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.
Tried to fix crushes on O+ androids. OMG, I so hate android env: Service, IntentService, JobIntentService, JobScheduler, WorkManger and nothing works well.