Back when Android manufacturers were adding wireless charging to their phones and Apple wasn’t, one popular theory was that the iPhone maker was skipping contact-based chargers and holding out for long-distance ones. In the end, of course, Apple finally opted for the same contact-based Qi chargers as everyone else.

But resonance chargers – which require the device to be placed extremely close to the charging coils – offer limited benefit. Sure, it’s convenient to be able to just put your iPhone X down on a wireless charging dock and pick it up again later without having to fiddle with the Lightning cable, but you still have just as many cables as ever – they are now simply attached to docks.

RF-based long-distance charging – promoted by companies like Energous – promises far greater benefit, allowing devices to be charged anywhere in the same room as the charger. So why didn’t Apple wait … ?

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With Energous and rival company Powercast both announcing FCC approval of their long-range wireless charging devices, we could be forgiven for expecting the vision of a wire-free world to be realized any day now. That if Apple had waited just a little bit longer, it could have skipped olde-world resonance charging and gone straight to RF charging – which is truly wireless.

But there’s a reason it didn’t, and why that ‘anywhere in a room’ charging vision isn’t going to happen any time soon.

Let’s start with a couple of basics. First up: distance.

There are only really two forms of RF wireless power: near-field and far-field. Energous uses the term ‘mid-field’ to describe the three-foot range device for which it received FCC approval, but rival firm Powercast dismisses that as ‘a marketing term’ and says that it actually falls into the near-field category. The vision we’re discussing is far-field charging.

The next obvious question is: what distance counts as near versus far? That’s a question almost impossible to answer. First, because experts disagree – there are various rival definitions being touted – and second because the definition is not measured in units of distance. Here’s Powercast’s take, for example.

We define the boundary as a distance of 2D2/λ where D is the largest dimension of the transmitting or receiving antenna and λ is the free space wavelength.

Er, yeah. But in approximate terms, a few feet would be considered near-field while far-field can, in theory, reach up to 80 feet. I’ll talk more about the real-life side of that shortly.

Second, let’s address another confusing issue: power limits.

Under what are known as Part 15 regulations, the Federal Communications Commission (FCC) limits the maximum power output of an RF power transmitter to just one watt. However, you’ll see references to three- or four-watt devices, and that’s because the one-watt limit refers to an omnidirectional antenna. By focusing the radio beam in a particular direction, you can effectively concentrate the available power.

The FCC allows a four-fold antenna gain (6dBi if we want to be really technical) to within a particular arc. One sample RF charger, for example, uses a directional antenna with a 70-degree arc. However, it would be misleading to equate this to 4 watts: the maximum power that can be transmitted remains at one watt. And since nothing is ever 100% efficient, the maximum power that can be harnessed is even less – even right next to the transmitter, that might realistically be half a watt.

But we can’t talk power without returning to distance. More specifically, the relationship between power and distance. Radio waves (and RF power is just a specific form of the same) are subject to the inverse-square law. This states that intensity is inversely proportional to the square of the distance from a source.

Get 0.5W one inch from the transmitter, and you’ll be down to a 0.125W two inches away from it (two times the distance = quarter of the power). At four inches, you’ll be down to 0.03W.

It doesn’t take much math to see that visions of powering a MacBook across the room from the transmitter any time soon are just pure fantasy. So what’s the reality of today’s tech?

Powercast says there are two realities: what is technically possible, and what is both safe and legal. Here’s what Dr. Charles Greene has to say about that ‘charge a device anywhere in a room’ vision.

The technology is real, the technology works, it is technically feasible. In a military application, we’ve delivered 5 watts of power to an autonomous device 20 feet away.

So yes, technically we could today fully charge an iPhone anywhere in a room. But not legally, and not safely. That military application was with no people present in the field.

And that’s with good reason. Put a human being in a powerful RF field, and it starts heating up their tissues, exactly like a microwave oven. Understandably, the FCC isn’t too keen on allowing companies to cook their customers, and that’s where that one-watt power limit comes in. Which means the amount of power you can deliver over any distance is very small.

How small? Greene said the peak power available – right next to the transmitter – is around half a watt. Within inches, it drops to tens of milliwatts. At three feet, you’re in single digit milliwatts. By ten feet, you’re measuring power in micro-watts.

Energous, however, points out that you can sidestep the power limits imposed by Part 15 regulations by opting instead to abide by a different set of rules, known as Part 18.

Powercast is limited in terms of how much power they can emit from their transmitter as their certification falls under FCC Part 15. Our certification was for FCC Part 18, a world’s-first for RF-based charging at a distance. Part 18 has no specific limit in terms of power, as long as you can meet other FCC safety/exposure requirements.

Powercast agrees, and says that it too has worked under Part 18, but that doesn’t change the fact that adhering to those rules doesn’t get you much further in terms of deliverable power – a point Energous acknowledges.

This first generation transmitter was certified to provide 120mW at about 1ft from the transmitter. That is able to charge many of today’s smaller electronic devices and also provides a trickle charge for larger battery devices.

The company of course says it plans to do much better – but can’t say when.

So no, we’re not going to be using this technology to charge our iPhones across a room anytime soon, let alone power a MacBook.

But, says Greene, that doesn’t mean the technology is useless. It’s all about being realistic about its applications. For example, a few hundred micro-watts are useless for powering a MacBook or an iPhone, but is a perfectly usable trickle charge for a low-powered device – like a keyboard or trackpad. A single transmitter behind your desk could keep a Magic Keyboard, Trackpad or Mouse operating without ever needing to connect a Lightning cable.

And in these days of the Internet of Things, we suddenly have a lot of very low-power devices around the home. Thermostats. Thermometers. Motion sensors. Hue switches. All these sorts of things are almost tailor-made for distance charging applications.

Powercast has been powering devices like these in the industrial and commercial sectors since 2010, and it’s no surprise that it now sees consumer applications for its technology. For something like a window sensor, it says, distances of up to 80 feet are possible.

Will that ‘anywhere in a room’ vision ever be realistic for iPhones, iPads and MacBooks? Greene says that’s something for “the very distant future.” The company is focused on what is possible today: and that’s motion sensors, not MacBooks.

Along the way, though, we’ll likely see short-range systems that can keep a phone topped-up when placed on a desk or nightstand. And that’s the point at which inclusion of the chip in an iPhone starts to make sense.


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