Most all phones get hot off a car charger and when u multitask .. And if it's charging playing music and using Gps it gets super hot lol my tbolt hit 120 degrees today and it felt every bit that hot.. So i wouldnt pay it no mind. It's amazing the things these phones can do and not have any type of cooling system..
But really it is going to get warm/hot because you're charging the battery. It'll be even hotter if you're using GPS or anything like that as you're making the phone work. If possible get a temp monitor of the battery, and it shouldn't go past a certain temp. Off the top of my head I think it was 115F? But again I'm not sure and hopefully someone chimes in and knows the normal battery temp.
I was using Navigation and charging 2 weeks ago and it got warm but certainly not hot. The one thing I did notice is that it was actually charging my phone (increasing the charge%). With my OG I actually lost charge while doing the same.
actually, I only use my car charger when I'm on 4g waiting for class to start. I unplugged it today and its seems 4g isbwhats causing phone to get hot.