[boxed]

I think the iPhone will warn you if your battery performance is significantly degraded?

[dml2135]

It's buried in the battery settings, so not something you will find unless you are looking for it, IIRC.

[rootusrootus]

You can find it under settings, but when it goes below 80% the phone will start telling you about it regularly using a big prominent notification. Especially if it causes a crash.