Lawn Mower Alternator / Generator

Hello, I'm a mechanical engineering student from canada I have a question I'm hoping someone can answer. I'm doing a senior project and I'm use a
lawnmower with the engine. I was looking at some of the Briggs and Stratton engines in the range of 6-10HP. I noticed some of the engines come with a built in alternator (eg 3amp). I'm wondering if theres a way to use the build in alternator to charge and external 12V battery or do I have to attach a car alternator to the motor using pulleys and belts. If anyone has any suggestions, drawings, schematics or anything they might think will help me, just reply back. Any answers would be greatly appreciated. Thanks
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If it's like the ignitions I have seen on small engines, the spark initiator is a magneto. Unfortunately they usually produce a high voltage (thousands of volts), which would be too great for battery charging. You would need to step the voltage down with a transformer and then rectify it to DC. You also have to regulate the current down to a trickle charge so that you don't overcharge the battery.
Don Kansas City
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On 23 Feb 2005 17:01:49 -0800, in sci.engr.mech snipped-for-privacy@eastlink.ca (Saulnier) wrote:

If there is actually an alternator as opposed to just a magneto for ignition, than that's probably what it is there for in the first place for things like electric start or lights.
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I think most mowers run the engine spark on magnetos, but the electric starter batteries are recharged using an alternator. I think the batteries are 6 volt, but I don't know if the alternators are stepped up from another voltage or not.
You might check with Toro in Bloomington, MN to get voltage and current, or with a local shop that sells parts to see what voltage battery one of those alternators charges.
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Thanks for the info. I did some more research on the magnetos and it does produce a lot of voltage but not much current. But now I have another question. How do I capture the amps from the magneto to the battery or if I need to, a step down device. Any suggestions would be appreciated thanks again for everyones help.
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I would not even try to get charge for storage (current) from a magneto - a magneto is designed to give a high voltage small current spike of short duration - a short tall peak and then nothing for a long time means the area under the curve has zip power. use a lawnmower alternator or the like, which is designed to provide constant steady current at a relatively low voltage.
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What about relocating a motorcycle charging system to the motor. Even looking at some m/cycle charging systems may give you some ideas. rob
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