Powering a TV from an exercise bike!

I am hoping to create an exercise bike with a 12v motor attached to a 12v battery that will get charged up when the bike wheel turns. It’s more complicated than this, there will be a media player too, but the tricky part is, I would like to have a relay that is also attached to the feed from the bike that only closes or passes voltage from the battery when the bike generates a certain amount of power. I also want to avoid it constantly going on and off as the bike fluctuates in voltage, some form of delay. Some form of display of the voltage would be great or even a way to display with LED the voltage created by the bike as it generates (red to green kind of idea). I understand this may seem a bit much, but a voltage regulator that only passes power when it generates the correct amount is the bit I need help with. many thanks. Scott