Shaggy, to expand a little on Kevs.
Is it possible to get external electrical power onto the aeroplane when the two aircraft batteries are flat?
Nop. The GPU contactor is controlled by the Auxiary Control & Protection Unit (ACPU) which takes power direct from the Battery bus-bar, as does most of the contactors in the dc system. Note;this bus-bar is 'live' even with the batteries switched
off (assuming there's any juice left of course)
Surely the aeroplane wasn't rendered unusable when it was in service if the batteries are inadvertantly allowed to get discharged?
Yep. We would reject the aircraft if the batteries continued to take a charge after 10 mins....unless we knew why or the fuzzy wuzzies were coming over the hill. Getting airborne with suspect batteries was bad ju ju.
Is there a way to apply ground power without battery power being available
Nop. Unless you've got another source of dc as Kev suggests. Either through the batteriy terminal connectors or jump leads direct onto the battery bus-bar, but you
must disconnect the battery terminals first...
....but I never said that.
and then allow ground power to charge the batteries in the normal way?
BIG BIG CAUTION
This is not the normal or the safe way to recharge NiCad batteries from flat. If your not aware please Google "NiCad Thermal Runaway"....I'd hate to see you blow the old girl apart!!!
if the batteries are disconnected while ground power is powering the aeroplane, ground power is dropped and the electrics die.
By 'disconnecting' I assume you mean pulling the battery terminal connector plugs, then yes; because you've removed the supply to the battery bus and the ACPU, the GPU contactor will drop out.
When powered by the generators the GPU contactor wasn't a player...unless the GPU contactor actually closed....but that's another story.
Would a Nimrod in flight, powered by the engine-driven generators, be rendered devoid of all electrical power despite it being available from the generators if the batteries failed?
No. Only the battery bus would be dead, which would take out the Engine Fire Extinguishers but not a lot else. Everything else on the A/c would remain powered.
In fact fail
all generators and
both batteries, the old girl was still perfectly flyable provided the pilots could see which way was 'Up'. There were many things that Mr de Havilland got right that would put a modern electric jet to shame.....but don't tell the holiday makers.
Oilcan
oops posts crossed.
ps. Sounds like she's in good hands.
...but watch for the thermal runaway....once the temp and current flow starts to build it can be impossible to stop it till it goes BOOM.
Hope this helps.
Oilcan