I found this kind of interesting. It's from ARCO Systems Alternator Tech Bulletin:
I never knew this but I suppose it makes sense......
Just because the alternator looks like it’s turning, doesn’t mean it’s turning fast enough to charge. Most
alternators do not start charging until they reach 1,000 RPM alternator shaft speed. 5,000 RPM alternator
shaft speed is normally required to reach maximum output. If you’re not sure what the alternator shaft
speed is, you can determine this with the pulley ratio. Measure the diameter of the crank shaft or drive
pulley and the alternator pulley. Divide the crank shaft pulley diameter by the alternator pulley diameter. This
figure would be the engine-to-alternator RPM ratio. A normal ratio would be 2.5 to 1. For example, let’s say
we have a 7 inch diameter crank shaft pulley and a 2.75 inch alternator pulley. We would divide 7 inches by
2.75 which equals 2.54 to 1. If the engine was turning 1,000 RPM we would multiply 1,000 by 2.54 which
would give us 2,540 alternator RPM.
Again, with today’s high amperage alternators, belt condition and tension are critical in proper