Car batteries are probably about 60Ah, or 720Wh@12V. At 200W, a cyclist could charge the battery in about four hours, maybe five considering various losses. A normal car alternator could be used to convert bike energy to 12V DC to charge the battery.

you need to also run a small dc generator, one that powers bike lights and connect it to the field on the alternator.
first it freewheels till you are going fast, suddenly you are peddeling against great resistance and generating eletrical power.