What is the minimum number of RPM an alternator needs to turn to charge a battery?

Let's assume we have a good, chargeable battery.

My truck idles at around 650 rpm. The battery does charge at that rpm. So, for my truck with my alternator, the answer is 650 rpm.

As each model of alternator is different, and is built for each model of vehicle, I think the answer would vary. BUT - I also believe that the minimum rpm for charging would be the idle state of the engine. ELSE - running the engine at low rpm would kill the battery. Most modern engines cannot run by alternator alone.

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1200 rpm or above to charge......assuming battery charge only....

Full output is at about 3500 - 4000 RPM.

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Thanks. The reason I asked is a friend of mine is considering running an alternator off the wheel of a horse drawn vehicle.

I couldnt even imagine a wagon wheel at 650... yikes

It would have to be done through a series of pulleys of various diameters. The idea is to have a small rubber wheel in contact with the tire of the wagon.

what is the application for? there's always something like this: bike generator

there are other lesser wattage generators that may also be enough for LED lighting

My friends and I are talking about building animal drawn vehicles, horses and donkeys, as R/V's. We're wondering about the feasibility of powering electric lights or even a small refrigerator by putting a small friction wheel against one of the wagon's rear wheels. The wagons would run on automobile tires.

For years I've been planning to build a 21st century version of a covered wagon and following fire trails around the west.

I think the limiting factor for many cars is to prevent the alternator from blowing up at engine redline. If the engine is going 6500rpm, the alternator will be turning a good bit faster.

Diesels have the advantage there. I wouldn't be surprised if the pulleys are sized so that the alternator works well even at idle speed, since diesels don't reach high rpm.

I don't think I've ever seen an alternator where the pulley wasn't a good deal smaller than the pulley driving it so the alt. RPM would be much higher than the engine's. I believe the old generators had larger pulleys.