Figure 1: Resonant wireless charging such as Rezence technology produces a wide field tolerating imprecise alignment between charger and device under charge. (Source: Alliance for Wireless Power.)
Today’s wireless charging is like commuting to work by bicycle: great in principle but a pain in practice. Cycling promises fitness, no gas bills and freedom from public transport schedules but the reality involves dodging cars, inhaling truck fumes and arriving in the office disheveled. Similarly, wireless charging has the potential (excuse the pun) to free consumers from the tedium of finding the correct charger from the dozens of incompatible units in the kitchen drawer and to cut through the Gordian knot of power cables lurking under the office desk. Yet wireless charging systems remain thin on the ground and compatible mobile devices are rarer still.
If you drink the analysts’ Kool-Aid you’d be forgiven for thinking the paucity of wireless charging technology is merely down to a lack of consumer awareness. IHS, for example, forecasts that the wireless charging market will expand to $8.5 billion by 2018, up from $216 million in 2013. And according to Technavio, the sector will reach 33% compound annual growth rate (CAGR) by 2020. The reality, however, is that wireless charging’s progress has been slowed by technical hurdles and––surprise, surprise––commercial infighting.
Conventional wireless charging systems suffer from limited throughput so charging laptops and tablets is impractical. Moreover, some devices have to be positioned on the charging platform with the precision of a moonshot for the systems to work properly. Such drawbacks limit consumer acceptance. But things are slowly improving.
One version of the technology uses inductive technology: a charger employs a coil, powered by AC voltage, which induces an AC current in a second induction coil in the consumer’s device. The AC current is then regulated by an AC-to-DC voltage converter to charge the device’s battery. With a maximum power transfer of about 5W, charging is limited to a single smartphone. Another drawback is that inductive coupling works over an extremely short range, around 2 to 4mm, meaning the charging station and the device need to effectively be in immediate contact with one another and precisely aligned. Multiple transmitter coils can be laid over each other in the base station to widen the optimal coupling field, but that adds complexity and cost. The upsides of inductive charging are its relatively simplicity, lack of expense, maturing technology and commercial adoption.
An alternative form of wireless charging attempts to address some of the main drawbacks of purely inductive systems. Dubbed “resonant wireless charging,” the technology uses a primary coil “transmitter” generating an electromagnetic field oscillating at a frequency of 6.78MHz (which is internationally reserved for this type of application while also avoiding the heating issues seen with tightly-coupled inductive systems using lower frequencies). As with inductive chargers, the secondary coil draws power from the primary’s electromagnetic field and the AC power is converted into a DC voltage to charge the battery. Resonant wireless charging gains its advantage because by ensuring both coils resonate at the same frequency, power transfer efficiency is markedly improved over purely inductive systems.
This greater efficiency means it’s not only possible to create a much wider charging field, but also more power can be transmitted, allowing multiple devices to charge from a single charging pad at the same time. The system can even tolerate things like magazines sitting between the pad and the mobile or the device not sitting level. The key downside of resonant wireless charging is complexity. For example, the system requires close control between charger and device being charged which must be achieved (obviously) wirelessly. One popular technology employs Bluetooth Smart to do the job which does require that the device to be charged is fitted with the low power wireless technology (which is often––but not always––the case).
While promoters of resonant wireless charging point to the technical advantages of the method, the inductive charging supporters argue that increasing adoption of wireless charging lies not in figuring out the fastest or most efficient connection, but in making the technology available to people where they need it most. Placing charging stations in public locations such as Starbucks is one way to do this, saving customers from the inevitable search for mains power that results from an intensive session at the coffee shop. Placement in airports and hotels are two more ways. Some manufacturers are pursuing such initiatives as part of their marketing strategy.
Technology choice aside, a major drag on wireless charging’s widespread introduction is the inevitable standards war. Two camps now dominate the space. On one side is the Wireless Power Consortium (WPC), a group of manufacturers including the likes of HTC, Panasonic and Qualcomm, who are the driving force behind a specification known as Qi (pronounced “Chee”). The specification supports both inductive and wireless charging with the organization claiming that each has unique use cases and benefits. The organization points out that Qi is built into Nokia, Samsung and Motorola smartphones. On the other are the recently merged Alliance for Wireless Power (A4WP) and the Power Matters Alliance (PMA) (now called the AirFuel Alliance), whose membership count Intel, Broadcom and Duracell among their collective number, and who promote a resonant wireless charging standard dubbed Rezence.
Figure 2: Audi offers an optional Phone Box with Qi Wireless Charging for the brand new Audi A4 & A7 models (shown, courtesy Audi.) Qi can also be found in the Toyota Prius, Avalon, Camry, Lexus NX, the Scion xB, several Cadillacs, Chevrolets and in the Jeep Cherokee. Ford is apparently waiting for the technology to settle. Reference WSj.com.
Unfortunately, the standards struggle is doing nothing to promote common functionality, interoperability or flexibility between the WPC and the AirFuel Alliance. The outcome of the battle is so uncertain that some blue-chip electronics firms, such as Microsoft, Qualcomm and Samsung, have hedged their bets by becoming members of both bodies and introducing products that support both wireless charging technologies – hardly a recipe to limit consumer confusion.
Competing standards wars are nothing new of course, and if the inevitable shake down happens anytime soon it’s entirely possible that the analysts’ forecasts could become reality. That’s something worth praying for. Imagine how universal wireless connectivity coupled with wireless charging anytime we place our mobile devices on a desk, coffee-shop table or bedside cabinet could herald a whole new era of productivity. Don’t hold your breath though. A statement that an engineer “[Has] discovered the essential principles of wireless charging and it only remains to develop them commercially” doesn’t come from a recent WPC or AirFuel Alliance press release; rather these are the words of Serbian-American electrical engineer and inventor Nikola Tesla … spoken in 1921.
Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.