Free Electricity # 2

[keyplayer]

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.

 

[nstreet]

connect a volt meter across the battery, spin the alternator, when the voltage is just above 12v, say 13.5, count how many rpm you doing at the alternator. That will be about your answer

 

[Screwi]

the rpm is usually stamped on a plate on the alternator and gives the o/p at a specified rpm.

 

[felix]

If you're going to the trouble of making a fan you might be better off hunting down a second hand windcharger. These things are often available cheap on caravan sites which have recently been converted to mains power. There is no gearing (too inefficient) and the generator is in the fan hub; fixed coils inside a rotating magnet system. One drawback is that you will need the regulator as well. It's a simple shunt regulator that dumps current when the voltage reaches 13.8V.

Incidentally, you'll have to watch the fan speed if you're using that bike alternator. The mechanical load will drop right off when the battery is full and a strong wind will have the blades screaming round - and possibly out!