Are you really saying that the basic circuitry needed to refuel one tank draws 260 Amps?
Nope. Boeing are saying that if you leave the refuel switch 'on' for too long there will not be enough power left in the battery to a, start the APU. b, supply other systems during power up.
An example: Many years ago we had a flat battery on a 737-200. It was so flat that there was insufficient power available to operate the external AC power contactor. Fortunately the 737 also had a DC external power connector in the EE bay.
787 does not have that option.
I don't have the schematics with me and I can't remember whether the refuelling battery switch is connected to the Main hot bat bus or the APU hot bat bus.
I imagine the BDM function as:
If MAIN batt. is switched ON., (normally should be ON because itīs supposed to be "healthy") and DC BUS power is lost BDM "connects" AUTOMATICALLY MAIN batt. to maintain DC BUS with nominal voltage. The automatic (fast in miliseconds range) transfer is ABSOLUTELY NECESSARY. Why? In previous a/c the DC BUS is ALWAYS connected to the battery. Not in 787. The immediate connection is done by the "valve" (BDM) you mentioned.
Not quite.
The BDM is between the battery and the HOT BAT BUS not the main DC Busses. As it's name suggests the Hot Bat Bus is always 'live'.
The Hot Bat Bus is connected to the Main DC Busses via the Hot Bat Bus Contactor.
Quote:
Depending on Main Batt charge state, there may only be enough power for a half hour tow.
You mean,APU batt. OK? (Not MAIN batt. state)
No. The APU Bat when towing only powers the Position (Nav) lights.
The Main Bat powers the essential towing items such as VHF1, brakes (& Anti collision beacons-I think).
There is a very simple indicator on the flight deck that shows battery state when the a/c is unpowered. It simply shows High, Medium, Low.
When beginning a tow, this will give the engineer/mechanic a reference to how long they have to complete the tow. I have also heard that automatic load shedding will occur if the bat state gets too low, IE. VHF1 is lost. etc.