@Lyman
Does the ACE perform other, occasional duty?
As I understand it, it is only designed to provide emergency power to the ACEs in the event of a loss of other in-flight power sources.
How many discharge/charge cycles per flight?
I would expect none since it would only be used if all other in-flight power sources failed.
Is it enclosed, with a tamper proof seal?
It is enclosed.
Is it not a true "Backup" storage battery, where the APU/MAIN batteries do extra duty?
Yes, it appears to only be designed for use in an emergency.
Do (we) know if the ACE storage battery is part of the AD?
It appears to not be as I have seen no mention of it in any of the FAA, NTSB or JTSB. Same with other Li-Ion batteries aboard the 787 like the ~30 for wireless emergency light units and those that power the DFDR boxes.