On initial activation of the APU starter the battery voltage will drop to about 21VDC.
It will then commence to climb from 21 to 22.5 VDC as the APU accelerates to 20% RPM. It will then remain constant until APU starter cutout (50% RPM?) at which time it will return to approximately 23.8-24VDC.
When the APU generator comes on line the voltage will then go to 28DVC as the TRUs power up.
Hope this helps