Regenerative braking works with a powerful alternator and li-ion or similar battery because a normal 12V agm 80Ah has a charging current of about 25A. So it needs 3.2h of regenerative braking which is ridiculous. I have that on my car and I donít think it does anything. On my previous car the system used to keep the battery always too low and in the winter you could nít use the rear defrost because of extra current needed to charge the battery (bellow a certain state of charge it charges continuously). So to work a 5kW electric starter/generator may work, it would charge a 80Ah in 12min at maximum power which is ok. This electricity is better than using the grid like in my op example because itís something that otherwise it would be lost, but itís harder to implement and costs more. In addition it needs data from the car, and a custom made generator, and controller.