# Hybrid Power System for the Range Extension of Security Robots: Prototyping Phase
## 2 Unit Testing
### Unit Test Results
Nonetheless, we found that the fuel cell could continuously meet the load demand up to 700 W, which required the power supply to be as high as 850 W. This safety margin is needed because of the uncertain conditions under which the fuel cell will be run, for example, when its performance decays over time or when the ambient conditions are hot and dry. The dynamic test was aimed at identifying the transient behavior of the entire system and its components. Two situations that are likely to occur in the HPS are sudden changes in the power supply or the load demand. With regard to the first situation, variations in the power supply were produced by a short-circuit unit (SCU) that is exclusive for the selected fuel cell. As its name suggests, the SCU short-circuits the fuel cell for 100 ms every 10 s to condition the fuel cell and thus enhancing its durability. Although the SCU might be beneficial to the fuel cell, it can be damaging to the entire system because it causes the fuel cell voltage to drop every 10 s. Such concerns proved to be unnecessary because we found that despite variations in the fuel cell voltage (high-side), the DC bus voltage (low-side) was almost unaffected (see Figure 2a). This was mostly because the battery in the HPS readily made up for the power loss from the fuel cell when the SCU was activated. In addition to the regular short-circuiting by the SCU, irregular voltage drops in the fuel cell were observed because of the infrequent measurements at intervals of 1 s. With regard to the second situation, sudden changes in the power demand may occur more often for security robots operating in unstructured environments. Because of the limited knowledge on the actual operating conditions of the security robot, its operation mode was defined using a modified version of a well-known drive cycle, specifically, a charge-depleting cycle life test profile for an electric vehicle battery [23]. A drive cycle in the form of power versus time, which is usually called a power profile, is more appropriate for unit tests based on an electronic load than a drive cycle in the form of speed with respect to time. The modified drive cycle lasts for a total of 12 min, with the security robot surveilling and patrolling at the same time for the first half of the cycle, and only surveilling for the second half. During the first six minutes of simultaneous surveilling and patrolling, we observed a clear difference in the dynamic characteristics between the two power sources, which is in agreement with previous simulation results (see Figure 2b). As the load increased, the battery started supplying power to the load before the fuel cell to ensure the responsiveness of the entire system. As the load fell, the fuel cell supplied the remaining power to the battery and promptly recovered its state-of-charge (SOC). Figure 1: Results of unit tests performed to determine the maximum output power that the fuel cell can sustain for 24 h. (**a**) Temperature and efficiency versus power, (**b**) Voltage, and (**c**) Temperature versus time. The voltage drops observed in the fuel cell are caused by short-circuiting with the SCU. The voltage drops occurred regularly but appear irregular due to the infrequent measurements at intervals of 1 s.