Power supply

A power supply is formed from a number of fuel cell elements (21, 22, 23; 31; 41) connected to a switch (SW1) to enable the number of elements operational in a fuel cell stack to be varied in order to sustain output voltage despite variations in the output load current. Typically, the output current is monitored to allow incremental adjustment by switching combinations of the fuel cell elements. The fuel cell elements themselves may be single fuel cells or groups of fuel cells connected in series or in parallel.