# Selected Aspects of Designing Modular PEMFC Stacks as Power Sources for Unmanned Aerial Vehicles
## 3 Results
### Electrical Tests of the 500-W PEMFC Stack
Hybrid power sources consisting of simultaneous (parallel) mutually independent PEMFC stacks and electrochemical batteries provide so-called double power, which ensures the redundancy of the power sources such that in the event of the failure of one, the other can still supply power to a limited extent, i.e., the second functional module can still operate and deliver the power at its disposal. Operation of this kind is of particular importance in aviation applications for such critical systems as avionics and the control and powering of servo systems governing control functions (the airframe remains under control and can be steered, which will enable it, for example, to make an emergency landing in glider mode  
Figure 20: Current vs. time waveforms recorded during the operation of a three-module 500-W stack of fuel cells integrated with an electrochemical battery (5s B) under current load within the range 0-15 A, where: green line—load—current taken from hybrid system (500-W fuel cell stack + LiPo 5s battery pack) by electronic load; red line—battery—current taken from LiPo 5s battery pack; black line—current taken from 500-W fuel cell stack (FC stack). after losing its propeller function). Low-temperature PEMFC stacks consist of many cells connected in series. The operation of a power source containing a PEMFC stack requires the simultaneous correct operation of all components in the stack. In extreme cases, damage to even a single cell in the PEMFC stack may lead to its complete shutdown. The reliability of such a power source can be enhanced by using independently operating smaller stacks (modules) of fuel cells. Figure 21 presents the variation in distribution of current flowing from individual modules connected in parallel under various electrical loads. This may occur in cases where some MEA cells in PEMFC modules are characterized, for various reasons, by lower values of electrical properties. In cases of nonuniform temperature distributions, MEA cells with modules with hot-spot operations are expected to exhibit poorer performance due to the potential presence of some pinholes which may occur under operation in such conditions. These defects may reduce the voltage of membranes, which may also lead to changes in the directions of polarization, or to electrical short circuits in the PEMFC stack. These unfavorable phenomena may, in a single-module PEMFC stack, lead to devices integrated within the energy systems (e.g., DC/DC converters) being turned off. Nonuniform temperature distributions also lead to the presence of various stresses. Under nonuniform stress the risk of cracks and pinholes in fuel cell components becomes relatively high. The flooding phenomenon may also result in unstable PEMFC stack performance. One possible reason for this was a suboptimal humidification process, in which more water was produced than expected. In such situations, poor performance can be observed, especially during an increase in current density [47]. In the case of PEMFC stack application, the reliability and monitoring of these parameters are very important for propulsion structures as well as for as the power source supplying BOP in the drones. From the analysis of the data presented in Figure 21, it can be seen that the intensity of the current drawn from the two modules (1, 2) is comparable and, at the same time, greater than the intensity of the current drawn from module (3). This variation in behavior  
Figure 21: Variation in the distribution of current flowing from individual modules (M1; M2; M3) connected in parallel in 500 W PEMFC stack under various electrical loads. is probably connected with the lower values of some MEA cells and differences in their humidity. In order to circumvent these problems, in the segmented power source, the electrical power generated by different sections in the modular fuel cell stack can be independently controlled in hybrid power sources.