Wireless charging technology in 2015: what you need to know

Wireless charging is a technology with plenty of potential, ranging from promises of cordless charging at your desk to boosting your battery as soon as you walk through the door. However, most Android OEMs only appear to be making a token effort to push this technology into the mainstream and many consumers are still waiting for a truly convincing implementation.

As is currently stands, there are three big groups each promoting their vision for the de-facto future standard of wireless charging: the Wireless Power Consortium, Alliance for Wireless Power, and Power Matters Alliance. Recent merger intentions between two of the industry’s largest groups may signal a big push into products later this year. In which case, we should probably know a little bit more about each of the big names.

Qi

Qi (pronounced chee) is probably the most well-known of the various wireless standards, having made an appearance in the popular Nexus 4 and 5 smartphones, Nokia’s (Microsoft’s) Lumia range, and a handful of flagship smartphones, such as the LG G3 and Galaxy S5. The Qi standard was developed by the Wireless Power Consortium, which was established back in 2008 and currently boasts 206 companies in its membership program, including big tech names like Microsoft, MediaTek, Qualcomm, Samsung, and Sony, among others.

Despite missing from the spec sheets, both the Nexus 6 and Galaxy Note 4 support Qi wireless charging. Although, you will require a compatible cover for the Note 4.

Despite early adoption, this generation has seen Qi wireless charging move from a standard feature in some models, to optional smartphone cases sold separately at additional cost. Many Qi phones no longer come with a charger by default either, see the Nexus 6 or Droid Turbo, turning it an option that many consumers aren’t even aware of.

Qi may have taken a backseat this generation for a number of reasons. Qi cases are slightly more expensive to produce than ones without a coil, which could be the first cut to make as price competition increases. Alternatively, the questionable practically of Qi may be hindering its popularity, but more on that in a minute.

Coils of wire create magnetic fields, which can be used to generate a current flow in a separate, insulated coil. This is the basis of transformer technology.

Qi has been around for quite a while and is based on the core principle of electromagnetic inductance. Essentially, this technology is powered by two coils of wire. The charging station contains one coil which produces an oscillating magnetic field, which induces an alternating current in the receiving coil located on the device being charged. Communication between the base station and receiving unit controls the output power of the transceiver if it is to be used with multiple devices.

The latest Qi specification states a transfer of around 5W of power when using the appropriate secondary coil, which is typical of older smartphone chargers connected to the mains supply.

An important point for later is that Qi uses tightly coupled coils for high transfer efficiency, but this necessitates that the coils are tuned slightly “off-resonance” frequency, as, oddly, two close tightly coupled coils don’t work very well with a matched resonance. This results in the most efficient transfer of power but at the cost of high sensitivity to coil misalignment.

Tightly coupled coils ensure maximum power transfer (left), while smaller loosely coupled coils operate at greater distances with reduced power (right). Source: WPC

The close nature of this connection ensures a good level of transferable power without the need for wires, but it does have its compromises. These drawbacks mainly boil down to the limited distance allowed between coils, which means that devices have to be lined up quite accurately in order to achieve a good connection and power can only be transmitted to one device at a time per transmitting coil. Even though the Qi standard ensures compatibility across all certified devices, different sized smartphones and docking stations sometimes fail to play nicely with each other.

Often it ends up being just as easy to keep a USB cable or standard dock at your desk than it is to buy a compatible, more expensive Qi docking station. The lack of any major benefit over normal charging is a probably cause as to why Qi has not become a more mainstream technology.

Rezence

Highly tuned, high Q resonance circuits create a higher power transfer at the base, allowing for larger coverage distances. Source: Hyperphysics

The Alliance for Wireless Power may not have established products like the Wireless Power Consortium, but its technology is just as promising. Just like Qi, you will find a wide selection of big tech names among A4WP’s members list, including Qualcomm, Samsung, and Intel. All of which are interested in its wireless charging standard known as Rezence.

Rezence, as the name implies, makes use of resonant inductive coupling, which is an ever so slightly different take on the technology used by Qi. The principle still uses coils of wire to create a usable magnetic field, but also tunes the frequency of the oscillation to precisely match between receiver and transmitter. This increases the maximum transfer distance before power diminishes, but looser coupling between the coils results in suboptimal power transfer.

Well coupled (similarly sized) coils allow for an excellent capture of the magnetic field at close range, but quickly fall off at a distance. A loose (smaller), on-resonance receiving coil can continue to make use of the magnetic field at greater distances and can also operate almost anywhere within the magnetic field, albeit with a decreased level of power.

Tight coupled, off-resonance coils (left) allow for maximum power transfer, while loose, resonant coils (right) can be placed anywhere in the field.

