Qi Phone Charging

I have just bought a wireless phone charging system based on the Qi Inductive Power Standard. I bought a charging device which connects to a standard micro-USB cable and receivers for the Samsung Galaxy Note 2 and Samsung Galaxy S3 phones I own. Both those phones have contacts in the back of their case that are designed for wireless charging so you can install a charging device inside them. The charging devices make the case fit a little tight, and the charging device is stuck to the phone battery with contact adhesive, this makes it impractical to change the battery on a phone with such a device and makes it a little more difficult to swap out a battery case. One nice feature of the Nexus 4 is that it has Qi charging built in, that saved me $19 and was also more convenient.

I believe that the main advantage of a wireless charger is to avoid the risk of damage to the phone if it’s dropped while connected to a USB charger. This allows the phone to be charged in situations where you might need to quickly or regularly unplug it to go somewhere. One example of how I might use it is when working at an office so I could charge my phone while at my desk and then quickly take it with me if I had to go to a meeting (sadly I have worked in many offices where they have so many meetings). Another example is for sysadmin work where I have to frequently visit devices to fix them.

The wireless charging mat that I bought from Kogan connects to a standard micro-USB plug, the good thing about this is that it’s easy to find cables and it can take power from any PC. The bad thing about this is that the resistance of the USB cable is a factor that limits the power that a phone can receive, when using wireless charging you have the limit of the cable resistance as well as some power loss from the wireless transmission. After any extended period of charging the charging mat feels warm to the touch and the phone that’s been resting on it feels warmer than usual. The warmth is an indication of energy loss which means longer charging times, a longer charging time isn’t necessarily a problem as the convenience of wireless charging can allow longer charging times, but if you want to charge your phone in a hurry before you go somewhere then wireless isn’t a good choice.

In the past I’ve discovered that the battery in a Samsung Galaxy S3 can’t be charged if the phone is at 46C [1]. 46C might seem extremely hot to people in some parts of the world (EG northern Europe and Canada) but the temperature in even southern parts of mainland Australia can get that hot and it can be hotter in central and northern parts, so phone temperature can be a real issue. Currently my house is at 21C according to a digital thermometer, the Galaxy S3 and the Note 2 are being charged from USB and report temperatures of 27C and 23C respectively. While the thermometer in my house and those in the phones probably aren’t really accurate it seems reasonable to assume that the battery of a relatively idle smart-phone that’s being charged will be a few degrees warmer than the ambient temperature. The Qi charger makes things a lot worse as it even feels warm to the touch. So maybe a phone on a Qi charger would be 8 degrees warmer than the ambient temperature or more. That implies that in Australian summer weather a Qi charger won’t be useful outside or in any building that lacks air-conditioning. So I think we can give up on the idea of using Qi devices to charge phones at a BBQ.

Picture of Qi charger on top of Samsung Galaxy Note 2

The final problem I have is that the Qi device is quite small, I took the above picture with my phone face-down because no part of the charger is visible in normal use. With that size I can’t just dump a phone like a Note 2 on top of the charging mat and expect it to work. I have to carefully place it so that it balances and so that the wireless receptor inside the phone matches the transmitter in the mat, if the phone isn’t placed correctly then the Qi mat won’t detect it and won’t supply full power to the transmitter.


I’m fairly disappointed in this device. The waste heat makes it unsuitable for Australian summer conditions and slows charging. The difficulty of correctly placing the phone reduces the convenience which is one of the major features.

The price was $19 for each charging card for the Note 2 and the S3 and $29 for the charging mat to give a total of $67. I think it’s worth the money for me to cover the risk of one of my phones having it’s USB port damaged. Using a Qi charger on occasion will decrease the probability of such damage and allow the phone to be used after receiving certain types of damage.

The prices of those phones nowadays are $389 for a Galaxy S3 (Kogan price), $250 for a Nexus 4 (when it was on sale in the Google store), and probably about $500 for a Galaxy Note 2 (last time Kogan offered them). So by paying $67 for Qi charging I believe that I’m getting some degree of damage insurance for just over $1100 worth of phones. It seems likely that the Nexus 5 will ship with Qi charging support and that the Galaxy Note 3 will also support an optional Qi charging card (which will probably also be $19 or some similar price) so the charging mat should be useful for a long time.

While I’m disappointed I don’t regret buying the device. But I would be hesitant to recommend it to other people and definitely wouldn’t recommend it to someone who doesn’t have a significant interest and investment in smart phones.

7 comments to Qi Phone Charging

  • Steven C.

    If USB power is DC, there must be some sort of inverter built into the charging pad, wasting power? You may have already wasted some power rectifying it to DC in the first place. Something that steps down from wall AC to a safe AC voltage for charging, with a nice thick cable, ought to be a more efficient way to do this and producing the least waste heat. Keeping the resistance of the charging and receiving coils low may also help, but would increase weight, bulk, and expense.

    I guess inside the phone you still have to rectify AC current back to DC to charge the battery in any case, wasting yet more power and probably explains the high temperatures when wireless charging. I wonder if anyone really thought this gimmick through before marketing it?

    What if the battery was – gosh – easily removable, so that you could swap it for a fully charged one from your bag or charging cradle in mere seconds? A very small backup battery or supercapacitor (like some people allege the NSA to have put in there already) might allow enough time to switch over without it having to power down.

    Maybe carrying a spare battery in one’s bag lacks the social allure of pulling out one’s gimmick wireless charger in a public place though.

