Competing Wireless Power Standards: Qi vs. Rezence

By Noah Imam, Inteleaf, for Mouser Electronics

Having to charge multiple devices can be frustrating. Whether it’s at an airport terminal or a coffee shop, power outlets can be a contentious commodity. Imagine if you could charge your device simply by setting it down. No more wires, dongles, or bulky chargers. With wireless charging technology, that’s likely to become commonplace.

Today, the average person may have a phone, laptop, tablet, and a smartwatch, which are all steadily losing battery power and waiting to be recharged. According to the Radicati mobile statistics report, there will be over 9 billion mobile devices in 2015. In the midst of it all, a tech standards battle looms over unplugging and charging wirelessly. We’ve seen these before, first with VHS vs. Beta, USB vs. Firewire, and more recently between HD DVD and Blu-ray Disc. With potentially billions of dollars at stake, Qi (pronounced “chee”) and Rezence (pronounced “REH-zence”) wireless power standards are vying for the same lucrative market share. How do these technologies compare, and which of these will emerge as the industry standard?


Figure 1: Qi-compliant Wireless Charging System (Source: Texas Instruments)

Physics Behind Wireless Charging Systems

Strong coupled inductive charging is analogous to how a transformer operates with a primary and secondary winding. Sending a current through the primary winding results in a magnetic field described by the Biot-Savart Law:


Biot Savart


Since r = distance, the magnetic flux (dB) is increased as distance is decreased. Inductive charging requires two planar coils of similar size, orientation, and in close proximity (within 5mm) for efficient power transfer. Although inductive charging is limited to a one-to-one relationship between the transmitter and receiver, it is a simple and efficient implementation of wireless power.

Weakly coupled magnetic resonance, although less energy efficient, reduces the proximity requirements of inductive charging. Nikola Tesla was famous for his experiments using the electromagnetic field (EMF) of the Earth’s ionosphere in an attempt to distribute energy. Similarly, magnetic resonance (non EMF) relies on magnetic fields but allows primary and secondary coils to have fewer magnetic field lines in common. This increases the system’s spatial freedom, allows the coils to differ in shape and size, and allows multiple devices to charge from a single source, establishing a one-to-many relationship.

Resonance is a property of an oscillating system which drives another system to oscillate with a greater amplitude at a designated frequency. For example, if you have two tuning forks that respond to the same frequency, by playing one fork the other tuning fork will begin to vibrate due to resonance. This same property is achieved in electrical systems through resistor, inductor, and capacitor (RLC) circuits. Efficiency in this case is modeled by the ‘Q’ factor which represents electrical resistance:

Electrical Resistance


Although this is independent of distance, for a field to propagate over a larger area requires greater power, which is why resonant systems are less efficient compared to inductive chargers.

Strong Inductive Coupling

In 2009, Qi introduced its wireless low power specification backed by the Wireless Power Consortium (WPC). WPC is a group of over 200 technology companies, smart phone manufacturers, and semiconductor suppliers. The technology is tried and tested and relies on strong inductive coupling, providing 5W of power between small distances of around 5mm. The Qi spec also includes a dedicated communication protocol allowing devices to specify the exact amount of power they need. Perfect for small devices such as phones, mp3 players, and portable Bluetooth speakers, the Qi standard has been adopted by various OEMs including Samsung and Nokia for the Galaxy and Lumia mobile phones. Since it’s introduction there have been over 800 Qi licensed products available on the market. Going forward, Qi continues to be an attractive solution for medium and high power devices from 15W up to 1kW.

Example Qi Wireless Power Consortium Compliant RX/TX configuration


Figure 2: Example Qi Wireless Power Consortium Compliant RX/TX configuration

Loosely Coupled Magnetic Resonance

Rezence approaches wireless charging through magnetic resonance technology, providing up to 50W of power. Supported by the Alliance for Wireless Power (AW4P), which merged with the Power Matters Alliance (PMA) on June 1st, 2015, Rezence is supported by over 250 companies and is strongly advocated by semiconductor giant Intel. The main difference between the two wireless charging methodologies stem from their inductor coil configuration.

Rezence implements loosely coupled magnetic resonance, which provides greater freedom for the charging device with respect to positioning. For example, a device can still be fully charged even if there is a book between the device and the charging mat. This added mobility comes at the cost of reduced charging efficiency, about 45% vs 65% for Qi. Furthermore, Rezence requires the device to have Bluetooth support in order to communicate its power needs and Bluetooth is common on most mobile devices. Although today’s Qi devices are for the most part tightly coupled, the Qi standard provides flexibility to support both resonant and inductive technologies, while Rezence remains strictly resonant and loosely coupled.

