I have been thinking about the B737 Classic starting off ground power, and I can't see how it works.
I remember, many years ago having a B737-200 with a flat battery. We used a DC ground power unit plugged into the E and E bay and started the APU. We were in a hurry to get it up the ramp.
But other times, on the line with a flat battery, I have plugged in AC ground power and charged the battery until it has enough volts to start the APU. I have never tried to start a B737-400 APU off AC ground power.
Now I need to hunt out a maint manual.