iPhone suddenly drains battery quickly, but only sometimes My iPhone 7 normally lasts about a week, but sometimes it suddenly starts draining very quickly. I have Background App Refresh completely turned off, and I wasn't using the phone at all when the activity shows 100% screen turned off. Why does it do this, and how do I stop it?

I wasn't able to find anything online, since searches only result in "turn off your apps" or similar.
You should be able to scroll down farther on that screen with battery level and activity and see specifically which applications are using energy.  Never seen the "not available message" however...