Old Engineer
I continue to be puzzled as the why the APU battery must remain connected after it has started the APU.
From what I have read, the APU Controller is powered by the APU battery only. So should the battery fail, that powers down the APU controller and the APU. I understand this is the scenario that happened with JL8 at BOS (once the APU battery failed, the APU controller and APU shut down).
I've been told this is a standard Boeing Commercial Airplanes design philosophy and is not unique to the 787. Boeing employees I have spoken with have explicitly said this is how the 777's APU operates, for example.