Description :
Alternator rpm and temperature are two main factors that affect output current. Output current is directly proportional to the alternator rpm.
Whatever the engine rpm, normally above 2000 rpm ( depends on drive/pully ratio ) of alternator speed is taken. If more rpm then more output current.
For a setting drive/pully ratio of 1:3 , engine rpm should be 1000 for alternator speed of 3000 rpm. Lets 12 volts battery & if 1500 rotor rpm is applied then output will be about 15 amps