I think the most you can ask for is just extending the time the radio will work before running out of batterys. Physics basically says you can't get more energy out then you put in.
In regards to having the right motor and pulley combo/ratio, the way I understand it is if you could find a combination that would work, the pulleys end up being large, and if you were to disconnect the battery after starting it up, the motor ends up not having the grunt to turn the pulleys, and comes to a stop. Get a bigger motor, need bigger pulley to drive it.
On to efficiencies. electric motors are about 90%, alternators are about 50%, so that can be a bit to overcome.
But I don't really know- been a long time since school.
I just imagine that if you could get 'free' power out of a motor and alternator with the right pulleys, than that technology would be everywhere, being put in all the latest electric cars, being retrofitted on electric motors worldwide, powerplants would be a joke.
Thats my 2c anywho
