Glad rag.
If you lose both engine's gennies then you are relying on the APU to be started, quickly. The Cabin Air Compressors (CACs) can draw up to 105KVa each, so a battery is no use there.
There is also a Ram Air scoop that opens in the left wing/body fairing. This Alternate Vent System (AVS) cannot keep the cabin pressurised though.
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