Pontryagin's Minimum Principle provides an efficient framework for Hybrid Electric Vehicle (HEV) energy management. The control is obtained by minimizing the Hamiltonian at each instant. The whole optimization problem is then transformed into a Boundary Value Problem (BVP). For a few problems, this minimization can be solved explicitly and for most of the others, numerical methods are used. An alternative method for any convex problem is proposed. As a result, the whole hybrid vehicle optimal energy management is turned into an extended BVP.