Hawker Siddeley HS-748

I have a question about the Hawker Siddeley HS-748 that I downloaded from this website. There seems to be an external power switch that has to be turned ON if the engines are not running, or so the manual says. So as far as I know, the external power is something like the APU - it gives power to the aircraft when the engines/generators are off. But even when I switch on the external power (and the battery) the avionics and most of the electronics go off after about 15 minutes! (This also happens to the default fs2004 aircraft when you leave them on battery without the engines running).

So am I doing something wrong or is the system not simulated at all? I believe the external power should work for as long as you need it.

Thanks in advance.