In searching for Android apps that could be counted upon to play or sing or chime something when the battery got too low or high enough <—- and NOT EVER FAIL— I settled on these two, though neither are totally infallible
FWIW, “Battery Charge Notifier” does well on an Axon 7 (2016), while “BT Free” seems best for a couple of older phones, a Moto G4 Play (2015) and a Sony Xperia Z3 (2014).
I mention those models in the hope that they can be a kind of sampling of other phones which I don’t have. Also, I wonder if the brand new phones even need such apps around anyway. In case they do, well...
I won’t go into why I want such apps around, except to say that when they work (which is about 99% of the time), they make pleasing noises that say if they need more juice or if they’ve had enough.
I hope you like. And if you have suggestions for any other apps that are as good or better, or can complement what’s here, please share!