Extra energh! WOHOOO!
Generate HHO!
Just kidding.
One would need to size the battery according to your driving situation. If you brake a lot, then a smaller battery may suffice as it will get charged a lot. If a lot of highway miles then the batteries would need to be bigger to be able to go longer without using the laternator.
The super capacitors (there are a couple of threads on this forum) are a bank of 6 rated to 15V when combined. They are ~3000F each and they can hold about 3 seconds of cranking power at most.
I think that the capacitor idea was referred to in conjunction with a deep cycle battery. At the end of a brake'less trip, if the battery is too low, it will not start the car. But if you have capacitor bank that may be able to push enough amps to start your car. Or it is very quick and easy to boost up the capacitor so that it is capable of starting your car again.