Chris Scott:
Was it fully charged on APU shutdown at NRT? Does recharging continue from the main electrical system? If not, presumably it would have leaked some of its charge during the 12-hour flight, but not much.
I cannot see the logic in limiting the power to the APU BCU to come only from the APU itself. At a minimum it would have to be rechargeable from shore power, and there would be no reason not to also power it from one of the general AC power busses.
In the Narita incident, it was admitted that there was some sort of unintended cross connect between the APU battery bus and other loads including the entertainment system. Whether that included any possibility of sneak charging around the BCU has not been mentioned, but the diode between the APU battery and the APU battery bus should have prevented that.
Single Point Of Failure anyone?