Cockney Steve
I am sure Turin can weigh in as well but there seems to be a slight misperception about what the purpose the aircraft main battery serves in a degraded power state. Indeed if it it a race to the aerodrome the battery should last for 30 minutes to provide absolute essential power-think in this case of the captain's flight instruments. This has been pretty much the standard on every Boeing that I have flown as well as MD, Lockheed, and narrow body Airbus.
What really should be going on however, is that the battery is providing power long enough until you can climb back out of the degraded power state. This might mean the RAT starts powering the busses, the apu comes on line, or you can get a generator back. It should be just an interim power source at most.
I know you realize this but I am not sure some that are following this thread do.
Btw. I have a colleague who landed in South America on battery power only in a 767. 40 minutes on this power source. 10 more than advertised but it was drained on the rollout....close...