Inductive charging, how efficient?

I have noticed an increase this kind of device recently. I seem to remember from when I played with this that its main issue was loss. IE there is no way to closely couple the windings enough to transfer enough power to make it worthwhile. so has there been some revolution here, or is it just some kind of clever marketing of power wasting rubbish? I mean, induction hobs could be repurposed as battery chargers using a plastic plate with a winding inside?
I was thinking about eye great leccy in bathrooms debate. One other odd thing is that Kitchens have power sockets and water and nobody seems to worry about that, and yes people do get killed due to this aspect due to accidents like power cables dropping into sinks or dodgy cowboy practices in fitted kitchens. Brian
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On 27/01/2017 09:11, Brian Gaff wrote:

I've no experience of these, but like you I would think them to be pretty inefficient. But it only needs a few watts to charge a mobile phone, so if the charger takes ten times that from the mains would anyone care? The energy loss would, presumably, just warm the room up a bit.

I think you've got a great idea there - hope you patented it before giving the idea away.

The argument that I've seen is that in a bathroom you might be wet all over, so the risk is larger. I'm not convinced by this: in a kitchen you often have both hands wet, and in the event that you touched one hand on a live wire and the other on say a tap you might get a serious jolt.
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Hasn’t been like that for a long time now. If it was ever true.

Nope, it was always viable.

Nope, it works fine for lower powered devices like phones, toothbrushes etc.

Makes more sense to get a dedicated battery charger.

That’s one of the reasons it was seen with toothbrushes early.

Because you have to be really stupid to get killed in that situation.

Almost no one ever does.

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Well they do, a person in this very borough got killed by it about three years ago and there are regularly some near misses in the local papers.
In the case of inductive charging perhaps they use an hf ac signal to increase the efficiency, aas 50 hz is always going to be ineeficient due to the low frequency. Brian
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On Friday, 27 January 2017 16:31:37 UTC, Brian Gaff wrote:

yes they start at about 100KHz for low power and up to 300KHz for higher power. I think I remmeber a a figure of GHz for charging over a distance.
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Sounds a bit unlikely given that those are microwave frequencys with the associated health problems with those.
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On 27/01/2017 09:11, Brian Gaff wrote:

It is a bit inbetween. Very useful for things like toothbrushes where water could get in the works. Mainly good for modest power.

Not easily since they would almost certainly see the load as abnormal.
A decent review on the chipsets and pros and con of inductive and resonant coupled wireless charging is online here:
http://www.digikey.co.uk/en/articles/techzone/2016/aug/inductive-versus-resonant-wireless-charging
There is no free lunch. A wire will always be more efficient.
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Martin Brown wrote:

True, but the convenience of inductive charging often outweighs that, my previous two phones (Nexus 4 and 5) had wireless charging, I had a charging pad on my bedside table, and a magnetic charging dock in the car.
My phone is typically going to be left by the bed overnight, so it could trickle charge over many hours rather than one for all it matters, and in the car it could provide sufficient power to use it in satnav mode without draining the battery, just by 'throwing' the phone at the dock.
I was quite annoyed that the Nexus 5X didn't have wireless charging, so didn't buy one ... until I put the Nexus 5 through the washing machine!
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I can't see the difficulty. If the item to be charged is not in the field of the charging winding, the latter will act as an inductance without any more loss than a transformer with unconnected secondary in a wired charger.
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"Brian Gaff" wrote in message

So if you have a wacking great 132 kV mega amps pylon line crossing your property, how large a tuned circuit resonating at 50 Hz do you need to pull significant (and undetected) power your way :)
Andrew
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On 27/01/2017 13:13, Andrew Mawson wrote:

A fluoro tube lamp stood on end will do nicely.
There was a case once in the dim and distant past where there were allotments around a MW transmitter and some enterprising radio Ham built a circuit to heat his greenhouse from it. This was OK until he built ones for others and the resonant load affected the transmitter.
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Urban myth? Just seems improbable to me that you could draw enough power from a MW radio transmitter to run any sort of heating but I dare so those who understand these things will put me right. ;-)
Tim
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On 27/01/2017 14:57, Tim+ wrote:

http://message.snopes.com/showthread.php?t 91
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On 30/01/2017 23:04, Vir Campestris wrote:

I tend to disregard snopes. I know of at least one incident that snopes puts down as an urban myth that wasn't. The one of there being rat in KFC. The shop in west bromwich was fined for serving rat to a customer that just happened to work for trading standards. The shop became a fish and chip shop shortly after.
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On 27/01/2017 09:11, Brian Gaff wrote:

The gf has an inductive charged vibrator.
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