With the standby power selector in the BAT position, as selected by the flight crew, the main battery provided power to the hot battery bus, the battery bus, the AC standby bus, and the DC standby bus. In addition, the main battery charger was not receiving power, and thus the battery was not being recharged. When main battery power was depleted, all 4 of the aforementioned buses became unpowered.
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It's not that they wouldn't have "main electrical power"
R and L AC were fine.
Battery's powering the standby AC/DC, batt bus and hot batt bus. As long as you have a battery you'll be fine.
But with the Stby power switch in the BAT position the battery charger is disconnected.
How long should you expect the battery to last? Uh, 30 minutes anyone? It lasted 90-100 minutes.
