## 5 Discussion  
We utilized a smartphone cover as a solar concentrator to enhance harvested light energy. The low-output harvested energy was studied as a supplement to primary battery power in order to perform some essential functions such as wireless communication. The presented scheme can be used in a variety of IoT nodes and wearables in the distant future [50,51] in home networking environments. Using both indoor and outdoor lighting conditions, a primary smartphone battery can be recharged using an energy harvesting technique with operating system (OS) support, where emergency calls and functions can be activated under a time limit. The energy harvested is discussed to show that when PV technology efficiency will reach a critical efficiency point, it will be able to support the functionality of smart devices with current load circuitry.  
In order to show efficiency, we first connected two solar cells in series. Then, two pairs were connected in parallel to form four PV cells, as shown in Figure 9. The four solar cells were connected with the BQ25504-EVM circuit board and tested in different light conditions, as shown in Figure 10, with the energy generated harvested in the 100 u capacitors. Within about 42 s of charging, inside a typical windowed office environment under different ambient light conditions, the combination of open windows with sunlight generated more power as compared to the electric lights only condition.  
Figure 10: Charging of the 100 uF capacitor by the BQ25504 converter under different light conditions (room light and hybrid light conditions).  
Figure 9: The testing scheme for powering the load during ZigBee communication, using light energy harvesting.  
As shown in Figure 10, the black line (i.e., room lights) showed charging in 42 s with only indoor room lights, and the blue line (i.e., room lights with sunlight) showed charging to a similar amount of power in the same amount of time. The red line (i.e., room lights with sunlight) made by combining these shows an increased amount of charge for 42 s when charging using indoor room lights and outdoor sunlight as a hybrid method. The hybrid method discussed here does not simply mean a mixture of room lights and sunlight, but is also the direction of the development of commercial products that can be highlighted as having the biggest advantage in this paper. It is natural that cells vary depending on the angle that they receive light from, and in order to develop this more scientifically in the future, ideal hybrid methodology must be adopted in conjunction with an optimal angle.  
Using the testing scheme in Figure 9, a 147 uF capacitor was connected to the output of the BQ25504 circuitry board. After charging the capacitor, the \(V_{BAT\_OK}\) signal was used to supply the energy from the storage capacitor to the ZigBee module. Initially, the capacitor charged to 2.8 V (576 uJ), as shown in Figure 11. This energy was enough to bear the inrush current, initialization, and data transmission of the module. After first-time data transmission, the capacitor voltage varied from 1.8 V, the lower limit of the under-voltage-lockout circuit, to 2.8 V, repeatedly. Thus subsequent data transmission requires less energy (338 uJ) as compared to the first-time power-up procedure of the ZigBee module.