There are several overheating and/or battery draining issues :
- weak signal
- 4G (LTE)
- faulty battery
- Android apps (Skype, games, ...)
- continuous usage of the phone for hours
- GPS, NFC, DATA, Wi-Fi, bluetooth, ...
- accounts sync (with the contacts list as first culprit)
- after a fresh reload (See Accounts sync)
- after setting the encryption to On
- Device full
- ...
So :
- never charge your battery in small increment
- never use your phone during charging
- don't let your phone in direct sunlight too long
- use imap instead of pop for your emails
- check your accounts sync settings
- customize your notifications
- avoid vibration mode
- enable bedside mode at night
- avoid Android apps
- close unused active frames
- check your headless app in Permission settings
- adjust your screen brightness
- lock your screen when not use or buy a leather pouch/holster
- customize your screen lock timeout
- stop the thing you don't always need : GPS, NFC, etc... try 2G for a while ;)
- stock your media on a SD card
- restart the phone and/or take off the battery for a while
For the Q10 users around here who want to reset the battery estimated time, just power off your phone, pull the battery, connect the official charger and push the power on button. Then, disconnect the charger, replace the battery and switch on the phone.