A belt tensioner for a belt drive is designed to connect an engine shaft of an internal-combustion engine to a reversible electric machine which has the function of starting motor and current generator; the belt tensioner comprises a fixed part and a pair of arms, hinged on a common pin carried by the fixed part and provided with respective idle pulleys, and a spring, which forces the two arms towards one another so as to load the pulleys elastically against respective branches of the belt. The two arms of the belt tensioner are provided with respective first arrest elements, which are designed to interact with the fixed portion to define respective positions of arrest of the arms themselves under the action of the spring, and respective second arrest elements, which are designed to interact with the fixed portion to define respective positions of end-of-travel of the arms under the action of the pull of the belt.