Within the "Industria 2015" Italian framework program, the HI-ZEV project has the aim to develop two high performance vehicles: one full electric and one hybrid. This paper deals with the electric energy storage (EES) design and testing of the hybrid vehicle. A model of the storage system has been developed, simulating each cell like an electric generator with more RC circuits in series. To take account of the heat transfer, a forced convection model has been used with the air speed proportional to the vehicle speed. The model had two calibration steps: the first has determined the electrical parameters of the model (open voltage circuit, internal resistances and capacitors); the second to calibrate the heat transfer model. The first calibration has been made on a climatic chamber at 23 °C discharging one single module with different constant currents from 5C (5 times the nominal capacity) to 25C and charging with currents in the range from 1C to 5C. (where C means the current rate the flow in the battery in 1 hour) The results show that for all the tested cycles the battery pack designed is able to run safely, not exceeding the limit temperature (50°C). The model developed and validated can be a useful tool in the design phase of a battery pack system. © 2015 SAE International.