flywheel battery charge need lesser time than normal battery, & it save more energy with lesser space. if it can easily to recharge in a city, is it possible to replace normal battery by flywheel on a city car like fortwo & iMiev?

It might work on a car like a Toyota iQ.

edit:

oh, electric car battery

Is this kind of an alternator for the drive battery? I'd imagine you can't just slap one on a car that wasn't designed for it without some special conversion kits if it is

Why not just rise a push bike? It's eco friendly and exercise . It will get you about as far as a flywheel battery powered car .