1 Introduction

The current drawn from a solar panel is closely related to the output voltage of the solar panel being used. When no current is drawn, the solar panel relaxes to the open circuit voltage (OCV) condition. The OCV for a fixed solar panel increases as the temperature decreases [1 - 3]. At the opposite end of the operation spectrum, if too much current is drawn, then the output voltage of the solar panel moves to a short condition. At this operating window, the current is called the short circuit current (SCC). The SCC current for a fixed solar panel increases as the irradiance increases. The MPP lies in between these conditions and requires careful selection of the input voltage limit to not only avoid operation at the OCV and SCC conditions, but to meet the MPP [4].

Typically, the MPP voltage lies between 75% to 85% of the OCV in fully irradiated sunlight conditions. This ratio of MPP voltage to the OCV is also known as the k-factor. Having the ability to modulate the input voltage around this ratio can help to achieve the MPP of the solar panel. The sample current vs voltage and power vs voltage curves for a solar panel are shown in Figure 1-1. In the power vs voltage curve, there is a clear MPP of 7W and 13.5V. Moving the voltage up or down beyond this point lowers the output power [5]. Finding a way to keep the solar panel voltage at the best point is critical to help charge the battery faster.

Figure 1-1 Sample I-V and P-V Curve