Abstract: A belt tensioner (20) for a belt drive (1) is designed to connect an engine shaft (6) of an internal-combustion engine (2) to a reversible electric machine (4) which has the function of starting motor and current generator, the belt tensioner (20) comprises a fixed part (21) and a pair of arms (23, 24), hinged on a common pin (31) carried by the fixed part (21) and provided with respective idle pulleys (25, 26), and a spring (27), which forces the two arms (23, 24) towards one another so as to load the pulleys (25, 26) elastically against respective branches (15a, 15b) of the belt (15).