Hi Lem
There is a
BIG difference between APU- starting 737CL vs. 737NG.
737CL use 28V DC to turn the starter on the APU. When 100% speed is reached you can get AC by the APU generator.
737NG APUs have combinated starter/generator. This means that during starting he acts like a starter. Later it acts like a generator.
The starter/generator needs AC to turn the APU. Now two other new parts come into action. The start power unit (SPU) and the start converter unit (SCU). The SPU provides the SCU with 270V DC. The SCU converts the 270V DC to AC and feed the starter/generator.
And here finally the answer to your question about AC ground cart:
The SPU can use batterie (28V DC)
OR "AC TRANSFER BUS 1" (115V AC)
HIH
Ingoxx