I bought a recumbent exercise bike last year to modify with one of my DC motors as a generator head but accidentally scrapped the motors when cleaning the garage. I do have some smaller ones and may just revive the project since I am trying to lose weight and kill two birds with one stone.
Most low speed high amperage gen-heads are pricey plus the actual conversion and regulation is not as easy as one may think, the more wattage produced also makes the bike very difficult to pedal making a 30 minute ride quite a workout and if not in shape would never be able to pedal the 10 hours needed to produce 10 cents of electricity.
You will need a good voltage regulator and a sufficient size deep cycle battery, if you do not have direct 12 volt appliances then you will also need a 12vdc to 120vac inverter which will cost you about 20% in efficiency i.e. every 100 watts pedaled will only result in 80 watts available for use. Deep cycle batteries need to be charged and discharged according to their specifications or you will kill them quickly, too high of a charge rate will boil them and too deep of a discharge will sulfate the plates.
For a quick and cheap test and if you can still find them is to get one of those small bike wheel generators that use to power a head light, put your bike on a stand and pedal away to see what kind of juice you can make (besides sweat). Remember the larger the generator and the more power you try to pull from it the harder it is to turn and can even become impossible for a human to overcome without large gearing ratios.
Most of these projects are novel and not meant for large production but mine will certainly make enough power for a laptop.
