# Selected Aspects of Designing Modular PEMFC Stacks as Power Sources for Unmanned Aerial Vehicles
## 3 Results
### Electrical Tests of the 500-W PEMFC Stack  
The second type of PEMFC stack mentioned was a three-module 500-W power source designed as a power supply for on-board electronic and executive devices (servomechanisms, motors, electromagnets). These devices, which also require an uninterrupted supply of electricity during their operation, are characterized by a variable profile of power demand throughout operation. Figure 18 presents the U-I and P-I curves recorded for the three-module 500-W PEMFC stack. As can be seen, the designed PEMFC operated within an approximate variable voltage range of 21-33 V, and within a current range of 0-25 A. The nominal power output P\({}_{\text{max}}\) of the PEMFC stack was recorded for a U-I pair (-21 V; -25 A), and corresponded to maximum power P of \(\sim\)515 W. No characteristic maximum point of power was observed in the P-I curve. The assembled PEMFC stack was also capable of supplying excess power for a short time. These results confirmed that the 500-W modular PEMFC stack could be tested in an electrical connection with a hybrid electrochemical power source (a 500-W three-module PEMFC stack with a parallel connection to a 2250-mAh 5 s lithium-polymer battery). The electrical behavior of the hybrid power source 5 s lithium polymer battery plus 500 W PEMFC stack was also studied. The combined effect of supporting the hydrogen-oxygen fuel cells from 5 s lithium-polymer battery during higher power demand is shown in Figure 19.  
Figure 17: **(a)** The U-I; P-I and F\({}_{\text{H}}\)-I curves recorded for the investigated 2-kW PEMFC stack integrated in propulsion unit (according to a scheme from Figure 4). **(b)** Dependence of the rotational speed of a 30 \(\times\) 12 propeller connected to a BLDC motor vs. power. In the case of a slow increase in electrical load within the current range I = 0-22 A, the power was mainly supplied from the modular PEMFC stack. When the current (I) was increased above 22 A, power was also supplied from the electrochemical battery power source. The total power of the hybrid power sources recorded in these conditions was 710 W; in the case of the PEMFC stack alone, 490 W. The increase in the power of the hybrid power source was confirmed by this study. Figure 19: Power (P)-current (I) distribution recorded for a hybrid power source consisting of a three-module PEMFC stack (FC) and 5s lithium-polymer battery (5s B). Figure 18: Voltage (U)-current (I) curve, or power P vs. current I recorded for the three-module PEMFC stack. In Figure 20 the current vs. time waveforms recorded during constant current mode operation of a three-module 500-W stack of fuel cells integrated with a lithium-polymer battery (5s B) is presented. The system was loaded in the constant current mode within a range of 0-15 A. Approximately 50 s after starting the PEMFC stack, the power source was loaded with a constant current of 8 A. At that time, a gradual increase in the current drawn from the 500-W PEMFC stack and a simultaneous decrease in the current drawn from the electrochemical battery was observed. In the case of the battery, the current stabilized below 0 A (\(-\)2 A), signifying recharging of the battery. The slow increase in the current produced from the PEMFC stack resulted from the need to warm the stack up to the set operating temperature and from the gradual increase in electrolyte hydration. The stepwise increase in the current electronic load to 12 (time from 250 to 300 s) or 15 A (time from 700 to 800 s) resulted in a similar response on the part of the cell's current; however, in these cases, the current flowing from the battery stabilized above 0 A and the current taken from the battery reached \(-\)2 A (time from 250 to 300 s) and \(-\)4 A (time from 700 to 800 s), which was associated with gradual discharge of the battery.