## 3 Results
### Autonomous Operation of Sensor Nodes with PVEH  
Building on our previous results, which demonstrated the feasibility of operating our system with the energy harvester alongside a backup battery for challenging scenarios, we were inspired to explore the potential for fully autonomous operation. To achieve this, we suggest removing the Lithium-Ion battery and substituting it with a supercapacitor. This supercapacitor will be electrically connected in parallel to the system and will play a crucial role in supporting the high switching impulses and radio transmission peaks in our BLE-based node circuits. The power management board already includes a 20 mF supercapacitor connected to the output. However, this capacity has proven insufficient to power the node systems effectively. As a solution, we recommend adding an additional commercially available 470 mF supercapacitor, which provides a nominal voltage of 5.5 V (the same as the capacitor used in [38]). Experimental tests have shown that this chosen capacitance represents the minimum threshold required to reliably operate the system at \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & \multicolumn{6}{c}{**Operation Mode**} & \multirow{2}{*}{**Results**} \\ \cline{3-3} \cline{5-6}  & & \multicolumn{2}{c}{**Battery + Harvester**} & \multicolumn{2}{c}{**Battery-Only**} \\ \hline
**MCU Platform** & **Broad. Interval [s]** & **I [\(\mathbf{\mu}\)A]** & \(\mathbf{\Delta V}\) [mV]** & **I [\(\mathbf{\mu}\)A]** & \(\mathbf{\Delta V}\) [mV]** & **r** \\ \hline TI-CC2650 + TPL5110 Timer & 5 s & \(-164.394\) & \(-18.08\) & \(-457.711\) & \(-0.37\) & \(2.78\) \\ ATmega328p + TPL5110 Timer & 5 s & \(-35.42\) & \(-4.307\) & \(-340.422\) & \(-25.68\) & \(9.61\) \\ Adafruit Feather + TPL5110 Timer & 15 s & \(-162.34\) & \(-15.04\) & \(-455.73\) & \(-3.86\) & \(2.81\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Experimental results of the expected battery life extension when operating the sensor node using the energy harvester and the battery as a backup plan in the worst-case scenarios. various broadcast frequencies. Any capacitance lower than this value results in a failure to power the sensor node adequately. We conducted experimental tests under identical conditions as those for the battery, with a duration of 10 min. The investigation encompassed the same broadcast time intervals, as illustrated in Figure 12. Building upon insights gained from previous investigations, we determined the maximum broadcast frequency for each sensor node, serving as a performance threshold. Consequently, this threshold served as the starting point for our current investigations. Our primary objective was to ascertain the minimum broadcast time interval (equivalent to the highest broadcast frequency) required for autonomous operation of the sensor nodes. To accomplish this, we leveraged the characteristics of the supercapacitor and introduced Equation 6 for our analysis:  
\[dI=C\times\frac{dV}{dt}, \tag{6}\]  
where \(dI\) is the current variation flowing into the supercapacitor, \(C\) is the capacitance, \(dV\) is the supercapacitor variation, and finally \(dt\) is time in seconds. In an ideal scenario, the current consumption of a sensor node should be at 0 mA. However, in practical situations, there is always a minimal current flowing through the capacitor. Throughout our tests, we observed a consistently stable voltage across the capacitor/load. You can find a summary of the experimental test data in Table 6.  
These results reaffirm our previous findings, indicating that the most power-efficient sensor nodes are those incorporating the ATMEGA328P and TI-CC2650 MCUs. We observed that the ATMEGA328P MCU requires a voltage supply higher than 2.7 V for reliable operation, so the corresponding tests were conducted at a 3 V voltage level.