Endless nags don't do end users much good. Battery status is a highly visible UI element on mobile. We don't need extra nagging from within an application. Also even less granular reporting is still an element to perform fingerprinting on.

App developers should be already working on these conditions by default. You should already have a game save system that can expect client shutdown or disconnection at any time.

That said, I could see a 15% or less warning but your mobile OS is doing that anyway, so why bother.