**4. Simulation Results**

In order to verify the operation of the proposed storage unit, simulations were carried out via the Cadence Virtuoso suite. The input voltage of the storage unit was set to 2.4 V, emulating the output voltage of a harvesting circuit, i.e., a step-up DC-DC converter. The charging and discharging voltage thresholds of the supercapacitors were set as follows:

$$\begin{array}{ll} V\_{\text{max1}} = 2.3 \text{ V} & V\_{\text{min1}1} = 2.25 \text{ V} & V\_{low1} = 2.15 \text{ V} \\ V\_{\text{max2}} = 2.3 \text{ V} & V\_{\text{min2}} = 1.8 \text{ V} & V\_{low2} = 1.6 \text{ V} \end{array} \tag{2}$$

The voltage levels in (2) were implemented with voltage dividers, each with a total resistance of 82 MΩ. An AVX BZ015A104Z\_B Bestcap supercapacitor of 100 mF was selected as SCsmall and an SCMT22C505PRBA0 supercapacitor of 5 F as SCbig. To extract realistic results, ESR and leakage resistors were added to the schematic models, according to the supercapacitors datasheets. Figure 5 shows the charging mode when both supercapacitors are fully empty. At startup, a 350 mV early-state charging phase is observed for SCbig, due to the insufficient power supply of the control unit. The duration of this phenomenon depends on the available input power as well as the selected supercapacitors values and their leakage current. For the components under consideration, the charging of SCbig stops after approximately 100 ms.

**Figure 5.** Supercapacitors charging mode and startup (inset).

The supply of the output loads is depicted in Figure 6a,b. A 100 Ω resistor is connected to Vout1 and a 22 Ω resistor to Vout2, drawing approximately 20 mA and 100 mA, respectively. As shown, the loads are connected when the voltage levels of the supercapacitors exceed the minimum thresholds. Figure 7a presents the activation of the bleeder resistor. Figure 7b shows the battery support mode. The battery input voltage is set to 2.5 V, providing energy to the critically discharged supercapacitors, with the defined charging

**Figure 6.** Connection of the external loads according to the predefined voltage thresholds of the supercapacitors. The output loads are disconnected in low-input energy conditions to preserve the operation of the system. (**a**) SCsmall voltage level and Vout1 load output. (**b**) SCbig voltage and Vout2 load output.

**Figure 7.** (**a**) Bleeder mode activation. The bleeder mode is activated only when both supercapacitors are fully charged. (**b**) Battery energy support. Initially, the battery charges both supercapacitors. SCbig charging stops when its voltage reaches the lower voltage threshold (1.6 V). SCsmall is charged to its high threshold (2.3 V).

The simulated total current consumption of the storage unit is 374 nA at 2.3 V. The voltage dividers of the comparators consume 56 nA (28 nA each) and the integrated control circuit draws 318 nA.
