In a fuel cell the power supply is uninterrupted while it is fed with the fuel and the oxidant, different from a battery in which the energy storage is limited by the amount of reagent stored in it. Insofar the theoretical thermodynamic efficiency of fuel cells are close to 90% in the thermal engines (Carnot), this efficiency is of only 40% under typical operating conditions for both systems. In practice, the efficiency of a fuel cell is usually less than 60% [112].