Owe to their high electrical energy density, lithium-ion batteries are the most employed technologies in electrified vehicles, whose market share is growing very fast. As a matter of fact, their thermal management is of crucial importance to keep the operating temperature within an appropriate range, as this might greatly affect performance and durability of such devices. Heat generation during cyclic charge and discharge processes, occurring during a vehicle mission, may cause critical temperature variations and, therefore, a suitable thermal management is indispensable. This is particularly true for fuel cell hybrid electric vehicles, where the battery undergoes more severe thermal stresses than in battery electric vehicles, due to higher operating C-rates. A hybrid energy storage system, which integrates the battery pack with a metal hydride hydrogen storage tank, may be a promising solution to store energy while implementing an effective, integrated and yet simple thermal management. In fact, if the system is properly designed, it becomes possible to exploit the endothermic desorption process of hydrogen in metal hydrides to remove heat from the battery during vehicle operation. In this work, starting from a battery electric L-class vehicle, a plug-in fuel cell/battery hybrid powertrain with a hybrid energy storage system is designed in order to improve its performance in terms of driving range, by enhancing the on-board gravimetric and volumetric energy densities. Due to the homologation constraint on the maximum vehicle weight, i.e. 450 kg without battery for L-class vehicles, particular attention is taken to the influence of weight increase associated to the fuel cell and metal hydride hydrogen storage tank. Simulation results are presented to demonstrate the effectiveness and potentialities of the proposed solution.