Hi, the problem looks is not difficult.
If we understand correctly, we need to 12v battery connected to a Brushless motor that is attached to the shaft through the car generator.
Get some output voltage and power. Output voltage should be 12v DC and stable. control voltage has to do PCB controller.
The scheme can be developed. To the best out scheme, we need to know the generator power (or current output of the generator).
it is necessary for selecting some elements of the scheme.
The tasks to design the circuit, and the need to provide drawings with the elements of the circuit board?
Sincerely, John.