A non ER 757 main battery will give you 30 minutes of power (40amphours). On a ER version, you also have the HMG, which powers the hot batt bus for unlimited time, thus the battery will be charged all the time.
For the EICAS msg you had, it shows the charger is defective. If the message is followed slightly later by the main batt disch message, you know the battery will be dead within 30 minutes.
@oceancrosser, the charger message will most likely be a charger, and not the battery. The charger has protective shutdowns built in, e.g. with an overtemp batt, which of course go away if you put a new battery in. Question is, how long before your defective charger eats up the newly installed battery...
For aircraft storage/prolonged parking, we disconnect the batteries from their busses. Also be aware that some solenoids do eat battery power, as some solenoids are energized when the cockpit switch is off. It doesn't mean that when a pilot switches a system off, that it is completely removed from the battery.
And as said the hot batt bus has no switch, it is directly connected to the main battery and its charger. The hot batt bus is in turn connected to the ground service bus, which gets power from either a GPU connected + ground service switch pressed (batt charging on ground), or from the right AC bus (batt charging in flight).
Last edited by Piper19; 8th Sep 2013 at 23:43.