  • etbe

    Converting from 240VAC to 5VDC does involve some inefficiency and power waste, that’s bad in terms of environmental issues (although really not significant when compared to home insulation and other things) but not an issue in terms of phone charging speed. Converting from 5VDC to a magnetic field is an inefficiency that limits the charging speed and is more of a concern for me in this regard. Also I suspect that converting to a magnetic field wastes more energy based on the fact that the charging mat feels so warm while the plug-pack that provides 5VDC doesn’t feel warm.

    Now a device that took 240VAC to power a Qi charger (which incidentally wouldn’t need a thick cable at all, the thinnest legal 240V cable can provide more power than a phone battery can accept on the charge cycle) wouldn’t be as portable and therefore might not get enough sales to give a low price.

    As for whether people thought it through, multiple generations of phone have supported it (I count the S3 and S4 having special terminals for the receiver and software to notify the user as “support”) so it seems that people have done it again after it’s been tried in the field.

    The solution to battery swap problems is to just have a bigger battery. The Galaxy Note 2 is 9.4mm thick, if they made it 15mm thick then they could have a battery with 3* the capacity or more and also give it a camera with a greater focal distance which means a larger CCD and better picture quality in poor light.

    Designing a modern smart phone with a battery that lasts 3 days of typical use (and one day of Ingress ;) wouldn’t be a challenge.

  • Steven C.

    If the plug pack provides 5VDC then the charging mat possibly needs DC-AC inverter circuitry inside it – I would think that creates much of the heat – I wouldn’t be surprised by as much as 50% of the input power. When talking about inefficiencies I was trying to infer that the lost power becomes unwanted heat; rather than out of environmental concern which is somewhat low for infrequent charging of a single phone (but a more important responsibility when you’re putting millions of these into consumers’ hands).

    A larger-capacity battery maybe doesn’t help with charging convenience when you eventually need to charge, if the charge cycle takes longer or must run faster and hotter. Maybe my idea would work better with two individually replacable batteries that can work in parallel at different levels of charge. Carrying one or more spares with you should be as portable as any charger imaginable, and time with the phone itself being ‘on charge’ is reduced to zero.

    But I guess the market will just give us fancier wireless chargers with ‘chilling zones’ or other insanity.

  • etbe

    Steven: In regard to environmental issues, people are making all sorts of wild claims about how much energy is consumed by Google searches and watching NetFlix videos (I might write an article analysing the claims about NetFlix power use), so we can’t just ignore such issues even if they don’t seem important. Also the word “infrequent” can’t be used when discussing charging smart phones as it seems that no-one is selling a smart-phone that’s designed to run for 24 hours without charging (as opposed to 5+ days for older phones).

    I would be shocked if 50% of the power was wasted, but I should try testing that. Even though phones tend not to be that accurate when estimating how charged their batteries are they are good enough that it should be possible to charge it twice and determine if a 50% power loss is plausible.

    In regard to charging convenience, most people can’t function well if they have much less than 7.5 hours sleep per night and that’s a lot of time for charging a phone. The rare people who need little sleep still require about 2-3 hours which when combined with time for a shower and breakfast should be enough to charge the biggest internal batteries that are currently available.

    You suggest that it would be insane to create a chilling zone, but given the temperatures in some parts of Australia and the charging temperatures of batteries it seems that some sort of phone cooling device is going to be needed.

  • Steven C.

    Yes I do care about environmental impact of such things at scale, I just meant that most people needn’t worry if a rarely-used low-power charger is being inefficient – except in this case I think it could be an issue due to lost power being converted to unwanted heat.

    And I meant that it’s insane to add cooling systems if only to remove waste heat caused by some inefficiency, but I’d really love to see it measured somehow. Not sure how exactly how that can be done. If a charger works inefficiently it may still be able to charge at the same speed, but only by drawing more power from the wall (the DC supply may increase its power draw+supply to maintain 5V). And those wall-plugging power meters seem to have very low resolution/accuracy when measuring at the low end of their scale (0-15 Watts).

    I suppose the most accurate measure would be from an ammeter in-line with a 5VDC supply when supplying the phone directly, or the charging mat. If both charge the phone at the same speed, you could directly compare Amps.

    If I owned a device that allowed for this type of charging, I’d certainly play around with making my own charging coil, perhaps supplied from a wall-plug AC-AC low-voltage power supply. Perhaps varying the number of windings would speed up or slow down charging, and thus be able to limit heat during charging.

  • Thanks for the post. I saw those for sale at Kogan but after adding them to my cart I decided that I wasn’t sure enough about it so I removed them. Glad I did :-)


  • etbe

    Steven: I just read the Qi Wikipedia page and it says that the “low power” specification (the only one that’s actually implemented now) is limited to 5W. That could be a big limitation given that some USB chargers supply up to 2.1A, or it could be a benefit given that some chargers are limited to 500mA and apparently some combinations of phone and charger will work at 500mA even if both are capable of higher current.

    I’ve done some calculations of USB power at the above URL, 5W through the air shouldn’t be a limitation compared to what I’ve been seeing. However 5W through the air might not be possible when the supply is from USB, particularly if I use my USB charger that’s rated at 1A.

    Chris: I think that anyone who really wanted one will find that it’s OK when they get it as I did. But anyone who is ambivalent about the purchase would probably be disappointed. So I think it’s good that you didn’t buy it.