You are referring to the 2010 test which was just that - for the purpose of gathering data and trying out various components.
This weeks tests are a massive step forward, (although not flight capable). integral with the wheel & powered off the APU.
Naturally the final version will be FAA certified & integrated - giving the pilot autonomous control.
It wasn't a battery cart. It contained the inverter & measuring equipment. A STANDARD APU has sufficient spare capacity for WheelTug's twin Chorus Motors. The detail is commercially sensitive.
The weight of the system is not more than the weight of the fuel that won't need to be loaded. The WheelTug system controls and data flow will not interface with any other aircraft systems.
The savings to be made come to about US$600,000 (or more) per year per aircraft. But the benefits don't stop there. Time, engine wear, safety & environmental. Imagine being almost at the runway when curfew hours end & in the air 5 minutes later!
WheelTug is planned to enter service late 2013. More detail is on the
WheelTug Website There is also a benefits calculator.