An Engineer in Wonderland – Wireless charging nonsense

I neither like or dislike the idea of wireless charging, providing it does not fill up my house with strong magnetic or electric fields.


However, I don’t like misleading comparisons.

The Wireless Power Consortium is proposing an inductive charger built into a pad on which compatible items would be placed to charge.

There are all sorts of interlocks included to stop metal things placed on the pad being a problem, and a communication protocol to allow the chargee to tell the charger what it needs.

All well and good so far.

The website is also good, until it discusses efficiency and decides that wireless charging is more efficient that wired charging.

The calculations all look pretty reasonable, until you get the bit that casually says: “And assume that the wireless charger replaces 2 wired chargers.”

After which I sort of lost interest.


Respond below, or to



  1. Hi Please.
    I may of missed your point.
    Do you mean that is a lot per year, or not much?

  2. Either way the calcs show a consumption of about 11 Wh/day.
    Any of you efficiency braniacs actually calculate how much that would cost per year????
    And a wonder why Timeguard would have “no interest.”

  3. More reasons to think before wireless charging.
    Thanks folks

  4. I’m sorry to say that I do not like the idea of wireless charging. (Let Tesla keep his secrets) I can see my wife coming home and placing her handbag on the pad. Her phone battery has charged. Then she finds her Oyster card has been fried and all her store and credit cards have been erased. (Solution Please)
    2 years ago I solve my problem on charging equipment and forgetting to turn them off. Being an engineer I always try to think out of the box and usual come up with an idea.
    In the end I modified a Timeguard Dusk to Dawn Timer (TG85). I removed the LDR and replaced it with a push button and changed the timing circuits from 1 / 8 hours to 1 / 24 hours. Now I set the timer for the correct charging time push the button, the charge takes place and then is isolated. The only current taken by the timer is the 0.47uF feeding the timing circuit. Circuit modification available.
    I past this idea to Timeguard, of course “No interest”.
    Regards Tim

  5. What worries me about these plans is the prospect of creating horrendous RFI/EMI problems. Transferring power through a short range inevitably creates all sorts of stray fields add in penny pinching or downright non-compliant manufacturing practices (it is known for filter components present in units sent for type approval to mysteriously disappear in production units) and radio/tv reception will be blighted.

  6. Jeremy.
    I think the only issue with a kettle-like connector is that ‘dock’-style charging needs the portable device to be physically supported.
    Otherwise a sideways physical knock during charging could damage the tiny connector that would be inevitable in the compact gadgets we carry around today.
    Although, I suppose, if all the gadget makers could agree a fixed distance between the charging socket (which always seems to be on the edge) and the back of the devices, I supposed they could be rested in a row on an inclined support with connectors at the bottom.
    Much like the bulk chargers that used to be sold by walkie-talkie suppliers.

  7. Sorry Yaqub, I have no wish to offend anyone.
    I agree that the detail of the discussion on the site is reasonable.
    And that even the assumed consumption figures are pretty good.
    – Although now there are least two manufacturers (CamSemi and Power Integrations) that have low-cost off-line switching components and wired charger reference designs that dissipate well under 40mW in standby.
    Both these companies sell in bulk to wall wart makers
    So maybe the calculations on the page could be bought up to date.
    My point is that the comparison of two wired chargers against one wireless charger is arbitrary.
    Why not assume that one wired charger is used to charge two devices during 24 hours, or three?
    Someone who just scan-read down to the ‘bottom line’ of this page could easily come away with the impression that wireless charging was more efficient than wired charging.
    It isn’t.

  8. Thanks Steve.
    I think major competition for wireless charging will come from standardised charging through micro-USB plugs.
    The European Union has just mandated that all phones from a certain date be chargeable through this interface.
    So I can imagine a wall wart with several micro-USB leads hanging out of it for all the gadgets in the house.
    What I can’t imagine is, equipment makers preferring to build a magnetic receiver and protocol stack into a torch/flashlight when they can install a USB socket and two wires.
    The USB interface does put a limit of 4.5V for simple resistor-controlled charging, but that is at least three NiMH cells or one Li-ion.

  9. Why not fit a physical connector to the charger pad like most modern kettles? Of course the voltage should be limited to something safe like 12 or 24V. A simple shutter mechanism could cover the socket when not in use. Microcontrollers could communicate charging requirements and limit standby power to something very low.
    Inteligent design is needed. The main thing is that it would be a standard for many devices.

  10. Alice,
    Clearly you didn’t bother to read the article.
    It says:
    “That link is not efficient as a copper wire (what can beat a copper wire?), but careful design made it possible to achieve at least 70% transfer efficiency.” [1]
    Which quite clearly is a statement saying that wireless is LESS efficient than wired power transfer.
    You said:
    “The website is also good, until it discusses efficiency and decides that wireless charging is more efficient that wired charging.”
    So where in [1] does it infer that wired charging is less efficient than wireless charging?
    It DOES provide some calculations (with the revelant assumptions supplied) that indicate that users who leave their wired chargers plugged in all day will benefit from reduced OVERALL power consumption. The assumptions made aren’t even unreasonable, in fact they’re commonplace.
    So the conclusion should be that people who have multiple devices to charge will benefit from reduced overall power consumption. Yet I read an article bashing the proposal, based on a statement that was never made or even implied.
    Perhaps you shouldn’t be so hasty in forming your opinions and consider information more carefully.

  11. There may be some people for whom this is a good solution. People who need to charge their mobile phone, mp3 player, digital camera(?), bicycle headlight, etc., might have multiple chargers plugged in continuously. They could use something to reduce the power used when not charging. Maybe they could just unplug the chargers when not in use??
    I do like the statement at the bottom of the first page: “Although wireless transfer is obviously not as efficient as transport over a copper wire, wireless power transmitters saves standby power energy when the wireless transmitter replaces multiple external power adapters.” At least they recognize the shortcomings of their design. Too bad this isn’t in the first paragraph of their press release.

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