I am trying to work out a way mathematically if it's worthwhile to place a dynamo or generator on an electric bicycle to convert the bikes movement to electricity. Some dynamos create very little drag. By doing this I hope to prolong the time before charging the batteries again. What I think I need to take into account is how much electricity the dynamo returns offset against the extra energy consumed from the battery to compenstate for load placed on the wheel by the dynamo. Is there any easy way to do this. I know no formulas except for how to work out kinetic energy. Even if someone could tell me what formulas would be required to mathematically test my idea would be good.