•Hull and eebay is at cabin altitude ~6000 feet
Fixed it for ya.
The ACE is single purpose, a true 'Backup', not integrated into the distribution system for general purpose.
The APU battery has a single purpose, mainly. It is not integrated into the system. It starts the APU (occasionally) or powers the Nav lights (very occasionally).
But still failed.
The main battery has extra functions compared to say, the 777 main batteries, and that is to power emergency brakes (hardly ever) or towing without a generator (rare).
And also failed.
General purpose? No.