BatteryAlert monitors and notifies you of Bluetooth battery levels via Email, Messages, Notifications or Alerts. It also features a live Dock tile offering easy visual indication of current battery status at all times.
I was looking for something that could report the battery status of both my Apple branded trackpad (2015 Magic Trackpad) and Logitech branded keyboard (K811). It couldn't read either of them. I don't know if the problem was it doesn't have logic for my devices or it doesn't work with 10.11. If I find something that does what I am looking for I'll comment here.