*5.1. Laboratory Measurements*

To test the fabricated design, laboratory measurements were carried out using a digital oscilloscope (RIGOL DS1052E), as depicted in Figures 9–11. Voltage dividers with a total resistance of 82 MΩ were utilized for the thresholds of the comparators, according to Equation (2). Additionally, the AVX supercapacitors mentioned in Section 4 were used. A voltage source unit provides 2.4 V to the storage scheme input.

Figure 9 demonstrates the supercapacitors charging mode. Initially, SCsmall begins to charge and only when it is fully charged at the voltage of 2.3 V determined from the thresholds of the comparators, SCbig is connected at the input. As shown, the early-state charging of SCbig stops at about 50 mV, lower than the simulation result, and the overall charging duration is longer. These phenomena are due to the charging path resistance (i.e., chip pins and breadboard PCB resistance), which reduces the charging current. The peak current drawn is 2.1 A in the simulation, while its experimentally measured value is 0.45 A.

**Figure 9.** Oscilloscope view of SCsmall (CH1) and SCbig (CH2) voltage signals during charging and startup (inset), following the predefined charging priority feature. The SCbig supercapacitor is connected to the input only when SCsmall is fully charged at 2.3 V.

**Figure 10.** Oscilloscope view of (**a**) SCsmall voltage (CH1) and Vout1 (CH2) load output. (**b**) SCbig voltage (CH1) and Vout2 (CH2) load output. The output loads are disconnected in low-input energy conditions to preserve the operation of the system.

**Figure 11.** (**a**) SCsmall voltage (CH1), SCbig voltage (CH2) and bleeder output (CH3) during bleeder mode activation. (**b**) SCsmall voltage (CH1), SCbig voltage (CH2) and battery input (CH3) during battery energy support.

The load's connection modes are depicted in Figure 10a,b. A 100 Ω resistor is connected to Vout1 and a 22 Ω resistor to Vout2, similarly to the simulation setup. The loads draw energy from the supercapacitors, as long as the voltage levels do not fall below the predetermined minimum thresholds. Figure 11a presents the bleeder activation, drawing the excess energy. Figure 11b curves are obtained without input power supply. The supercapacitors are discharged, and two 1.2 V Ni-MH AAA batteries, connected in series (measured at 2.5 V), provide power to the system.

The measured current consumption of the integrated control unit is 376 nA at 2.3 V. Combined with the two 82 MΩ voltage dividers consumption, the total current consumption of the proposed unit is 432 nA at 2.3 V.

In Table 1, this work is compared with other state-of-the-art implementations. As shown, the proposed design offers many advantages such as low-complexity, small size integrated solution, ultra-low-power consumption and wide voltage and current range. Thus, this design is suitable for a wide variety of autonomous energy harvesting applications.


**Table 1.** State-of-the-art storage units implementations.

<sup>1</sup> Actual value not available.
