1DC:
I'm no expert, but the 28v battery is normally there just to keep essential
services such as flight instruments running when all other power generation
systems have failed. It would only have a fraction of the power capability
of the main generators, which may be 100's of Kva or more on a large aircraft.
If the battery is larger, then it means that there are more essential services
to keep alive, greater power consumption, or longer run time. I would assume
that the apu battery is just there for starting the apu and is intentionally
separate for redundancy reasons.
Quite interesting that they use a 270v dc bus for some systems and shows just
how much power is consumed. For equivalent Kva, increasing the rail voltage
decreases the current, which allows thinner cable to be used, saving weight.
Introduces a whole lot of problems for the designers athough and not something
you would want to get your hands on :-(...