The fuel cell together with a finite amount of fuel and oxidant is conceptually a battery. The stationary fuel cell in many applications is anticipated to be a 40, 000 hour primary battery. Fuel cells are used for longer term operations or missions, the common battery for shorter term. Because of the short battery life, regenerablility is an important and desirable battery feature. Like a battery, some fuel cells are reversible or regenerable or rechargeable (electrolysis), but the development of regenerable fuel cells is not as imperative. While the perfect fuel cell would undergo no degradation, practical fuel cells, like batteries, will degrade. During battery discharge in a single cycle, degradation cannot be readily separated from power output. In the battery degradation can be seen definitively, macroscopically cycle-to-cycle. In the fuel cell, perhaps unlike a battery, the mechanism of degradation may not depend on current density of operation. The life, and hence capacity, of a degrading fuel cell depends on how the fuel cell is operated.