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Bearings in most small motors are pretty lite, likely not last long under the load of the turbine. Need to add a thrust bearing, or a jack shaft to take the load off. With a jack shaft you could also gear the motor up for higher rpm.
Good simple idea. Been looking for a cheap portable AUX. power pack for my GPS. (Internal bat. good for about three hours) Out stomping around in the boonys you can use that up fast. I used a 4 "D" battery pack rigged with a belt clip. Works Soooopoib! Thanks.