Wireless charging is a way to transfer electricity from a charger to a device through 2 nearby coils, without a charging cable between them. The charger creates an alternating magnetic field, and the phone's receiver coil turns that magnetic energy into electrical power for the battery. Qi and Qi2 are the main wireless charging standards used for this process.
Wireless Charging at a Glance
| Feature | How it works |
|---|---|
| Transfer method | Magnetic induction |
| Charger component | Transmitter coil |
| Phone component | Receiver coil |
| Charging standard | Usually Qi or Qi2 |
| Physical range | Very short, usually through a thin case or cover |
| Main limitations | Misalignment, heat, metal objects and device power limits |
| Does the charger need a cable? | Yes. The charging pad still connects to a power adapter |
How Wireless Charging Works Step by Step
1. The Charger Receives Electrical Power
A wall adapter supplies power to the charging pad. The pad changes the incoming direct current into rapidly alternating current, which flows through the transmitter coil as part of an inverter and resonant circuit.
2. The Transmitter Coil Creates a Magnetic Field
Alternating current through the transmitter coil creates a changing magnetic field. The field is strongest close to the pad, so a phone normally needs to sit directly on, or very close to, the charger.
3. The Phone's Receiver Coil Detects the Field
The receiver coil inside the phone responds to the changing magnetic field. That field induces a voltage in the receiver coil, much like a transformer transfers energy between two coils. Wireless chargers use separate coils with a weaker magnetic connection than a conventional transformer.
4. The Phone Converts the Power for the Battery
The receiver coil produces alternating electrical power. Power-conversion circuitry changes it into direct current, regulates the voltage and sends it to the battery-management system. The battery-management system controls the charging process.
5. The Phone and Charger Communicate
Qi charging includes communication between the phone and the charger. The phone reports whether it can accept power and how much power it needs. The charger adjusts its output and can stop transmitting when charging is complete.
What Are Qi and Qi2?
Qi is the main wireless charging standard for smartphones and other portable devices. The Wireless Power Consortium develops the Qi specification and certifies compatible chargers and receivers for safety, interoperability and energy efficiency.
Qi2 is a newer version of the Qi system that uses magnetic alignment on supported devices. The magnets help place the phone's receiver coil over the charger's transmitter coil, which makes placement easier and can reduce losses caused by poor alignment.
The main Qi2 power levels are:
- Qi2: Up to 15 watts for certified products.
- Qi2 25W: Up to 25 watts for compatible certified devices and chargers.
The phone, charger and power adapter must all support the relevant power level for the phone to charge at its maximum rate. A Qi2 25W charger does not make every phone charge at 25 watts.
Why Does Alignment Matter?
Alignment matters because wireless charging works best when the transmitter and receiver coils are close together and centered over one another. Efficiency falls when the coils are misaligned, farther apart or different in size and shape.
Poor alignment can cause a phone to:
- Charge more slowly near the edge of a flat charging pad.
- Stop charging if it moves too far.
- Become warmer during charging.
- Charge less reliably than it would on a magnetic Qi2 charger.
Qi2's magnetic ring holds the phone in the correct position over the charging coil.
Can Wireless Charging Work Through a Phone Case?
Yes, wireless charging can work through many thin, non-metallic cases. Magnetic fields can pass through materials such as plastic, glass, wood and air. Thick cases, metal cases, magnetic accessories and objects between the phone and charger can reduce performance or stop charging.
Remove metal objects such as coins, keys, cards with magnetic strips, jewelry and metal plates before charging. Metal can interfere with the magnetic field and become warm. Qi systems use foreign-object detection to identify unsafe objects and control or stop power transfer.
Why Does Wireless Charging Generate Heat?
Wireless charging generates heat because some energy is lost during power conversion and transfer. Misalignment, extra distance, unsuitable cases and inefficient hardware can increase those losses. A phone may reduce or pause charging if its temperature becomes too high.
For better charging performance:
- Center the phone on the charging pad.
- Use a compatible power adapter.
- Remove a thick or metallic case if charging is slow.
- Keep the phone and charger in a ventilated location.
- Keep metal objects away from the space between the phone and charger.
Apple says iPhone charging can slow or pause when the device becomes too warm. It also recommends removing the case if the case contributes to excess heat.
Is Wireless Charging the Same as Charging From a Distance?
No. Standard Qi wireless charging uses near-field power transfer, so the phone must remain very close to the transmitter coil. It does not send useful charging power across a room like a Wi-Fi signal.
The Wireless Power Consortium focuses on near-field wireless power because far-field charging faces efficiency, cost and safety problems for common consumer devices.
Wireless Charging Versus Wired Charging
Wireless charging is convenient because you can place a compatible device on a pad instead of inserting a connector. Wired charging usually provides a more direct connection and may support higher speeds, depending on the phone, cable, charger and wireless standard.
Wireless charging is useful for:
- Overnight charging.
- Desk and bedside charging.
- Car charging mounts.
- Charging several compatible devices on one charging station.
- Reducing wear on the phone's charging port.
A cable is still required between the wireless charger and the wall adapter. "Wireless" describes the short connection between the charger and the phone, not the whole power system.
Wireless Charging in One Sentence
Wireless charging uses induction: the pad's transmitter coil creates a changing magnetic field, the phone's receiver coil converts that field into regulated electrical power, and Qi or Qi2 controls the transfer.