Figuring your pulley ratio is easy. Fist measure the alternator pulley overall diameter.
Measure The Crank
Next, measure the overall diameter of the crank pulley.
The alternator has an OD of about 2.43" and the crank has an OD of about 4.43" so we simply divide 4.43" by 2.43" and we have a 1.8:1 pulley ratio.
This means that for every revolution of the engine crank the alternator makes 1.8 revolutions.
By knowing this we can determine that at 1000 engine RPM our alternator is spinning at 1800 RPM. Once you know this you can compare it to the curve for your alternator to see where you need to run your engine at to develop the current you want..
Compare It To The Output Curve
This is the Electromaax performance curve for the 140A alternator. Electromaax uses a pulley ratio of 2.25:1 for this power curve. By knowing your ratio you can backwards engineer the performance off this chart.
Personally I would like to see Electromaax use alternator shaft RPM on their curves but they don't. Just know that their curves are based on a 2.25:1 pulley ratio.
Based on this performance curve we would need an engine RPM of 1675 to attain a 3000 alternator shaft RPM which puts us at near max performance for the alternator. So on this particular Volvo engine pushing much beyond 1675 RPM really gains you little in the way of added alternator performance.
If you run your engine at anchor for charging, or engine driven refrigeration, this is good information to know so you don't run the engine faster than is necessary and annoy everyone with unnecessary noise pollution. Sea Frost engine driven refrigeration systems attain max output by about 1500 RPM so this is a pretty close match up.
Even at an idle of just 1000 RPM this alternator can produce over 100A of current....
One last item. Do not forget that the neg wire for the alternator is equally important in this circuit as the positive wire. They should be the same size. These are 1/0 UL1426 battery cable.
I find the best location for a heavy neg wire on these alternators is to the adjusting arm ear. What can't be seen here is the alternators temperature sensor, which is also very important, especially if feeding large banks or banks of AGM batteries.
At the time I did this article Electromaax did not offer these alternators in an isolated ground option, I generally prefer this, and they now can offer that as an option.
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