Engineers' reader

wireless charging: phones that charge in your pocket

The short-range charging pads of yore are getting supercharged with long-range capabilities: phones that charge in your pocket are coming, and soon

Getting energy to consumer gadgets without the use of wires has been a Next Big Thing since the very earliest days of mains electricity, when 19th Century New York was lit up by the crackle of Nikola Tesla’s arcing coils.

You may not realise it, but wireless power transfer is all around us, safely beaming energy to electric toothbrushes and battery-less RFID payment cards every time you tap to get on the tube.

Now, however, Nokia and Qualcomm are selling phones and watches with wireless charging built-in, and Samsung has add-ons for its latest Galaxy handsets too. It's taken 130 years, but are we finally cutting the cables?

The kind of madness that underpins almost everything in the modern world. The most common form of wireless energy transfer is ‘magnetic induction’, which you’ll remember from third year physics, yes?

Right. Induction happens when you make two electromagnets by coiling wires around iron cores. Hold them close to each other, but not touching. When you pass a current through one of those coils, the magnetic fields interact to produce a similar current in the other, non-powered one.

By varying the number of coils around each core, you can create a lower or higher voltage in the receiver - which is why this is the basis behind every transformer in the world, from your laptop charger to the sub-station down the road.

Nope. The laws of physics don’t allow that - increase the power and you reduce the current flow, or vice versa. And some gets lost in the transfer - the amount that’s retained is what we call ‘efficiency’. Remember that - it’s going to crop up again in a couple of paragraphs.

Actually it’s arguably safer than wires. Remember, this isn’t arcing bolts of lighting seeking a human ground we’re talking about, this is power transfer over magnetic fields - so unless you believe that wireless radio waves a health risk, yes its safe.

Plus, these waves can pass through non-conductive materials, so all the live parts of a circuit can be encased in plastic. That’s why it’s popular when an electric device has to be used near liquids - like a toothbrush or water pump.

The area that could be really big for wireless charging is wearable technology. Take Qualcomm’s Toq smartwatch, for example.

It makes so much more sense to take it off at night and throw it on the charging dock next to your bed than it does to take it off, fiddle around in the dark with a tiny USB port cover, find the plug, drop the plug, curse, wake the dog, step on the cat, sit on the kids etc.

Mainly because it’s not very efficient - in every meaning of the word. Theoretically, wireless transfer can mean less waste energy than a normal transformer - Fulton Innovations, a firm that's been at the leading edge of wireless power research - demoed a system at CES which it claimed was 98% efficient or more.

That's way more than a standard wallwart, which are usually 70-80% efficient. Most current wireless chargers – including the one in the Nokia 920 – transmit energy at less than 40% efficiency, which is a lot of wasted power. Also, there’s the question of cost and convenience efficiency.

Adding a second power adaptor adds cost and bulk to a gadget and for what? If you need a separate wireless charging pad which has to be plugged into the mains for every gizmo you own, it’s much easier to stick with wallwarts. Also, with magnetic induction coupling the charger and chargee have to be very close together.

I didn't say that. The big breakthroughs are already happening. The Qi standard which has been adopted by most manufacturers (including Qualcomm, Nokia and Fulton) has just been enhanced to cover 'resonance induction coupling'. That's the secret sauce which will make wireless charging the norm.

It's a similar principle to magnetic induction coupling, but a lot more powerful. By using coils that have a natural vibration – or resonance – of the same frequency the flow of energy between the two can be tightly focussed, so it can be thrown across longer distances and with almost 100% efficiency.

What's more, resonance coupling means you can have a very large charger – like a wall – which can broadcast energy across a room to multiple devices. The new Qi standard only increases the charging range to about 4cm, but it's a start.

The big issue is about standards, because until any device can use any charger development costs for each gadget will be high – plus you'll need mulitple wireless chargers in your home.

The industry does seem to be settling around Qi, however – but not necessarily. A firm called Ossia has been pitching for investment in yet another form of charging tech, which it's called Cota.

Cota is different again in that it uses microwave beams at the same frequency as WiFi. Ossia reckons it can throw power over 10 metres with this, and will have early devices for sale by next year. Cota is promising, but it does add an element of uncertainty. If it becomes the new standard, what happens to all the Qi kit currently in use?

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