Firstly, I haven't worked the -800, but I know about the classics. I would imagine that they are not too different from each other, from what I've read so far.
My understanding, is that under normal conditions, ie. Batt on, Stby Pwr switch auto, in Air mode, the hot bat buss was fed via the battery, but the battery bus, confusingly, is fed via some relays from TRU3 (115V Bus2). I think the reasoning is that if you lost one power source, it's so configured, that you wouldn't lose all the instruments, radios, lighting etc. in one go, so giving you the chance to start the APU and get some power back, or reconfiguring the DC system according to the QRH.
Hope that helps a bit.