I seems inordinately warm while it's charging. I assume some warmth is expected but this is pretty hot.
Are you using it while charging as well? Do you feel the back cover expanding or anything? If at any point you feel the back cover expanding or being stretched, stop using your phone immediately and turn it off.
The s4, like any other phone I've used, does get warm under heavy use over an extended period of time. As for charging, yes it gets a little bit warmer than just sitting idle as the battery is getting recharged so that's understandable.
How hot exactly are we talking about here though? Hot to the touch, or too warm to hold in your hand comfortably over a moderate amount of time?
It's normal for the charging pad and the phone's receiver for the wireless charging system to heat up - some.
The wireless charger system uses two coils of wire...one in the charging pad (the transmission coil), which by being connected to an electric current via the two ends of the coil moves electrons in a flow through the coil and creates a magnetic field. Then the other coil on the phone (the receiver coil), when placed inside the magnetic field created by the pad's coil gets excited in sympathy and in respect creates a flow of electrons through the coil to the contacts, generating the electric voltage and current necessary to charge the battery.
Other than the fact there is no iron core between the two coils, what the two coils together create are essentially a transformer. Since the coils are formed from one long wire wrapped into a continuous closed loop with only two ends, and since both ends meet up with the source of power on the transmission side (the wall adapter), and the destination of power on the other side (the battery's charging circuit), there is nowhere for the electrons to go but in one end of the coils and out the other. As the electrons move through the coil, they meet up with a certain amount of resistance inherent in the wire used to make the coil. This resistance creates friction which then causes the coil to heat up.
When the two coils are put together and the second coil completes the transformer, the electrons moving through the second coil as a result of the magnetic field also meet up with resistance and likewise heat up. In addition, as the power is absorbed by the battery, it demands more power from the transmission coil, which forces more electrons through the two coils, and with the demand for more electrons comes increased resistance. It's not too much different than trying to force more cars through a tunnel than it was meant to carry...it would result in some cars scraping against each other and the walls of the tunnel...and that would create heat from friction.
All transformers heat up as a part of their normal function, and since the coils of this transformer are sandwiched between the phone - whose battery also heats up while charging (and for the same reason...friction), and the pad's case and the table they lay on, there is no air flow through, under or around the coils...so the heat from these three sources is contained, builds up and is concentrated. The result is the phone gets hot.
I believe that the manufacturers of the charging pads take that into consideration and build them so that they won't push the heat to critical levels. Also, the phone and battery have safety mechanisms in place to monitor the temperature and shut down the charging if the battery gets too hot. The combination of the design of the builds and the safeguards in the phone should prevent the battery and phone from heating up to a point where any damage occurs.
The short answer...don't worry about it, they should be fine (LOL!)
Last edited by FoxKat; 07-24-2013 at 11:10 PM.
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Thanks for that great explanation. One thing I did notice, and I'm not sure if it's my imagination or not, but if I turn the phone completely off instead of just standby while charging, it doesn't seem quite as warm as when I leave it on standby.