Abstract: Fuel cell systems that provide backup, or auxiliary, power to one or more energy-consuming devices normally powered by a primary power source (PPS). Operating methods and controllers for auxiliary fuel cell systems and/or for power systems that include a PPS and an auxiliary power source in the form of an auxiliary fuel cell system (AFCS) are also disclosed. In some embodiments, the fuel cell system includes, or is in communication with, a controller that selectively initiates the production of an electric current by the AFCS responsive to a triggering event. An illustrative triggering event includes a predetermined voltage drop across a diode or similar current-regulating, or flow-regulating, device that is electrically between the AFCS and the energy-consuming device(s) and/or the PPS. Another illustrative triggering event includes the voltage (or state of charge or readiness to satisfy an applied load) of a battery or other energy-storage device associated with the AFCS.