe or Qi2 (Apple)
Almost 1 / 0.5 or less	Rough measure of the link between coils (coupling factor): transformer / Qi (WPC)
10 mm	Center offset at which efficiency drops sharply with the earlier method (TDK, 2016)
About 70%	Example prototype efficiency (5 W, 5 mm distance; Lu et al.)
13.56 MHz	Frequency of the NFC magnetic field
30 cm	Suggested distance from medical devices while charging wirelessly (Apple)

The limits are maximums when both the pad and the phone support them. The power that actually flows changes during charging, depending on the battery level and temperature. Charging slows down especially as the battery nears full, by cable or wirelessly.

Learn more

Related articles: "AC vs DC Explained" (transformers and resonance) / "What Is a Diode?" (rectification) / "How Lithium-Ion Batteries Work" (charging steps and aging) / "What Chips Are Inside a Smartphone?" (power chips and NFC)
Terms: transformer / resonant circuit / Q factor (how sharp a resonance is) / rectifier circuit / synchronous rectification / USB Power Delivery (fast charging by cable)
See the differences in a table: Converter vs inverter / PN junction vs Schottky diode / LDO vs switching regulator
Try the math: series and parallel resistance and capacitance calculator / DC-DC converter calculator (intermediate)