## 2 System Overview
### Electrical System Schematic with Fuel Cell Stack and Supercapacitor  
The above section provided details about the mode of operation when a battery is used as the sole power source. However, this can cause certain issues, e.g., if there are very violent gusts of wind which require an enormous amount of current from the motors, the battery voltage may momentarily drop, which may cause the flight computer to suffer interruptions, or, in severe cases, restart. Further, because the flight computer, GPS, telemetry and other electronics feed off the same power source as the motors, the flight time of the drone is potentially reduced. So, as shown in Figure 3, a battery is only used to power the motors, and a fuel cell stack is used to power the APM and all electronics connected to the APM. The fuel cell stack used is a Horizon (H-30) proton exchange membrane (PEM) fuel cell stack, rated at 30 W, 8.4 V, and 3.6 A. The fuel cell stack is powered by a metal hydride fuel stick. Other details are available from the manufacturer's datasheets for the fuel cell stack, and details about the metal hydride fuel stick are available in Tables 1 and 2. It must be noted that, as these are commercially sold products, all details may not be available because such information may be proprietary. It is also  
Figure 2: Electrical system schematic without fuel cell stack and supercapacitor. worth noting that although Table 2 shows the fuel stick weight to be around 105 g, our measurements of both empty, and fully charged metal hydride fuel sticks showed that their weight is around 92 g. The entire stack consists of 14 individual cells. As shown in Figure 3, the fuel cell stack is connected to a battery eliminator circuit (BEC), which includes a buck converter module, so that the 8.4 V supply from the fuel cell is converted to the 5 V level required by the APM. A supercapacitor rated at 5 V, 5 F is connected in parallel to the output of the BEC. This 5V, 5F supercapacitor module is obtained by connecting two 2.5 V, 10 F supercapacitors in series. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Description** \\ \hline Fuel Cell Type & Proton Exchange Membrane (PEM) \\ No. of cells & 14 \\ Rated output power & 30 W \\ Typical voltage and current output & 8.4 V, 3.6 A \\ Purge valve voltage & 6 V \\ Controller voltage & 5 V \\ Reactants & Hydrogen and air \\ Maximum stack temperature & 55 \({}^{\circ}\)C \\ Hydrogen pressure & 0.45–0.55 bar \\ Humidification & Self-humidified \\ Cooling & Air (via integrated cooling fan) \\ Dimensions & 8 cm \(\times\) 4.7 cm \(\times\) 7.5 cm \\ Flow rate at maximum output & 0.42 L/min \\ Start-up time & \(\leq\)30 s \\ Stack efficiency & 40\% at rated power \\ Stack weight & 280 g (\(\pm\)30 g) \\ Controller weight & 90 g (\(\pm\)10 g) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: H-30 PEM fuel cell specifications. Figure 3: Electrical system schematic with fuel cell stack and supercapacitor. The supercapacitor module is used for the following reasons: (i) The fuel cell operates by using hydrogen from a fuel stick, and oxygen from the air. In the process of generation of electricity, there is some water which routinely collects within the fuel cell unit. This collection of water is detrimental to the operation of the fuel cell, and is periodically removed by a purge controller. The purge cycle lasts for a very short duration (of the order of milliseconds), but during this time period, the fuel cell output voltage drops sharply to a very low value. Such voltage interruptions affect the output of the BEC, as shown by the solid blue curve (thick line with diamond shaped markers) in Figure 4, and thus, interrupt the operation or cause a reboot of the flight controller. So, a filtering capacitor (in this case a supercapacitor) is placed in parallel across the BEC output terminals; the result of this is obvious from Figure 4, where the smoother red curve (thin line with circular markers) does not show any drop in voltage.