Calculating Alternator RPM
When considering idle output of an alternator, pulley ratio must be taken into account. All measurements of output of an alternator can only be taken at the shaft of the alternator. Thus, idle amperage is calculated at 2,400 alternator rpm. On a street vehicle, the average pulley ratio is 3:1, meaning that 2,400 alternator rpm is equal to 800 engine rpm.
The connection between the alternator and the battery is very important. An undersized charge wire or improperly attached terminals could result in voltage loss. When upgrading an alternator there is an increased volume of current, and in most cases that means the size of the charge wire should be increased in size as well.