There are several ways to get more power at lower engine speeds. If you only require a little more power at lower engine speeds you may be able to use a smaller pulley. The smaller pulley turns the alternator faster at lower speeds thus making more power. When using a smaller pulley care should be taken not to over rev the motor. If you over rev the alternator with a smaller pulley the alternator can fail. Installing a higher output alternator that is the same size as your original alternator will not give you more power at idle. The voltage regulator contains
circuitry that uses the residual magnetism in the alternators fields to determine when
to turn the alternator on, the regulator does this by sensing the RPM the alternator is turning.
When the alternator gets to a certain rpm the voltage regulator "turns on".
If its not broke, Dont fix it. Have it tested ?