Rezence’s latest specification states a maximum power transfer of 5W at greater than 80 percent transfer efficiency, again quite typical of older smartphone chargers, with a maximum of eight devices charging at one. However, this power will decrease with the number of connected devices and distance. 5cm has been stated as a typical operating distance, although no measurement is declared in the specification.

What Rezence may lose in maximum power transfer it makes up for in product flexibility. Rather than producing single use transmitter docks or complex and expensive multi-coil transmitters, Rezence circuits can charge multiple devices, without having to worry about alignment. It’s worth noting that Qi has its own resonant prototypes in the works and chargers can be built to simulate the effects of resonance coupling with existing Qi products, which will appear in the upcoming 1.2 specification. Guided Positioning and Free Position modes for devices are already in the latest Qi specification, suggesting that Qi is also interested in this approach.

Personally, I find the idea of being able to place multiple devices on a charging mat or desk far more useful than requiring a dedicated docking station for each device. That said, Qi may be the better solution if you want the fastest possible wireless charging, although mains charging via Quick Charge 2.0 is faster still. WiTricity is also a member of the A4WP and bases its technology on the same magnetic resonance principle, which it says can work up to several metres in specific scenarios.

The additional distance of a resonance based technology means that charging mats can be placed under tables and other surfaces to create discreet charging stations. Rezence charging accessories and compatible mobile devices will be rolling out throughout 2015.

Power Matters Alliance

The Power Matters Alliance is the last of the big three groups involved in wireless power standards and is overseeing a couple of interesting technologies. The PMA and A4WP are the big two that recently agreed to merge, in order to help promote their standards as an alternative to the more established Qi brand.

Until recently, the PMA was promoting its own inductive charging standard, which is very similar to Qi. In fact, some smartphone vendors had chosen to adopt PMA instead of or alongside the Qi standard in the US, and some chip makers have begun proving dual solutions for both technologies at little extra cost. Last year, Starbucks announced plans to adopt PMA chargers in its stores. In addition to this, the group also manages an extended layer of digital monitoring and policy APIs for wireless power.

However, following the agreement with A4WP, the PMA will now be shifting its focus to include resonance based charging alongside its tightly coupled inductive standard. Both groups will be adopting and supporting each other’s standards for future design specifications, meaning that wireless charging is likely heading for dual-mode operation, offering the benefits of inductive and resonance solutions. Qi also looks to be heading in this direction with its latest prototypes, but it remains to be seen if this more complicated solution is economically viable.

The Power Matters Alliance is also working with a new company named Energous, which own the unique WattUp technology. This is quite a bit different from the magnetic field based technologies of Rezence and Qi. Instead, WattUp uses a radio frequency system to transmit power up to 15 feet from the charging station. Using the 900MHz (5.7-5.8GHz) RF spectrum, WattUp targets your device’s receiving antenna with lots of small signals that add up to a decent current with which to charge your device. Power levels, charging times and schedules can all be managed with the company’s software and up to 12 devices can be charged at a single time.

However, Energous’ WattUp only delivers 1W of charging power at 15 feet, increasing to 4W nearer the charging station, which isn’t as fast or reliable as the competition, especially when you consider that Quick Charge 2.0 mains power typically operates at around 10 watts and can reach 36 watts on supported devices. On the plus side, WattUp can power your devices constantly without having to leave them on a dedicated pad, so your smartphone can start charging as soon as you walk in the door. Bringing WattUp to market is more challenging, as the expensive hub is clearly a bigger investment than a small docking station, but home and business use are both on the cards.

What to expect in 2015

You can now probably see why the industry has been so indecisive about making a commitment to any one standard. There is no perfect solution to wireless power yet. Rather, there’s an assortment of workable ideas to choose from, each with their own pros and cons. Not to mention that better technologies are still being worked on and some ideas haven’t even been presented to consumers yet.

With competition over standards expected to pick up this year, we may see a wider range of smartphone accessories to ensure compatibility with various technologies.

Qi hasn’t taken off in its current form, instead resonance or dual-mode charging is likely to be the next stop

As for where things are heading, Qi clearly hasn’t taken off in its current form, instead resonance and/or dual-mode charging is likely to be the next stop. This should overcome most of the practical and performance issues associated with the early first generation technologies. Ideas like Energous are also promising, especially in the business space where entire offices could be built to operate on wireless power. But this is likely further away and requiring substantially more investment and time to perfect than smaller gadget accessories.

The big question still to be confronted by wireless power is: why should the majority of consumers choose it over the convenience of a bedside USB cable? The first group to solve this issue both simplistically and economically could unlock a market with hundreds of millions of potential customers. Wireless charging still has a lot to prove, but the technology is right around the corner.