It would be really great to incorporate a user adjustable low and high battery state of charge alarm to the BatteryTracker app. For example, the user could set a low alarm of 50% and a high alarm of 80% to alert when the battery was at those levels during discharge & charging.
There is a background battery alarm app on the App Store, but it doesn't work reliably at all.
Example app below:
Low Battery - Phone battery power glance & notifications by Edi & P AB