Thank you guyes for your help. Got the APU running now. I think the Problem is, when you turn on and off many switches, especially the bus-switches, something seems to run out of normal. When I reload a complete new flight and proceed the startup procedure from a fresh loaded cold&dark cockpit everything works well and the apu starts with the first try. If you only reload the panel-state --> strange, sometimes working, sometimes not.

If you want to start the APU from cold&Dark just turn on the battery and press the Apu start switch. It's that simple. Fred.

The APU starter generator takes power from switched hot battery bus if AC XFER BUS1 is not avaiable. The SW. HOT BATT. BUS is avaiable with the battery switch ON, no other links are required to power the APU OFF/ON/START switch.A thing about fuel pumps, they are ALL 6 AC fuel pumps, so they needs AC power that is not avaiable in DC mode. Stby power doesn't power the pumps, they takes power from main buses.There is a DC fuel pump that automatically starts when the APU switch is not OFF and main pumps are in low pressure. This is the case of the battery start.The APU will be cranked by the starter generator, after starting is complete, the starter generator revert to generator and is able to supply power.If AC is avaiable the apu will start from the AC XFER BUS 1, I checked this on NGX but I've not found any increased AC AMP drainage from GEN 1, neither from GEN 2 with GEN 1 not avaiable (XFER on AUTO), and there is also no current drain from the battery, it simply starts without any current drain!