Just had a quick look at some logs I had taken first thing in the morning when my battery would have been fully charged and it looks like the 4th byte is the Battery Level

F5 fully charged

F3 half charged

as my phone has been between 55% and now 43% while testing this evening that fits neatly into a 5 Bar display. I expect to see the 4th Byte drop to F2 once my phone drops below 40%

Ran phone with the "torch" on and played a few you tube clips and got it below 40% and "Bingo" that 4th byte dropped down to F2 as expected.

Thats a bit of success for the evening, got signal strength and Battery level determined.