Mine does the same thing but I have an android.
You see.. I lost it in the car for over a week not to long ago. It pretty much got stuck inside the car somehow and so whenever the car started it got pretty hot I guess..
Now the sounds really low even at max and my charger only goes up to 20% or 40%. But its more of a bluff because it stayed at 20% or 40% For just as long as it would at 100%