I’ve searched how to do that and finally found it. Source (seriously, check it out if you’re using Arch)

On Arch Linux, it should be enough to run:

cp /usr/lib/systemd/system/bluetooth.service /etc/systemd/system/
sed -i -r 's/ExecStart=.+/& -E/' /etc/systemd/system/bluetooth.service
systemctl daemon-reload
systemctl restart bluetooth

And the headset battery level should appear. To get the battery level programatically, you can then use UPower’s DBus API.