How can I find if I have a rogue background process killing cpu and battery?
Enable Developer Options on your phone (usually done by tapping the Build Number in Settings &rarr; About phone 7 times) and look for an option named "Running Services" in it. It shows a list of all applications which are running, for how long, and the amount of RAM they are using. If you tap on the app it will also list those services of the app which are running and allows you to force stop each service.