How do you assume you should loose battery power? Since Bat. power is primarily our backup system in flight you would first have to loose your engine driven generators, natural action would then be to turn on the apu and get it on gen bus 2 (power to transfer bus 1+2, main bus 2, AC stdby. bus, TR1+2+3..), if it also fails you would have to rely on bat power thru the standby power system and get the bird on the ground within 30 minutes (fully charged bat.).
I do not think loss of all electrical AC/DC power is a failure that has been taken into account by Boeing. Can't see it happen unless you lose both generators, apu generator and deplete the battery... talk about having a very bad day..