## 4 Experimentation and Testing  
To test the proposed system presented in Figure 2 for directly charging the smartphone battery, the PV cell has to charge the high-capacity smartphone internal battery. The battery's open-circuit voltage was measured as compared to the battery status shown by the two tested smartphone models (commercially available, LG-F180K and SHW-M420S) in Figure 3. With the four PV cells used, it will take about 8 h at 2500 lx lighting conditions to recharge the battery from 1% to 10% battery status, so that the smartphone can be powered up and some emergency phone calls can be made, as shown in simulated graphs in Figure 3. Both smartphone model batteries were different and have different voltages, thus, we compared them using the battery status displayed on the smartphones as shown in Figure 3. Furthermore, to validate and analyze the solar energy harvesting technique, four Type-1 PV cells, similar to PV on a smartphone's top surface, were installed on a normal wristwatch, as shown in Figure 4.  
Figure 2: Using mirrors as solar concentrators to enhance the incident light on PV cells. Furthermore, for testing the system presented in Section 3, the commercial smartphone models LG-F160K and SHW-M420S with internal ALS sensors were used. We did not calibrate the sensor, and the error of the ALS sensor was found by comparing values with a TES-1330A light meter under different indoor lighting conditions, as shown in Figure 5. Under very close distances from the cathode fluorescent bulb (CFL), the maximum light intensity of 2500 lx was found in an office environment. The system for using a smartphone cover as a solar concentrator shown in Figure 2 was experimentally developed, as shown in Figure 6. By manually adjusting both mirror angles at the same magnitude, \(\theta\), the light intensity from the smartphone's internal ALS sensor was compared with the reference light meter and open-circuit voltage of the four PV cells, as shown in Figure 7.  
Figure 4: Four Type-1 PV cells were fitted on this wristwatch for light energy harvesting. Figure 3: Two smartphones’ battery voltage w.r.t displayed battery status. Figure 3.0 A comparison of smartphone internal ALS data vs. light meter data. Figure 3.1 The experimental platform for the solar concentrator system with an angled (\(\theta\)) setup. Figure 3.2 Light intensity and PV output voltage variations due to changes in mirror angle \(\theta\). The main objective of this experiment was to use the internal ALS sensor data as an input for the maximum power point calculation. However, due to its location inside the smartphone, the ALS was only able to estimate CFL light intensity at angles greater than 80\({}^{\circ}\), making it impossible to use with MPPT. Therefore, to attain the maximum power point while using a smartphone cover as a solar concentrator, open-circuit voltage or resistive set-point techniques are more practical, where the former will occupy less space without reference resistors. The solar concentrator was able to enhance the light intensity by about 1.85 and 1.43 times at angles of 90\({}^{\circ}\) and 71\({}^{\circ}\), respectively, so that more ambient light energy could be harvested at 2500 lx light intensity in a typical office environment. For the idea presented in Section 3, to attain maximum light energy harvesting, the optimum place is on the top of the smartphone cover where a maximal area is available for PVs. Two types of PV were used in this scheme as shown in Figure 8; one was four PV cells (Type-1) as previously discussed, and another was low-cost polycrystalline 60 \(\times\) 65 mm\({}^{2}\) with 15% efficiency (Type-2). The two types of PV have different output voltage levels. For maximum power harvesting, each requires a separate controller and converter. The rest of the available space was used for a small 3.7 V, 160 mAh, 31 \(\times\) 24 \(\times\) 2.9 mm\({}^{3}\) Li-Polymer battery.