When device goes to a low coverage area, it keeps scanning in the back ground for network connections at regular intervals. This leads to your device using more power, in turn draining your battery faster and causing the device to heat.
Normal Accepted Heating: Normal accepted heating of any devices is Ambient Temp + 20 Degrees (up to 50 Degrees).
Top Battery Consuming Apps can lead to battery drain. To uninstall such apps: Go to Settings -> Battery and look for top battery consuming apps. It is recommended to uninstall such apps if not used frequently.
Check of GPS is turned ON. Turn off GPS, Bluetooth and/or Wi-Fi if these are not used.
Practical VoLTE talk time: 3 to 8 hours (based Phone battery and display size).
Practical expected Video Play back: 3 to 5 hours (based Phone battery and display size).