Control Boxes  Back The control unit is fitted with a pressure regulator, which maintains a set pressure inside the stack. In addition, it has two functions:  one function is to drive the cooling fans for the convection stacks; the second function is to regulate the purging cycle for hydrogen in the dead-ended mode of operation of the fuel cell. The dead-end time can be varied from 4 sec to 40 sec.  The control units can be used for both convection and forced-flow stacks. A convection fuel cell stack when used with a control unit becomes a fuel cell system capable of independent operation at high energy efficiency. Batteries are not required for the startup and continuation of the fuel cell operation. Additional Information