Wireless charging: The story so far...

Rupert Goodwins looks into the technology behind wireless charging and whether it is good enough yet for wider adoption

As more and more IT escapes its tethers to run free in a high-speed, mobile world, one function remains chained to the wall – power. 

No tablet, laptop or smartphone can go for more than a day without being tied to a plug socket for hours. So, there is a lot of commercial pressure to find ways to charge up batteries over the air - even before the potential for electric vehicles kicks in.

The dream has been power over radio, beaming the energy to charge a device alongside the signals themselves. Various people have claimed to have achieved this, most famously inventor Nikolai Tesla in the early days of the last century. 

Unfortunately though, the structure of space and time itself says no; radio signals follow the inverse square law. Power dissipates exponentially with distance, so 100W at the transmitter will provide, at the very best, a retrievable watt by the other side of the room. Things must be tightly coupled to prevent that senseless dissipation.

Inductive charging

One answer is inductive charging where the charger creates a powerful, short-range magnetic field bound closely to itself and the device being charged has a pick-up coil. 

These form two halves of a transformer and if you tune them to identical frequencies, you can get efficient enough power transfer to go through a centimetre or so of plastic - no contacts or alignment needed. Drop your device onto the right surface and it starts to suck up useful amounts of juice. Or forego the resonance and align the magnetic field and you have a simpler, but still quite efficient system, although you have to be more careful about how you align charger and chargee.

That’s nothing new though; electrical toothbrushes have been doing it for years. You’ve been able to buy inductive charging mats too, which work either with special sleeves you put around your device or, if you have the right device, with the circuitry already installed by the manufacturer. It all works, but it is also a bit quirky and there is not much point in having yet another gadget to remember if all it does is act like a slightly more laid-back dock on your desk.

What is new - and what’s made wireless charging a hot topic in the last six months - is the potent combination of hundreds of millions of portable electronic devices. 

On top of that is the realisation if you make wireless power available like Wi-Fi, with lots of free charging surfaces scattered around cafes, public transport, libraries and offices, it becomes feasible to keep things going indefinitely while you’re out and about. The dead mobile becomes history. The restaurant keeps you there for an extra half an hour while your mobile - and you - top up for the journey home and, in turn, the handset manufacturer has a much more flexible device.

Several players competing for the standard

But, like Wi-Fi, this will only happen if critical mass is reached and the industry aligns around a single standard. As with any new pursuit in technology though, there are several in play.

One is the Alliance for Wireless Power (A4WP). It uses a high-frequency signal at nearly 7MHz, which is well-suited for giving the charging device quite a lot of freedom over positioning, relative to the charger. Another is Powermat, which uses a low frequency signal centred on 300kHz, needing tight coupling to the charged device.  

However, the longest established contender is the Wireless Power Consortium (WPC) with its Qi standard.

Qi uses a combination of techniques. Originally, it could only operate over a 5mm distance and needed the device to be carefully positioned on the charger. However, it has now created a multicoil configuration that can adapt the exact resonant frequency it uses - it is based around 150kHz, not far from Radio 4 Longwave - to cope with any detuning due to nearby lumps of metal or human, which is a big problem with resonant tuning.

This gives the charging device a workable service over separations as far as 5cm, which makes it a lot easier for retailers, airports and the like to securely and safely embed the chargers in furniture surfaces. 

But you are unlikely to be able to listen to Test Match Special in a room so equipped unless you do so over the internet - America doesn’t use longwave or like cricket, so is seemingly immune to this danger.

The contenders

The two big players are the WPC and Powermat, and until recently it looked as if Powermat was the underdog. 

Since it was founded in 2009, the WPC has got Qi into hundreds of device models and over eight million actual devices; it scored the Nokia Lumia 920, Google’s Nexus 4, and a Samsung-branded adaptor for the Galaxy S4. With 120 partners on board, it had all the industry relationships sewn up.

Powermat was founded in 2007 and had modest success. However, last year it formed the Power Matters Alliance (PMA) and progress ramped up. Instead of concentrating on IT industry figures, it wooed - and won - Starbucks, which took a seat on the PMA board in October. 

Starbucks is more than a high-profile retailer; it is widely credited with giving Wi-Fi the advantage in the turn-of-the-millenium wireless networking wars when it chose that technology for public internet access in its cafes. On the same day, AT&T and Google also joined up, and since then the momentum has gone PMA’s way.

General Motors and WiTricity have signed up to the PMA too. WiTricity may be the most important of those; it’s an MIT offshoot that’s been working on - and has intellectual property rights (IPR) in - the adaptive resonance systems that will give the PMA the same sort of flexibility and capabilities as Qi has now.

The most recent high-profile addition to the PMA is a Finnish company called PowerKiss, notable for two reasons. 

Earlier this year, it was in the Qi camp, but has now gone PMA-only, and it has a deal with McDonald’s and others to put wireless power charging technology in over 1,000 locations across Europe. This nearly doubles Powermat’s existing 1,500 locations in the US, as well as giving it a few hundred million potential extra users.

These aren’t insurmountable numbers, given that 2,700 locations globally is only around 5% of the total worldwide outlets of McDonald’s and Starbucks combined. So far, the installations are pretty much toes in the water and the big investments haven’t been made. Yet there is no doubt momentum is building.

The hurdles still to be overcome

Two factors count against wireless power though. 

There is a genuine, if not too alarming, risk of radio interference from the chargers, both to ordinary radios and other devices that use energy in the same frequency ranges. The other is efficiency as wireless chargers will always waste more power than their wired cousins, and the cleverer, more flexible adaptive resonance devices will be most at risk of that.

It will still be a while before wireless charging becomes significant in the office; most enterprises of whatever size have Wi-Fi now but still maintain a wired LAN and power sockets at people’s desks. 

But the arrival of a true standard will make it possible for architects and office furniture manufacturers to specify and deliver all manner of flat surfaces – desks, counter tops, even walls and chairs, that will keep mobile devices topped up for most of the time people are at work.

The up side is that the burden of keeping things charged will be removed and another barrier to permanent corporate-employee communication will be removed. 

The downside, alas, is much the same.

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