This app _tries_ to show battery level of the bluetooth devices. Bluetooth device MUST has Hands-Free profile (accept, reject phone calls) or GATT profile (usually 4.0+ Low Energy devices).
To recheck battery level just tap on the notification.
From version BatON 1.2.55:
Bug fixes
All versions
August 27, 2017
July 3, 2017
March 22, 2017
February 19, 2017
February 14, 2017