The electrical system is really quite complicated, isn't it? I've always found electrics one of the hardest areas. Only got Classic manual to hand at the moment- this is what I dredged up:
Normal power for the Battery Bus is from TR3 via the Standby Power Control Unit. In the event of failure of AC Transfer Bus 1 or DC Bus 1, power is supplied through the DC Battery System, I think the Batteries are supplying Battery Bus and Hot Battery Bus whether the Transfer Buses are powered on not powered whatever the position of the Standby Power Switch. 737 Ops Manual 6.20.13. There seems to be an inconsistency there!