Why does my iPhone 4s get hot when I'm charging it?

Why does my iPhone 4s get hot when I'm charging it?

It's common, I'm not sure what the reason for it is but don't worry because many people have reported this happening but it's been said to be harmless.

Because your filling it with power, and that power is naturally hot. So it is completely normal for it to get heated up.

All batteries het hot when they are being charged, it is perfectly normal and nothing to worry about.

It is possible phones will become warmer during heavy use. This is normal behavior especially if you are transmitting data, talking for an extended period of time, or playing games or music on the device. In these conditions, the phone is transmitting and utilizing a lot of power on the device which will generate heat.

iPhone Battery
The iPhone battery is composed of lithium ions in a sodium solution that react with metal electrodes. When you charge the battery, these ions move from one side to the other. This reaction creates heat. A little bit of heat is normal, but the iPhone should never be so hot as to become painful to the touch. In such a case, there are likely other factors involved that are still within your control.

iPhone CPU
The iPhone contains a processor that runs its operating system and carries out the complex computations required to run applications and even rudimentary functions such as receiving calls. Every calculation the iPhone makes requires energy, and some of this energy is wasted in the form of heat. Thus, if you use the iPhone extensively while charging it, you run the risk of making it even hotter than it would have been otherwise and pushing it toward a critical temperature.

Using iPhone when charging
"Apps" are any application that come with the iPhone or that you download from the Apple store. You can use apps while the iPhone is charging, but if you run too many of them while the device is charging, you will increase its thermal load. If you drive its thermal load too high and too often, you will shorten the life of the battery and you can even damage the delicate internal components of iPhone.

Maximum temperature envelope
The phone was designed so that it would generate and dispel a certain amount of heat. If the iPhone becomes too hot, you can actually receive an error message that says "iPhone needs to cool down before you can use it." This error message appears once the iPhone crosses critical temperature threshold. You can prevent this from occurring by using your iPhone for only the most basic functions when it is charging. You may want to limit yourself to one app and taking and receiving calls, for instance.