# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 2 Unit Testing
### Unit Test Results  
The constructed HPS prototype was bench-tested under static and dynamic loads. The static test was aimed at discovering the maximum output power that the fuel cell can sustain for 24 h. This goal time was set in consideration of the main development objective of the security robot, that is, to achieve 24 h continuous operation on a single charge. For prolonged and stable operation, the voltage, current, and temperature of the fuel cell must be maintained within the safe operating area (SOA). A typical SOA is defined mainly in terms of the under-voltage (UV), over-current (OC), and over-temperature (OT) conditions. In the selected fuel cell, an UV fault occurs if the average voltage per cell is less than 0.5 V, thus, 24 V per stack. An OC fault occurs if the current exceeds 30 A. An OT fault occurs if the internal temperature exceeds 65 \({}^{\circ}\)C or if the ambient temperature is higher than 45 \({}^{\circ}\)C. If one of the above fault conditions is met, the operating point of the fuel cell falls outside of the SOA, the controller will then immediately disconnect the load and sound an alarm. The alarm can be interpreted by counting the number of bleeps or by reading the text in the status display window incorporated into the controller. The \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Property** & **Function** & **Model** & **Installation Position in Power** \\ \hline \multirow{4}{*}{Power} & DC electronic load & Kikusui, PLZ1004W & \\  & DC power supply & Kikusui, PWR800L & Low-side \\  & Charge-discharge system controller & Kikusui, PEX2512 & \\ \hline \multirow{4}{*}{Signal} & Controller & NI, cRIO-9035 & \\  & Voltage & NI, 9229 & High-side \\  & measurement & Current & LEM, DHAB S/145 & High-side \\  & measurement & NI, 9239 & Low-side (2 ea) \\  & H\({}_{2}\) pressure & Keller, PA-23 & \\  & measurement & NI, 9239 & \\  & Air temperature & NI, 9212 & Inlet \\  & measurement & NI, 9212 & Outlet \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Apparatus in unit test bench. controller will attempt to reconnect the load every ten seconds. Once the fault condition that caused the initial disconnection has passed, and the operating point of the fuel cell consequently returns to the SOA, the load can be connected again. Using the electronic load, the load was increased from 500 W to 900 W in steps of 100 W. We observed that the power supply from the fuel cell increases with increasing temperature and definitely with lowering efficiency (see Figure 1a). A part of the fuel cell power is consumed by its auxiliaries, mostly, the fuel cell blower and controller. Therefore, the fuel cell must supply approximately 150 W more power than the given load. For instance, the fuel cell needs to deliver nearly 1050 W to meet the maximum load demand of 900 W (see Figure 1b). To maintain a constant output power, the heat generated by the fuel cell should be counterbalanced by the heat dissipated through its thermal management system. At the output power of 1050 W, which slightly exceeded the nominal power of the fuel cell, the fuel cell appeared to reach thermal equilibrium at 56 \({}^{\circ}\)C. However, the fuel cell abruptly shut down after only 2.24 h with an OT alarm and could not recover until the load demand was reduced (see Figure 1c). The operating points of the fuel cell at the moment it stopped were measured to be 25.1 V and 35.8 A by our data acquisition system and measured to be 23.9 V, 38.7 A, and 57 \({}^{\circ}\)C by the fuel cell controller. The temperature within the fuel cell, which triggered an OT alarm, could only be monitored internally. Therefore, according to the user's manual, either an UV (23.9 V < 24 V) or OC (38.7 A >> 30 A) alarm should have gone off, whichever alarm condition occurred first. However, in reality, the fuel cell went down with an UT (57 \({}^{\circ}\)C << 65 \({}^{\circ}\)C) alarm, despite the temperature margin of 8 \({}^{\circ}\)C. Such discrepancies between the fault conditions were often observed throughout the test.