Abstract: In a vehicle having a voltage regulated electric alternator driven by an engine for supplying power to drive electrical loads and to charge a vehicle battery, a method of maintaining battery state-of-charge includes adaptively learning a system voltage set-point during a first mode of engine operation probabalistically resulting in an electrical system operative in set-point regulation. The method further determines from the adaptively learned system voltage set-point and a present system voltage during a second mode of engine operation that the electrical system is operative in auto-regulation. Idle speed is controlled in response to the regulation state of the electrical system to maintain the system in set-point regulation to ensure adequate battery charge or minimal battery discharge during the second mode of engine operation.