This MR is a proposal to support those Bluetooth devices which report their battery status via AT+XEVENT.
It's been tested with Plantronics Focus (AT+XEVENT) and Xiaodu Du Smart buds (IPHONEACCEV).
The following output show the AT commands for a device using AT+XEVENT:
< receive
b'AT+BRSF=25\r'
> send
b'+BRSF: 879'
> send
b'OK'
< receive
b'AT+CIND=?\r'
> send
b'+CIND: ("call",(0,1)),("callsetup",(0-3)),("service",(0-1)),("signal",(0-5)),("roam",(0,1)),("battchg",(0-5)),("callheld",(0-2))'
> send
b'OK'
< receive
b'AT+CIND?\r'
> send
b'+CIND: 0,0,0,0,0,3,0'
> send
b'OK'
< receive
b'AT+CMER=3, 0, 0, 1\r'
> send
b'OK'
< receive
b'AT+NREC=0\r'
> send
b'OK'
< receive
b'AT+VGS=07\r'
> send
b'OK'
< receive
b'AT+VGM=15\r'
> send
b'OK'
< receive
b'AT+XAPL=047F-127-ec,6\r'
> send
b'+XAPL=IPHONE,2'
> send
b'OK'
< receive
b'AT+XEVENT=USER-AGENT,COM.PLANTRONICS,PLT Focus,127,2.36,bc1ba8c8faf83548866aab1252336934\r'
> send
b'OK'
< receive
b'AT+BTRH?\r'
> send
b'ERROR'
< receive
b'AT+XEVENT=BATTERY,6,11,461,0\r'
Battery level for BC:F2:91:8A:E0:E6 is 55%