Alternator? No, no, far too inefficient. Think generator -- a DC motor with permanent magnets. Apply juice and you get torque; apply torque and you get juice, even at very low speeds.

And ideally, you should also have a gear or pulley setup to increase the speed to the generator and absorb all the weird directional forces that might cause premature wear.

And even more ideally, if you could find a common, mass-produced source for such items, designed to run in the 12-18 volt range, and often available for free when the batteries konk out, you'd really be in the sweet zone.

And if you said "hey, that sounds like a cordless drill," give yourself a gold star.