APU Start without ground power connections non possible ? or issue ?

Hi pilots,

I have a question about starting APU :

I tryied to start the APU with battery power online only, no ground power connection.

Wonder myself that this way it won't start.. if you listen from outside u can ear the spool up whistle but suddenly it goes down and APU won't stat..

Tryied second attempt : i set up ground connection dual power but without putting online with the overhead switches.. (you can read two green switches telling "AVAIL") only connected but not activated !

In this case the APU starts without problems..

ISSUE or PrOCEDURE ?

anyone knows it ?

Thank you a lot !