 Fuel cells are classified by the type of electrolyte they use and by the difference in startup time, which ranges from 1 second for proton exchange membrane fuel cells (PEM fuel cells, or PEMFC) to 10 minutes for solid oxide fuel cells (SOFC).A related technology is flow batteries, in which the fuel can be regenerated by recharging.Individual fuel cells produce relatively small electrical potentials, about 0.7 volts, so cells are "stacked", or placed in series, to create sufficient voltage to meet an application's requirements.