The Science Behind Wireless Charging: Induction, Qi, Qi2

The Science Behind Wireless Charging

Wireless charging can feel like a trick: you set your phone on a pad, nothing plugs in, and the battery starts filling up. But what’s really happening is classic electromagnetism plus modern control systems working together.

At the heart of nearly all phone “wireless charging” is near-field magnetic power transfer. A charger creates a changing magnetic field, a coil inside your phone captures that changing field, and electronics convert it into usable DC power for the battery. The reason it works smoothly (and safely) is because standards like Qi define how devices detect each other, negotiate power, and shut down when something isn’t right.

This article breaks the topic into four layers:

• The physics (how energy crosses the air gap)

• The system design (what’s inside a charger and a phone)

• The standards (Qi and Qi2, and what they actually control)

• The real-world factors (alignment, cases, heat, and foreign objects)

Table of Content

1. The Science Behind Wireless Charging
2. What wireless charging really is
3. The physics: induction and coupling
4. Induction vs resonance: what changes (and what doesn’t)
5. How Qi charging stays controlled
6. Qi2 and magnetic alignment
7. Why wireless charging can be slower (and warmer)
8. Foreign objects and safety features
9. Safety guidelines: what standards are designed to respect
10. Where wireless charging is going next
11. FAQs
12. Reference

What wireless charging really is

Most consumer wireless charging is not “power through the air” like Wi-Fi. It’s a short-range magnetic link between two coils:

• A transmitter coil in the charging pad

• A receiver coil in the phone (or case)

The Wireless Power Consortium (WPC) describes Qi charging as magnetic induction: alternating current in the transmitter coil creates a magnetic field, which induces a voltage in the receiver coil.

Because it’s near-field, distance matters a lot. The closer and better aligned the coils are, the more efficiently power transfers.

The physics: induction and coupling

Faraday’s law in plain language

Faraday’s law says: a changing magnetic field induces a voltage in a nearby circuit.

Wireless charging uses that on purpose:

1. The pad drives an alternating current through its coil

2. That makes an alternating magnetic field

3. The phone’s coil “picks up” that changing field and produces an AC voltage

4. The phone converts that to DC and regulates it to charge the battery

Lenz’s law and why the phone affects the pad

Lenz’s law says the induced current flows in a direction that opposes the change that created it.

In practice, that means the phone and charger behave like a coupled system: when the phone draws more power, it changes what the charger “feels,” and the charger adjusts accordingly.

Induction vs resonance: what changes (and what doesn’t)

Inductive coupling

Qi phone charging is primarily built on inductive coupling: strong transfer when coils are close and aligned, weaker when they aren’t. The Qi intro document spells this out: coupling (and efficiency) drops with misalignment, extra distance, different coil shapes, and foreign objects.

Resonant coupling (the “tuned” version)

Resonant systems tune both sides (transmitter and receiver) so they naturally exchange energy more effectively at a particular frequency. A famous demonstration by Kurs et al. showed efficient non-radiative transfer using strongly coupled magnetic resonance (in a lab setting) and is often cited as a milestone for resonant WPT.

For phones, though, the big constraints are still the same:

• The system stays near-field

• Alignment and separation dominate performance

• Heat and safety limits shape real-world power levels

How Qi charging stays controlled

A wireless charger isn’t just two coils—it’s a controlled power system with communication.

What’s inside a charging pad

A pad takes input power and uses electronics to drive its coil with alternating current. During operation, it must also monitor conditions (like power draw and temperature) and respond to the phone’s requests.

What’s inside the phone

The phone has a receiver coil plus power electronics that convert the induced AC into regulated DC. The phone then decides how much power it can accept based on battery state and temperature.

Power negotiation: why Qi is interoperable

Before charging begins, Qi devices communicate so the charger can confirm the device is compatible and determine how much power to send. The Qi v1.3 intro describes negotiation phases and explains that the receiver requests power and the transmitter delivers the requested amount.

Qi also defines common power profiles:

• Baseline Power Profile (≤ 5 W)

• Extended Power Profile (up to 15 W)

Qi operating frequency (and why it’s in the kHz range)

Qi typically operates in the 87–205 kHz range. The spec notes that transmitters can use operating frequency as a control method and that resonance in the system often sits near the lower end of this range.

Communication while charging

Qi isn’t “set it and forget it.” It has a defined communication link:

• The receiver sends information by modulating reflected impedance (load modulation / ASK)

• The transmitter can send info back using frequency shift keying (FSK)

That’s why a phone can tell the pad to:

• Increase or reduce power

• Stop when the battery is full

• Shut down if something looks unsafe

Qi2 and magnetic alignment

Qi has been around since 2010 and has expanded into a large ecosystem—WPC says there are over 13,000 Qi Certified products.

Qi2 is an evolution aimed at making the experience more consistent:

• Qi2 introduced faster 15 W charging for certified devices

• It includes magnetic attachment technology to align devices and chargers for better efficiency and usability

Wired explainers often describe Qi2’s key feature as the Magnetic Power Profile, based on Apple’s MagSafe approach (a ring of magnets that improves alignment).

