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Monday, June 5, 2017

What Does It Take To Switch a "Phone Charging" Light On? Pt II

Following uBeam's demo, EEV Blog contributor Howard Long made a very interesting video showing how you can turn on a phone charge light with ultrasound. It's about 4 minutes long, with audio commentary, and gives more info in that 4 minutes than in the entirety of uBeam's demo. If this subject interests you at all, I encourage you to watch this.

From his comments (edited for brevity, read the whole thing here):

I could get it to light visibly with about 1mA at a distance of 2cm ... At 2cm distance, I had about 2mW, giving it a 2% efficiency. However, ... perhaps only 15-20% of the transmitted power appears at the rx anyway. So beam forming and reasoanably sized apertures on the receiver are essential facets for this to work.

... That camera thing is an Nvidia Jetson which looks like it's for visual device tracking. ... If it needs visual indication of where the target device is, and the sensors are on the rear of the phone, the phone will have to be used face down for a ceiling arrangement, and you won't be able to hold it in a normal fashion to make a call or use the screen. Even wall mounted, assuming nothing's in the way, you'll have to figure out new ways to hold your phone.

In its current form and key use, as a phone charger, this remains practically speaking a non-starter.

It seems an engineer reproduced a basic version of the uBeam demo in a day with about $20 in parts.

The phone charge indicator lights at 1mA, which implies 5mW (5 Volts supply) and so would take about 1000 hours (~6 weeks) to charge the phone - if it weren't for the pesky fact that a phone requires around 500mW to operate, on average, so it would make no appreciable charge effect at all. 

Now of course there's only a single element here, not a full array which could emit more power, but the key point is that a charging symbol tells you nothing about whether it is practically charging. You need voltage and current to know the actual power, and you need it at both transmitter and receiver to get efficiency (which he's calculating as 2% in this setup, pretty good actually for through air). Those are key numbers you need to have. From Howard's numbers 100 transmitters will get you that 500mW and maintain charge at a constant level, in an ideal world setup - possible but very large and introduce many questions on practicality and cost.

I like the way Howard also brings out a key point in this video - of course you can send power through the air by ultrasound. That's never been doubted or questioned, here or on the EEV Blog. What is questioned is how much power can be received, the efficiency of that, the safety aspect, the cost of transmitter and receiver, and the practicality for the user.

None of those points were addressed by uBeam, other than the emphasis on slow "trickle charging", implying the "faster than a wire" claims of 2015 aren't going to be happening.

Anyway, bravo to Howard Long for showing how to put together a short, clear, technically accurate demo from which you can actually learn something.

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