Wireless charging of mobile phones

Smartphone, Tablet and Smartwatch are our daily companion today. While mobiles had power for a couple of days, smartphones might be empty in the afternoon today. Besides the network connections also the high resolution display and GPS consumes quite some power.

Every smartphone has to be recharged sooner or later to get new energy. And it is not alone as a lot of electronic devices need a charger, too. While these are kind of hidden at home or at work behind the desk one discovers on journeys what ballast has to be carried. On the other hand with network connections by LTE, WLAN or Bluetooth the cable of the charger is often the only thing hanging on the perfectly designed notebook or smartphone like a foreign object.

The solution is actually simple. What works for the electric toothbrush should also work for the mobile phone – inductive charging. As early as 1891 Nikola Tesla experienced with wireless energy transfer. Besides the mentioned toothbrush the technique did not succeed for a long time. The first system being used in mass production was the Touchstone named system in 2009 within the Palm Pre. Disadvantage of this and similar solutions from third party accessory manufacturers was that they only worked with a few devices from the same company. As every company built these chargers differently they consisted of many ICs, an individually designed coil, many different additional parts and an own technique to transfer the power. Except for the integrated Palm solution the third party accessories were all thick batteries with new back cover which did not look good on the smartphones which aimed to get thinner with each new device. Because of the missing mass production these solutions could not be offered affordable and failed at the consumer with prices around $100. The solution could only be an interoperable standard which would be fully integrated and produced affordable in masses.

The Wireless Power Consortium (WPC) was established on 17. December 2008 and consists of over 100 members in diverse industries, including cellular technology, consumer electronics, batteries, integrated circuits, wireless technologies, network providers and manufacturers of furniture and car equipment. The WPC standardized a specification called Qi and develops different wireless charging techniques. The Standard was named after the Chinese language word Qi respectively Chee (air or spiritual energy flow). Devices of different companies equipped with the Qi logo may now be charged with standardized Qi chargers. As the standard is now found in many devices, the service is also emerging in public areas (airports, hotels, cafes, restaurants, etc.) and the charger can stay at home more and more often.

Qi uses a magnetic induction between sender and receiver. Both exchange data with 2 kilobits per second to maintain an optimal energy transfer. The sender modulates the sending field. RFID like technology is used by the receiver to send data to the sender. The transmitted power is 5 Watt (Low Power) or 120 Watt (Medium Power) at a transmission frequency from 110 to 205 kHz (Long Wave). Multi-Layer Foreign Object Detection (FOD) ensures metallic objects may be placed on the charging pad without becoming hot due to the magnetic field. The first known Qi compatible mobile phones were Nokia Lumia 920 and Google Nexus 4. In 2019, the Meizu Zero was introduced as the first smartphone that had no longer any buttons or ports and relied on Qi for charging. Apple iPhone 15 series offered the first smartphones with Qi2 in 2023. With a software update, all iPhones back to series 12 were subsequently made compatible with Qi2, as they already contained Qi with MagSafe. The first Android smartphone with Qi2 was the HMD Skyline in 2024.

Qi Versions

The standard was developed as follows:

• Version 1.0
  • Qi transmitter delivers 5 Watt power into a Qi phone.
  • Choice of transmitter designs include single coil transmitter, coil array transmitter, and moving coil transmitter.
  • High flexibility in design of Qi receivers
  • Limited flexibility in the design of Qi transmitters
    
    
• Version 1.1
  • Increased design freedom for transmitters. You can now choose between 12 different transmitter specifications.
  • Increased sensitivity of Foreign Object Detection. This prevents heating of metal objects in the neighborhood of an active transmitter.
  • The possibility to power a Qi transmitter with a USB charger.
    
    
• Version 1.2
  • Fast charging. The possibility for transmitters to deliver up to 15 Watt power and the option for receivers to obtain up to 15 Watt.
  • An improved thermal test for transmitters
  • Improved timing requirements
  • Changed limits for Foreign Object Detection improve the sensitivity.
  • Charging of Qi 1.1 receivers in up to 30 mm distance
  • Charging of Qi 1.2 receivers in up to 45 mm distance
  • Alternate resonant charging method added
  • Charging of multiple receivers with one transmitter
  • Optional unique identifier for power receivers (WP-ID)
    
    
• Version 1.3
  • Hardware-based authentication between transmitting and receiving equipment mandatory for power transmission above 5 W
  • FOD improved
  • Reclassification of applications according to performance level
    • <1 W - RF
    • 15 W - Low Power
    • 60-300 W - Medium Power
    • 2 kW - High Power
    
    
• Version 2.0
  • MPP magnetically aligns transmitter and receiver (based on Apple's MagSafe technology)
  • Less heat generated due to optimized alignment
  • More flexible placement of devices on the charger allowed by resonant charging
  • Support for simultaneous charging of multiple devices

An example for a mobile charging pad with Qi 1.2.2 standard and up to 15 W charging power in the Extended Power Profile is the Belkin BOOST↑UP P-F7U014.

Examples for chargers which are integrated into furniture with Qi 1.2.2 standard and up to 15 W charging power are Qinside Qi1015 and ZENS PuK 3.

Since the iPhone 8, iPhone 8 Plus and iPhone X Apple also supports the wireless charging standard Qi version 1.1 with up to 7.5 watts.

Besides Qi there is another standard from the AirFuel Alliance which resulted from a merger of Duracell founded Power Matters Alliance (PMA) with its standard PowerMat and the Alliance for Wireless Power (A4WP) in mid 2015. The first two smartphones with built in PowerMat were the Samsung Galaxy S6 and Samsung Galaxy S6 Edge which both feature Qi in addition. PowerMat has the benefits of being able to charge multiple devices and this with higher power. However, Qi is found in more smartphones today than PowerMat and even Ikea already sells furniture with built in Qi chargers. The AirFuel Alliance also realized this early in 2018 and joined the WPC.

Overview

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