This site does a great job in breaking down a basic charging system, as described below:
Current Demand and Flow:
If you have an alternator that can produce 120 amps of current (max) and the the total current demand from the electrical accessories (including the battery) is only 20 amps, the alternator will only produce the necessary current (20 amps) to maintain the target voltage (which is determined by the alternator's internal voltage regulator). Remember that the alternator monitors the electrical system's voltage. If the voltage starts to fall below the target voltage (approximately 13.8 volts depending on the alternator's design), the alternator produces more current to keep the voltage up. When the demand for current is low, the full current capacity of the alternator is not used/produced (a 120 amp alternator does not continuously produce 120 amps unless there is a sufficient current draw).
So the car would not be considered a great tool to charge the battery, as it would take forever... Depending on what the battery is demanding. RPM and loads are irrrelevant, as teh alternator will make do as needed as long as it is not overloaded. The reason a battery charger works best, as Bob described, is because it forces a set amount of current through the battery. This allows for faster, and better charging.