[Phantom]

Back to the air tools idea, if you get a tool that has a good amount of torque but is not fast enough that can be solved. The test with the die grinder was effectively running 1:1 being directly connected to the alternator. Now if the grinder is geared say a 2" pulley on the grinder and 1" on the alternator this places it at 1:2 so if the grinder is running at 200rpm the alternator will be at 400rpm.

An easy way to test what ratio would be needed it to use the sprocket from a bike so all that would need to be done is move the chain to the next gear to change the ratio till a suitable one is found.