WPC also brands Qi2 25W (Qi v2.2.1) as a step up in power, launched in July 2025.

The important science point: magnets don’t “add energy.” They reduce wasted energy by keeping the coils aligned.

Why wireless charging can be slower (and warmer)

Alignment and distance: the biggest performance levers

The Qi documentation gives a practical rule of thumb: coils should ideally be aligned, but a few millimeters of misalignment usually isn’t a problem—still, it reduces coupling and efficiency.

That “a few millimeters” is why real-world wireless charging varies so much between:

• Different phone coil locations

• Different pad coil designs (including multi-coil pads)

• Different placement habits

Cases and accessories

Extra thickness increases coil separation, reducing coupling. Apple specifically advises that if your iPhone is charging slowly and you have a thick or metal case, try removing it.

Apple also warns not to place objects between the phone and charger because they can reduce performance and may affect magnetic strips or RFID chips.

Where the heat comes from

Heat is mostly energy loss showing up as temperature:

• Resistance losses in coils

• Losses in the pad’s switching electronics

• Losses in the phone’s rectifier/regulators

• Extra losses when coils are misaligned (more power wasted as heat)

This is why a phone can feel warmer on a pad than on a cable, especially if placement is off-center.

Phones also manage charging around temperature. Apple notes iPhones may get slightly warmer while charging, and if the battery gets too warm, software may limit charging above 80% until temperature drops.

Foreign objects and safety features

One of the most important safety challenges in inductive charging is unintended heating of nearby conductive objects.

The Qi spec explains the mechanism clearly: the alternating magnetic field can induce eddy currents in conductive materials, heating them up. “Foreign objects” like coins or keys aren’t shielded like internal phone/charger metals, so they can heat more.

Qi requires that systems detect foreign objects and take action, but it doesn’t mandate one single detection method. Instead, compliance testing verifies a transmitter doesn’t overheat reference foreign objects in reference scenarios.

Practical takeaway:

• Keep the pad surface clear (coins, keys, metal rings/plates)

• If a charger gets unusually hot, stop and check for trapped metal or misalignment

Safety guidelines: what standards are designed to respect

Wireless charging uses non-ionizing electromagnetic fields. Major health and standards bodies publish exposure limits and guidance:

• WHO describes how exposure limits are set by national/international organizations and notes ICNIRP’s role and literature review basis.

• ICNIRP’s 2020 RF guidelines cover 100 kHz to 300 GHz (relevant because Qi operates in the low hundreds of kHz).

• IEEE C95.1-2019 covers 0 Hz to 300 GHz for human exposure safety levels.

Consumer advice stays straightforward:

• Use certified chargers from reputable brands

• Follow device maker guidance (especially about cases/accessories and pad cleanliness)

Where wireless charging is going next

Most near-term progress is about consistency and interoperability, not room-scale range:

• Qi2’s magnetic alignment improves everyday placement reliability

• Qi2 25W raises the ceiling for certified fast wireless charging within the Qi ecosystem

Beyond phones, wireless power is also growing in structured environments like vehicles. Standards such as SAE J2954 (for light-duty EV wireless power transfer) and IEC 61980-3 (requirements for magnetic-field WPT off-board equipment for stationary EV charging) show how the same core physics scales up with stricter alignment, EMC, and safety requirements.

FAQs

Is wireless charging really “wireless”?

There’s no cable connected to the phone, but power still transfers through a tightly controlled near-field magnetic link, not across a room.

Why does wireless charging sometimes stop or start?

Small shifts can reduce coupling. Apple notes that vibration (like notifications) can move the phone enough to interrupt charging on some pads.

Why does wireless charging warm up my phone?

Misalignment and extra conversion stages create losses that show up as heat. If a phone gets too warm, it may reduce charging power.

What’s the difference between Qi and Qi2?

Qi is the broad standard ecosystem. Qi2 adds magnetic alignment for more consistent placement and efficiency, and it supports certified higher-power options (including Qi2 25W branding for newer versions).

Is it safe to use wireless chargers around cards or key fobs?

Apple warns that objects between the phone and charger can reduce performance and may affect magnetic strips or RFID chips (credit cards, passports, badges, key fobs). Keep them away from the charging interface.

Reference

• Wireless Power Consortium: Qi Specification Introduction (v1.3)

• Wireless Power Consortium: Qi Wireless Charging (Qi2 / Qi2 25W overview)

• Wireless Power Consortium: How Qi Works

• Apple Support: How to wirelessly charge your iPhone

• Encyclopaedia Britannica: Faraday’s law of induction

• Encyclopaedia Britannica: Lenz’s law

• Kurs et al. (Science, 2007): strongly coupled magnetic resonance WPT

• WHO: Protection norms and standards (EMF exposure guidance)

• ICNIRP: RF Guidelines 2020 (100 kHz–300 GHz)

• IEEE: C95.1-2019 overview

• SAE: J2954 wireless power transfer standard overview

• IEC: IEC 61980-3:2022 overview