A hydrogen powered, auxiliary power unit (APU) was integrated into a late model commercial pick-up truck. The alternator and lead acid battery was removed and a fuel cell and a “Super Capacitor” combination installed to provide electrical power for combustion engine starting, ignition maintenance as well as powering the vehicle accessories and corresponding electrical loads. The fuel cell requires a small amount of chemical based battery charge at start-up and again during the shutdown cycle.

This paper discusses the electrical analysis of the electrical loading and peak energy requirements, sizing of the fuel cell and capacitor. Further, to optimize and minimize the small battery this unit is sized and an innovative bootstrapping battery charging scheme presented. This scheme uses the residual voltage left o the fuel cell stack at the termination of a user cycle.

At the termination of a user cycle the fuel cell stack (in our system) reaches an open circuit voltage of > 42 volts. The typical output voltage during ˝ load is 25 Volts DC and 29 volts at “idle”. A charger using a switching topology and a voltage detector controls the charging of the small cycle battery. By using the residual energy stored on the fuel cell stack as the primary charging source, overall system efficiency may be obtained.