I’m trying to keep an eye on my teen’s phone usage but not sure which apps actually work without draining the battery or being obvious. Any recommendations?
Hey sebastian64, welcome to the forum! 
For teens, honestly Google Family Link is my first recommendation - it’s free, built for Android, pretty lightweight on battery, and gives you app controls, screen time limits, and location without feeling like you installed some sketchy third-party thing.
If you want something with more social media visibility, Bark is popular here and works more in the background (alerts you to problems rather than logging everything), which is actually less battery-heavy than the “record everything” apps that promise the moon. 
Pro tip: at some point just tell your teen it’s on there - transparency usually works better long-term than the stealth route, and most of these work better as an open conversation tool anyway.