1. Depends on the condition of the BAT -> min voltage for APU start is 22 VDC With the meter selector switch in "BAT" pos youŽll read the actual voltage (fully charged 28VDC)
2. Voltage drops down to Žbout 19 Volts
3. Yes. YouŽll rather see the indication ligts (warning ligths) because they go dim.
The B737NG has a nice feature to keep the draw from the BAT "low". The start power unit changes 115v ac or 28v dc electrical power to 270v dc power. Transfer bus 1 or the battery supplies power to the start power unit (SPU). And the start converter unit (SCU) changes the 270v dc power to AC and sends it to the starter-generator.
hope this helps a bit. For more Information fell free to send me a private message.
Yours,