## 3 Results
### Sensor Node Operation with an Energy Harvesting Scheme as a Power Supply and a Battery as a Backup
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Configuration**} & \multirow{2}{*}{
\begin{tabular}{c} **Broadcast** \\ **Interval** \\ **[s]** \\ \end{tabular} } & \multicolumn{2}{c}{**Battery Readings**} & \multirow{2}{*}{**Configuration**} & \multicolumn{2}{c}{**Battery Readings**} \\ \cline{3-4}  & **1** [**\(\mu\)A**] & & & **I [\(\mu\)A]** & & **AV [mV]** \\ \hline \multirow{3}{*}{**TI-CC2650 + Internal Timer**} & 5 & \(-\)88.545 & \(-\)4.54 & \multirow{3}{*}{**TI-CC2650 + Internal Timer**} & 54.464 & 7.59 \\  & 15 & 152.693 & 12.52 & & 280.695 & 23.03 \\ \cline{1-1}  & 30 & 205.009 & 20.98 & & 343.178 & 28.26 \\ \cline{1-1}  & 60 & 216.362 & 20.789 & & 352.485 & 29.683 \\ \cline{1-1}  & 600 & 292.24 & 23.613 & & 236.76 & 15.783 \\ \hline \multirow{3}{*}{**TI-CC2650 + TPLS110 Timer**} & 5 & \(-\)236.792 & \(-\)20.8 & \multirow{3}{*}{**TI-CC2650 + TPLS110 Timer**} & \(-\)164.394 & \(-\)18.08 \\ \cline{1-1}  & 15 & 90.608 & 6.52 & & 154.443 & 12.43 \\ \cline{1-1}  & 30 & 180.759 & 13.92 & & 260.329 & 21.12 \\ \cline{1-1}  & 60 & 201.763 & 15.753 & & 281.232 & 22.206 \\ \cline{1-1}  & 600 & 273.542 & 23.98 & & 311.848 & 27.82 \\ \hline \multirow{3}{*}{**ATmega328p + TPLS110 Timer**} & 5 & \(-\)125 & \(-\)7.85 & \multirow{3}{*}{**ATmega328p + TPLS110 Timer**} & 35.421 & \(-\)4.307 \\ \cline{1-1}  & 15 & 134.347 & 13.934 & & 237.984 & 24.69 \\ \cline{1-1}  & 30 & 185.271 & 16.045 & & 288.483 & 22.36 \\ \cline{1-1}  & 60 & 224.398 & 20.82 & & 340.553 & 29.53 \\ \cline{1-1}  & 600 & 346.33 & 35.962 & & 354.36 & 36.703 \\ \hline \multirow{3}{*}{**Adafruit Feather + TPL5110**} & 5 & \(-\)595.385 & \(-\)95.428 & \multirow{3}{*}{**Adafruit Feather + TPL5110**} & \(-\)1147.73 & \(-\)176.6 \\ \cline{1-1}  & 15 & \(-\)91.645 & \(-\)8.488 & & **Timer** & \(-\)162.34 & \(-\)15.04 \\ \cline{1-1}  & 30 & 64.184 & 7.047 & Vibration mode: Mode II & 99.94 & 9.82 \\ \cline{1-1}  & 60 & 152.909 & 11.351 & Frequency: 77.78 Hz & 232.06 & 16.1 \\ \cline{1-1}  & Acceleration: 0.5 g & 600 & 400.08 & 84.002 & Acceleration: 0.5 g & 440.46 & 97.258 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Detailed overview showing the performances of the sensor node, which uses different MCU platforms at different broadcast frequencies using the reference energy harvester’s power generated at each mode, backed up with a Lithium-Ion battery for the worst-case scenarios. Figure 9: Battery’s used current level (**a**) and its equivalent voltage difference (**b**) experimentally measured at different broadcast time intervals while operating the different sensor nodes using the energy harvester (outer and inner patches) as a main power supply and a battery as a backup for the worst-case scenarios. As described in Table 4, when using a short broadcast time interval (5 s for the TI-CC2650 Launchpad and ATmega328p MCUs, and 15 s for the Adafruit Feather), negative battery current values are observed. This indicates that the backup battery is discharging. Over time, this continuous discharge will deplete the battery, eventually resulting in a system failure. Figure 11 provides a visual representation of the EH-ON signal recorded in such cases. In this scenario, it is apparent that the system struggles to recover and switch back to the energy harvesting source. Consequently, the EH-ON signal continuously flickers around the Low value. In such cases, the harvester serves as a battery life extender, meaning that it prolongs the time it takes for the battery to fully discharge, delaying the need for replacement. To empirically determine the battery life extension coefficient, we propose conducting a specific test. First, we operate the system for 30 min at a 5 s broadcast time interval, exclusively relying on the battery (a \"battery-only\" mode) while excluding the energy harvesting source. Subsequently, we switch the system to a \"battery + energy harvester\" mode.