Alliance for Wireless Power (A4WP) compliant hardware overview of Rezence Wireless Power demonstrating loosely coupled resonator and wireless data communication.


Figure 3: Alliance for Wireless Power (A4WP) compliant hardware overview of Rezence Wireless Power demonstrating loosely coupled resonator and wireless data communication.

Don’t Put Your Eggs All in One Basket, Just Yet

Technology alone won’t determine the winner. If we recall the Blu-ray and HD DVD showdown, Blu-ray ultimately won because its hardware was more accessible via the Playstation 3 and Blu-ray content dominated the retail outlets and distribution channels. Microsoft attempted to even the playing field with an HD DVD add-on for the Xbox 360, but there was little incentive for the average user to purchase additional hardware. However, unlike the rigid divide between Blu-ray Disc and HD DVD, the companies backing Qi and Rezence have not taken a firm position on one standard over the other. Both Qi and Rezence have shared backers suggesting that manufacturers haven’t yet reached an inflection point that would indicate which technology they will choose to support in the long term. IC manufacturers such as Texas Instruments and IDT want to supply to the largest market possible. OEMs like Samsung, Toshiba, and Lenovo want to avoid obsolescence. Their goal is to maintain compatibility with future versions of wireless charging platforms for as long as possible. Neither IC manufacturers nor OEMs currently benefit by alienating themselves from one technology or siding with the other.

Attrition Over Market Share

Qi undoubtedly has a lead in market share among mobile phone manufacturers and other wireless electronics, with products such as the Google Nexus 6 and Samsung Galaxy S6 topping the list. Nonetheless, the adoption rate of wireless charging has been slow. According to the IHS Consumer Awareness of Wireless Charging report, only 20% of mobile phone users are charging wirelessly. However, in 2015, knowledge and awareness of wireless charging more than doubled to 76% of US consumers. More importantly, the same report states that the market for wearables that charge wirelessly is expected to rise from just over 400 million USD in 2015 to 1.1 billion USD by 2019. Adoption of wearables may give Qi an insurmountable advantage. Publicly, Apple has remained passive between the Qi and Rezence struggle. Nonetheless, the Apple Watch charger, while not officially a Qi licensed product, is Qi compliant and leads the wearables market accounting for over 70% of total revenue. With such a dominant presence, Apple may cause other manufacturers to follow suit in the same way they have with previous technologies such as touch screens and fingerprint readers.

It’s Not Over ‘til it’s Over

Rezence and the A4WP specification are newer and manufacturers haven’t had a chance to release products that implement this technology. Despite Qi dominating existing wireless charging products, mass public awareness of Rezence could quickly change that. Select locations of Starbucks, Coffee Bean & Tea Leaf, and McDonald’s are now providing Rezence Powermats. More importantly, Intel is pushing for next generation laptops, Intel Atom powered tablets, keyboards, mice, and other PC peripherals to adopt Rezence wireless charging. Since Intel heavily influences marketing and advertising for OEMs such as Dell, HP, and Lenovo, it’s very possible that we may see a rapid shift to Rezence wireless chargers.

Medical device and automotive technology markets, which are more risk averse and focused on reliability, are less likely to standardize on a technology until one becomes predominant. However, should one standard evolve to focus in on a specific industry, they could certainly solidify their position in the market as a standard for many years to come.

To The Victor Go the Spoils – The Key to Market Success

Ultimately, the wireless power standard format war will be largely determined by three factors:

  1. Charge time
  2. Ease of use
  3. Availability

Consumers don’t care about how a solution is implemented as long as it works flawlessly with their devices. Furthermore, in order for wireless charging to become common, Qi and Rezence will have to push device manufacturers to bundle wireless chargers as a standard accessory instead of a costly additional purchase. Chargers that require additional applications, manual pairing, or external charging cases will be less popular than devices that seamlessly integrate wireless charging. Many already know the abysmal user experience of wall chargers, with tangled wires and outlet hunting. Qi has demonstrated an easy-to-use solution that is already available in numerous devices whereas Rezence currently offers faster charging rates. In the future, the success of either technology will depend on how fast industry heavyweights such as Intel and Apple influence and integrate wireless charging solutions widely into their end products.

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