What is your goal?
Are you trying to turn a car into a generator?
and why can't you use the battery?
If you must do it that way you should get a REAL generator dynamo.
(Don't try to spin a DC MOTOR)
A real dynamo generator has wire coiled on the outside and the magnets
spin on the inside. They can't create as much power as a energized field unit but they are easy to use and can be spun by anything from a old 2 stroke engine to a windmill.
But seriously, What are you trying to do ?
-Mike