Disclosed are methods, circuits, and systems for implementing an AC voltage adapter and battery charger system in a closed loop topology. Embodiments of the invention include methods for controlling an AC adapter charging a battery with steps for using a closed loop configuration to monitor one or more selected parameters in the charging loop. Feedback to the AC adapter is used for dynamically controlling the adapter output voltage. A preferred embodiment exemplifies a closed loop circuit for battery charging. The circuit includes an AC adapter and a monitoring circuit for monitoring the closed loop and for providing feedback to the AC adapter, controlling the adapter output voltage.