Since about half a year I own Bose QuietComfort 35 headphones.
They work well since bluez 5.42 (previous versions have problems with the headphones Low Energy mode, see Debian Bug #840983). While the battery last quite long (I guess roughly 20 hours), I would like to know the current status. The Android application displays the battery state, so it can be read via bluetooth.
While working on the kernel driver for Nokia bluetooth chips I coincidentally found a message from Marcel, that the headphones my support Apple’s battery status HFP AT commands. After struggling to use the proprietary bluetooth low energy API (which should also provide battery information), I gave this a try.
On a typical Linux desktop the HFP AT commands are processed by pulseaudio in backend-native. It would be nice, if there was some processing directly in bluetoothd, so that battery information could be provided there. For now here is a quick hack, which outputs battery level into pulseaudio’s log: