Not really. As the car idles for an extended period of time, there are several things happening. The alternator is less efficient at lower RPMs, and therefore is generating less current. This has nothing to do with the length of time that the engine has been idling. As the battery approaches a full charge, it will accept less current from the electrical system. At some point, the battery and alternator will reach a balance. At the same time, there are electrical loads from the vehicle that are also drawing current from the alternator. These loads have priority, and will draw from the alternator and battery as required. In a case where the lights, climate control (heater, a/c, and fan), heated windows, etc. Are all running, and the engine is at idle, the current draw could exceed the output from the alternator. In this case, the battery could be discharging as the engine idles.
In short, there is no hard and fast "rule" or time limit on how long the alternator will operate, or for how long or how much it will charge the battery. Load versus output will determine whether the alternator is charging the battery, regardless of time.
Saturday, June 20th, 2009 AT 6:34 AM