You're getting charge at idle, which usually only takes a few amps. Keep in mind the alternator provide juice for the electrical system and charging the battery. The battery is a starting battery, and only provides power if the load at any given time exceeds the capacity of the alternator at a given output. Alternator output increases with RPM, but also realize that load on the engine increases with amperage draw as well. Some folk want higher output from the alternator for such things as winches, sound systems, and portable welders. For normal use though the stock alternator is fine, unless it needs to be replaced. In that case, the $129 180 Amp unit looks REAL good compared to the OEM $$$!