Iterative procedure for solving the problem of optimal battery sizing and control of an EVT PHEV powertrain. For a given power threshold P_th the engine on/off sequence is decided. Then, for a given on/off sequence, the problem in Table 2 is solved assuming a constant vehicle mass. After optimal battery size n_b is obtained, the vehicle mass is updated and the problem is solved repeatedly until vehicle mass stops changing. The search for the optimal threshold P_th then continues by repeating the entire process.