What is Qi Wireless Charging and How It Works

Combining the old with the new, the Wireless Power Consortium (WPC) has established Qi (the Chinese word for “energy flow“, pronounced like the “chee“ in “cheese“). Qi is the reigning standard for wireless charging of battery operated devices like cell phones, iPods, MP3 players and cameras.

The concept and first practical application for inductive energy transmission was developed in the 1830’s by Michael Faraday, and it’s discovery is credited to him as “Faraday’s Law“. This was further refined by James Maxwell. (Related Article: Engineering “Laws” – Moore’s, Rock’s, Butter’s and others)

At it’s core, wireless charging (also know as inductive charging), requires a transmitter coil and a receiving coil linked by a magnetic field. The current Qi wireless charging standard has a maximum power transfer output rated at 15 watts, which is fast enough for normal use. For the new iPhone, Apple will be following the previous Qi version 1.1 standard, using only half of that at 7.5 Watts. The Galaxy S8 support version 1.2, charging at 10W. (Related Article: Best iPhone 8, X Alternatives)

For an illustrative example to help describe the workings of a Qi charger, at the left is the Belkin BOOST UP Wireless Charging Pad for iPhone 8, iPhone 8 Plus and iPhone X ($59.99 Amazon). The 7.5W transmitter coil (#3) is built into the charging pad (more generally called a “base station”), which is connected to a power source (usually to the electrical outlet) to provide the electromagnetic inductive power.

The mobile device (or whatever is needing to be charged) has the receiving coil to generate electricity. By generating an oscillating magnetic field in the base station, an oscillating magnetic flux passes through the receiving coil. This magnetic flux induces an electric current in the receiving coil which can be used to power the receiving device (or more commonly to charge it’s battery).

This is where the “inductive” term of this charging comes from; An electric current is induced in the receiving coil from the transmitter coil, over a small distance.

In order to get enough power transferred from the base station to the receiver, the distance between the two must be short and the two coils need to be aligned. To minimize the distance, the base station generally has a thin, flat surface directly on top of the transmitting coil. The receiving coil, which in the example of the iPhone is on the back of the phone just under the back plate. Adding on a small protective cover to the iPhone will not noticeably interfere with the charging. However, a ruggedized case like the OtterBox COMMUTER SERIES ($29.95 Amazon) may be sufficiently thick to decrease the inductive charging power.

Alignment, or positioning, of the receiving coil relative to the transmitting coil is critical. In the ideal case, both the transmitting coil and the receiving coil are the same shape (e.g., a circle or oval) and the same dimensions (e.g., 3 inch radius circle) and the coils are placed directly one on top of the other. This allows for the maximum magnetic flux in the receiving coil, and hence the maximum power transfer. The requirement for precisely placing the receiving coil onto the transmitter coil is referred to as “Guided Positioning”. This is the simplest, and most cost effective way to build a base station. To be effective, the base station typically has a graphic, image or other visual indicators to let users know where to place the device for optimum charging.

A more complicated, and which also adds expense, is a “Free Positioning” type base station. In this implementation, the user does not need to place the receiving device in direct alignment with the transmitting coil. The base station will either use multiple transmitting coils, possibly of different sizes (e.g., concentric or offset circles and/or ovals) or a mechanical system to move the transmitting coil to align with the receiving coil, or possibly some combination of all of the above. Free positioning implementations also require some sort of feedback from the receiver to the base station so that the base station can adjust for best performance. The majority of consumer electronics will use Guided Positioning rather than Free Positioning due to cost constraints and simplicity. Expect to find the Free Positioning systems in Medical and Industrial applications.

Which brings us to some consumer applications, and rationale for developers to implement inductive charging into their devices. Apple is not the first to build Qi wireless charging into their products, but with the recent announcements for the iPhone 8, 8 Plus and X this will help expand the current market for both chargers and devices. Smart phone makers Samsung, Nexus and Google have already had this technology available for their past few generations of devices. Third party developers have also been selling accessories to enable wireless charging on existing devices. Similar to (and sometimes including) a battery pack, a Qi back case (receiver) attaches to the smart phone and then can wireless charge when the case is placed on a charging pad.

As the transfer power increases, this will migrate to notepads, laptops and game consoles and all other battery operated devices. Imagine that you’ll no longer need any cables to tether your devices for charging. Simply place the device on a charging pad! Coffee shops (e.g., Starbucks) are looking to embed the chargers in their counter-tops; Ikea has already launched a line of desk lamps with Qi charging pads. Automobile designers are figuring out how to incorporate this into the dashboard or center console areas; Furniture makers are developing powered tables and chairs with charging surfaces.

Advantages of using Wireless Charger

  • Reduces the mess of wires in the home as well as in offices.
  • Eliminates the need to constantly change or plug in the charger to the wall sockets.
  • Compatible with all Qi enabled devices including Smart watches, Battery Packs, Game Consoles.
  • Simply Usage – put the device on the base station and it will start charging instantly.
  • Vendor Agnostic – wireless charging pads are universal in design irrespective of the model or brand.
  • Eco-Friendly – some of the chargers will automatically turn off the power when not in use, reducing standby or “vampire energy” consumption
  • Charge up to 3 devices simultaneously on a single wireless charger.
  • Portability – Wireless chargers are also considered as portable as they are easy to carry and take up less space on the table.
  • Device Life – reduces the chances of damaging the universal port of your device which is used for charging, data synchronizing, head-phones, etc

Want to learn more about wireless power? Check out these excellent books available through Amazon:
Wireless Power Transfer for Electric Vehicles and Mobile Devices ($110)
Wireless Power Transfer ($95)

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