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From that thread "in every part in the Ultegra DI2 group is a micro-controller so it can communicate with the other parts." answers your original question.
If you are talking about the pre-CANbus 7970 Di2, that's a different matter.
"7970 u can just cut off the battery mount and hook up any 7.4v Lipo... 6770 is the first using CANBUS, so u cannot, without putting the battery mount inline as well"
hippy
Brommers
@user136820
This thread says otherwise ..
There has to be a PCB somewhere, to manage the CAN Bus protocol