I am a bit of an android noob and I am not sure how can I run my app as long as the phone is on. Well I don't sure how can I open my app on startup and make it so it will run in the background. By the way, my app is notifing me when I have a certain amount of battery power left.
You should learn Android Service and then implement battery application. First read android life cycle and its basics then you should try application developent use this Link