Right, so I've ripped out an operational charging system from an otherwise knackered motorbike, and I have a new 12 volt battery to hand. All well and good, so my question is what is the minimum rpm a bike alternator has to travel at to successfully trickle charge a battery which will only be used for occasional lighting of a shed? (out of acorns and all that). I'm hoping to get the wind to charge it for me, so I'm pondering what sort of gearing and fan blade length is required.