## 5 Results
### Power Balance and Autonomy Evaluation  
#### 5.1.1 Power Tests Under Different Profiles  
To assess the power tests, the hydrogen-battery hybrid propulsion system was tested indoors by connecting it to a programmable electronic load with four variable demand profiles, representing the four driving modes linked to four working scenarios. The tests were conducted at 20 \({}^{\circ}\)C, 60% RH, and 1 atm.  
Profile 1, in Figure 15a, represents a continuous duty scenario with little variation in motor speed/torque. Profile 2, in Figure 15b, represents a drastic working scenario with  
Figure 14: Connection diagram between the FS-iA6 receiver module, Arduino Mega 2560 Pro microcontroller, and BLDC motor controller.  
severe changes in wheel speed/torque on a very frequent basis. Profile 3, in Figure 15c, represents a typical outdoor in-field working scenario with power peak demand. Profile 4, in Figure 15d, represents a stationary demand profile.  
Figure 15: Cont.  
As shown in Figure 15, the passive control-based power balance is capable of operating under the four profiles without exceeding the established safety limits. For Profile 1, Figure 15a shows that with a smooth power profile, the bus voltage remains stable, except for some voltage peaks caused by the internal operation of the PEM fuel cell. For Profile 2, Figure 15b shows that even with an extreme profile, the power system is still able to maintain the DC bus voltage value at \(\pm\)1.5 V around 36 VDC, which is within the set limits. With respect to Profile 3, Figure 15c, the platform has been tested with two different initial voltages (34.5 V and 36 V). It can be observed that the DC bus remains within the safe values; it even tends to stabilize at 37 V, verifying the effect of the passive control. Finally,  
Figure 15: Power test for the load profiles. **(a)** Profile 1. **(b)** Profile 2. **(c)** Profile 3. **(d)** Profile 4. **(left)**: Hygrobot power profile and DC bus voltage in single intervals of 0–240 s. **(right)**: Hygrobot power profile and DC bus voltage, built from multiple repetitions of a single interval.  
for Profile 4, Figure 15d shows that when the demand is zero, the DC bus voltage increases because the PEM fuel cell is charging the battery subsystem until it reaches 38 V. Then, the power balance system activates the overload protocol, disconnecting the PEM fuel cell from the DC bus.  
For more details, Figure 16 shows the power-sharing between the PEM fuel cell and the battery subsystems for Profile 3. The PEM fuel cell subsystem is intended to operate continuously, supplying energy to the battery subsystem and meeting Hygrobot energy demand. The authors define the sign power as positive (\(p>0\)) when power is injected into the DC bus and as negative (\(p<0\)) when power is drawn from the DC bus.  
Based on the passive power balance (with the PEM fuel cell and battery subsystem directly connected to the DC bus, as in Figure 6), the power delivered by the PEM fuel cell remains at \(P_{FC}=170\) W. This corresponds with the available PEM fuel cell power at nominal DC bus voltage, \(V_{FC}~{}=~{}V_{buf}~{}=~{}V_{bus}=36\) V, as in Figures 6 and 8.  
When the battery subsystem is discharging, the DC bus voltage drops, increasing the power provided by the PEM fuel cell and recharging the battery subsystem. Conversely, when the battery subsystem voltage is above 36 V, the DC bus voltage rises and the power supplied by the PEM fuel cell decreases, provoking battery subsystem discharge.  
The difference between Hygrobot demand and PEM fuel cell generation is supplied by the battery subsystem, which is available for charging and discharging according to the net power balance between the PEM fuel cell supply and Hygrobot demand.