Primary European car-makers agree on a 48 V embedded-supply to support the growing current-load requirements in modern vehicles. The functionalities required to the starter-generator are the engine start up, launch assistant torque, regenerative braking, battery charging and early fuel cut off. The dynamic load profile and the harsh environment working conditions make thermal issues the key challenge in the machine design. The paper presents the simulation and experimental test results of a belt driven starter-generator for 48 V mini hybrid powertrains. In particular, a double three-phase 10 kW induction machine has been selected as the suitable candidate for the application. In detail, the authors illustrate the experimental calibration and validation of the steady state thermal model, as well as the thermal characterization of a liquid cooled prototype. Finally, the paper deals with the temperatures prediction of the motor working in different load conditions.