Hello! i'm happy to answer your question. The Fast charger will only active when the device is below 50%. The built-in LED indicator light will alert you if the device is not properly aligned and when a good charge alignment is acquired. If using a Galaxy S6, Galaxy S6 edge, Galaxy Note5 or Galaxy S6 edge+, the pad will also alert you when your device is fully charged with a green LED indicator light. Thanks for Being a Samsung Customer! ^Rod