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Explainer: How Apple's AirPower Qi wireless charging mat works

Written by techgoth

Apple surprised few people by announcing it was bringing wireless charging to the iPhone 8, iPhone 8 Plus and its high-end iPhone X – the rumours have been around since the company became a member of the Wireless Power Consortium – but it did raise eyebrows with the unveiling of its own charging plate, the AirPower.

At launch, the iPhone 8, 8 Plus and X will work with third-party wireless chargers, namely those from mophie and Belkin, but from 2018, Apple will sell its own Qi-enabled power plate in stores and online designed specifically for its next-generation handsets as well as the Apple Watch Series 3 and AirPods.

The Apple Watch already charges wirelessly, via a magnetic charging sensor on back of the watch, and a dock. This dock uses MagSafe technology, rather than Qi. The brief glimpse at AirPower showed that the Apple Watch 3 will charge via Qi, and hinted that a Qi-enabled second-generation AirPods case was on the cards, too.

Wireless charging: What is it?

As the name suggests, wireless charging lets you charge devices without wires – although it’s not quite that straightforward.

You do, of course, need a wire to connect the charging mat to a power source, so wireless charging still currently requires you to carry a cable. The Apple Watch is a case in point – it charges wirelessly on a magnetic charging dock, but you still need to carry the charging cable and MagSafe dock with you, and plug it into the wall or a USB charger.

Instead, the ‘wireless’ element of wireless charging refers to the fact you don’t need to connect the cable to a port on the device. It charges without a physical connection.

How does wireless charging work?

Wireless charging uses electromagnetic fields to “transfer power from a transmitting source to a receiving device” to charge a battery and is based on the principle of magnetic resonance.

 Magnetic resonance is also known as Inductive Power Transfer (IPT), and, as a result, wireless charging is often referred to as inductive charging. Put simply, IPT is the process of transferring an electrical current from one device to another through coils that generate an electromagnetic field.

Delving deeper into this process, the voltage from the plug or mains is converted into a high-frequency alternating current. This current is sent to a transmitter coil via a transmitter circuit to create a magnetic field in the coil. The transmitter coil is what you find in the charging plate or dock.

When a smartphone, or the Apple Watch, is placed on this wireless charging plate, the magnetic field spreads to the coil on the device, known as the receiver coil, which is then converted back into a current on this receiving device. This current inside the receiver coil is converted into a direct current which flows into the battery to charge it.

This process only works when both coils resonate at the same frequency, which is why it’s known as resonant, or magnetic, coupling. Wireless charging and inductive charging is also often referred to as resonant charging, but they’re not the same thing.

Inductive charging vs resonant charging: what’s the difference?

As explained, inductive charging passes the charge between a transmitter and receiver coil when the two devices are close together, typically touching or separated by less than 7mm. The coils have to be aligned, too, so inductive charging is used predominantly for so-called “direct-contact applications”.

Some charging plates, or transmitters, use multiple coils in their technology to make sure that no matter where the phone, watch or other device is placed, its always in the “charging area.”

Resonant charging takes things a step further. Seen as the more advanced of the two technologies, it uses similar methods to those used in inductive charging but removes the need for the two devices to be touching or in close proximity.

On resonant chargers, the transmitter can detect and charge devices through non-metal surfaces up to 45mm away. Resonant chargers are, as a result, often placed under a table or inside larger or thicker devices where the receiving coil can be better embedded.

There is a third wireless charging technology that uses radio waves. This type of wireless charging is used to charge devices that run on smaller batteries and which require little power, such as hearing aids or wireless mice. 

In the case of radio wave wireless charging the transmitter that is connected to a socket. When the device receiver is set to the same frequency as the radio transmitter, the battery will charge.

Qi charging: What is Qi?

Qi, Chinese for “energy flow”, is a wireless charging standard set up by the Wireless Power Consortium (WPC) in 2008.

In 2012, WPC member Powermat spun out its own standard under the Power Matters Alliance (PMA). Similar to Qi, PMA runs on a different frequency and is less widespread than Qi. The same year Powermat set up PMA, Samsung and Qualcomm launched the Alliance for Wireless Power (A4WP) with a type of resonance charging that has greater flexibility and freedom of movement, letting people charge multiple devices at once. 

Qi uses both inductive and resonant chargers “because inductive and resonant technologies have unique use cases and benefits,” according to the WPC and “Qi products are incorporating both to offer the innovative services and features in the most energy-efficient and cost-effective possible.”

What phones support wireless charging?

Samsung Galaxy phones since the Galaxy S6 and S6 Edge have offered a form of wireless charging. As have a number of BlackBerry devices, including the Blackberry Priv and BlackBerry Z30.

Other devices include:

• Google Nexus 4-Nexus 7 • Samsung Galaxy Note 5 and Note 8 • Samsung Galaxy S7, S7 Edge, Galaxy S8, Galaxy S8 Plus, Galaxy S • Sony Xperia Z3V and Xperia Z4V
• Microsoft Lumia 950 and 950 XL
• Motorola Droid Maxx, Mini, Turbo, Turbo 2, X Force
• Nokia Lumia 735, 830, 920, 928, 930 and 1520

Additionally, the HTC 10, LG G5 and LG G6, Samsung Galaxy S3 and S4, Sony Xperia Z, Z2, Z3 will support wireless charging via an adapter. It is also possible to add wireless charging to existing Apple devices using either a mophie case and charging pack.


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