## 3 Experiments and Results
### Experimental Results
#### 3.2.2 Results of Cooling Batteries Onoard Drones with/without a Fuel Stick  
This subsection provides the results that compare the temperatures of uncooled batteries and ones that are cooled by an onboard fuel cell stack and a fuel stick. Figure 10 shows the results of a drone flight (Flight test 1) carried out with the drone powered entirely by a battery. The 3000 mAh battery used provided a flight duration of around 6 min, on average. It is to be noted that on all days during which flight tests were conducted (with or without the fuel cell stack), there were 4-6 mph winds. The left half of Figure 10 shows the initial temperature of the battery pack. During temperature measurements, the battery was laid on a piece of paper, so the grass below did not affect its temperature. As seen in the left half of Figure 10, the battery surface was initially at a temperature of around 30.11 \({}^{\circ}\)C. The right half of Figure 10 shows the battery laid on a piece of paper immediately after the drone landed. The surface temperature observed at the center at this point was 52.63 \({}^{\circ}\)C, with the highest surface temperature being around 53 \({}^{\circ}\)C.  
Figure 11 shows the results of a drone flight carried out with the drone powered by a battery and a fuel cell stack supercapacitor unit, as shown in Figure 3 (flight test two). The drone was attached to the fuel cell holder containing the fuel cell components and the aluminium jacket with the fuel cell stick and the battery, as shown in Figure 7. The left half of Figure 11 shows the initial temperature of the battery pack. The battery surface was initially at a temperature of around 30.33 \({}^{\circ}\)C. The right half of Figure 11 shows the battery laid on a piece of paper immediately after the drone landed. The surface temperature observed at the center at this instant was 48.27 \({}^{\circ}\)C, with the highest surface temperature being around 48 \({}^{\circ}\)C. This shows that just by using the fuel cell stick with the aluminium jacket for cooling, the battery temperature reduces by around 5 \({}^{\circ}\)C.  
Also, Figure 12 presents details related to the two flight tests described above. It must be noted that both missions which produced the results in Figures 10 and 11, were altitude hold missions, with the drone required to hold altitude at 15 m. From the different curves shown in Figure 12, it can be concluded that with the fuel cell handling all the power requirements for the flight controller, telemetry and GPS, and the battery supplying only the motors, there was a gain of around half a minute of flight time compared to the case where a single battery was used to power all electronics and the motors. Several of the previously described tests were carried out with batteries of different capacities from different manufacturers to verify the observations reported above. Table 4 shows the results of these tests. Every odd numbered test in Table 4 represents a test where the drone was powered solely by the battery. Every even numbered test in Table 4 represents a test where the drone was  
Figure 10: (**a**) Temperature of battery before flight; (**b**) temperature of battery after flight. Flight test 1—increase in battery temperature without any cooling, captured by thermal imaging device 1.  
powered by a battery, fuel cell stack, supercapacitor, and the battery onboard the drone was cooled using the onboard metal hydride fuel stick and the prototype cooling assembly developed in this work. Tests 1, 2 were performed with a 3000 mAh battery. Tests 3 to 6 were performed with a 2700 mAh battery and tests 7, 8 were performed with a 2500 mAh battery. The fourth column in Table 4 shows the difference between the temperature of a battery after no onboard cooling, and the battery temperature measured after a flight with fuel stick based onboard cooling. The average temperature decrease across all tests (1 to 8) was 5.19 \({}^{\circ}\)C with a standard deviation of 1.712.