## 3 System Implementation  
### Solar Energy Harvester  
The proposed energy harvester is based on the direct charging scheme [6], in which a dynamic comparator compares V\({}_{\mathrm{bat}}\) with V\({}_{\mathrm{pv}}\) in a timely manner. The dynamic comparator and required control signals can be seen in Figure 3.  
After turning off the switch between V\({}_{\mathrm{pv}}\) and V\({}_{\mathrm{bat}}\), the first inverting gain stage gets reset by activating the reset control signal, to establish a common mode voltage of V\({}_{\mathrm{cm}}\) at the V\({}_{\mathrm{C\_Top}}\) node. Meanwhile, V\({}_{\mathrm{C\_Bot}}\) is connected to V\({}_{\mathrm{pv}}\) by activating the Sample_V\({}_{\mathrm{pv}}\) control signal, and the difference between V\({}_{\mathrm{pv}}\) and V\({}_{\mathrm{cm}}\) is sampled on the sampling capacitor. In the next step, by deactivating the reset and activating Sample_V\({}_{\mathrm{bat}}\) control signal, V\({}_{\mathrm{C\_Bot}}\) is connected to V\({}_{\mathrm{bat}}\), and as a result, V\({}_{\mathrm{C\_Top}}\) changes to \(V_{cm}+(V_{bat}-V_{\mathrm{pv}})\). Outputs of the first and second gain stages (V\({}_{\mathrm{o1}}\) and V\({}_{\mathrm{o2}}\)) change according to the new V\({}_{\mathrm{C\_Top}}\), and V\({}_{\mathrm{o2}}\) is latched and stored as V\({}_{\mathrm{Comp}}\), by enabling the latch_enable control signal. If V\({}_{\mathrm{Comp}}\) gets 1, it means that V\({}_{\mathrm{bat}}\) is higher than V\({}_{\mathrm{pv}}\), and the switch should be kept turned off to avoid  
Figure 3: Circuit diagram of dynamic comparator. battery discharge, but if V\({}_{\text{Comp}}\) gets 0, the PV module starts to charge the battery by turning on the switch. After the switch is turned on, V\({}_{\text{pv}}\) will follow V\({}_{\text{bat}}\) during battery charging. As the operating voltage of the PV module is determined by the battery, end-to-end efficiency from the PV module to the battery is reduced when battery voltage diverges from the V\({}_{\text{mpp}}\) of the PV module during battery charging. Different maximum power point tracking (MPPT) strategies have been deployed in energy harvesting applications to maximize the amount of harvested energy. As the V\({}_{\text{mpp}}\) of the PV module varies with incident light conditions, an efficient MPPT scheme ensures that the maximum power is extracted from a PV module at any given time. Many complex and accurate MPPT schemes have been investigated for large solar harvesting systems [15]; however, when the PV module is small and can only provide a few micro-watts under reduced light intensity, only low-overhead schemes that incur very little power overhead can be good candidates. In [16], several low-overhead MPPT approaches, including fractional open-circuit voltage (FOC), fractional short circuit current (FSC) and hill-climbing techniques, have been discussed that can be deployed in inductive and SC DC-DC converters. Although in direct charging V\({}_{\text{pv}}\) always follows V\({}_{\text{bat}}\) and no MPPT scheme can be deployed, nevertheless, it achieves even higher efficiency than competing SC and inductive DC-DC converters [6]. In addition, the die area is much smaller than SC and inductive DC-DC converters, as large passive components, such as pumping capacitors, or inductors have not been used in this architecture. When the target V6 HR NiMH microbattery is fully charged, the battery voltage is close to V\({}_{\text{EOC}}\); as the battery is discharged by a low current, V\({}_{\text{bat}}\) drops to its nominal value and remains almost constant, up to getting close to the fully discharged state. However, if the battery is discharged by a high current, as can be seen in the discharge curve of the battery in Figure 4[2], V\({}_{\text{bat}}\) drops immediately and voltage drop depends on the remaining charge of the battery. During wireless data transmission, the battery is discharged by a high current, close to 5 CA (C being the 1 hour charge or discharge current).