Solar energy is an attractive energy source for powering portable devices. For some time, it has been widely used in applications like calculators and spacecraft. More recently, solar power is being considered for a much wider range of consumer applications including mobile phone chargers.
However, power available from a solar panel is heavily dependent on the operating environment. This includes such things as light intensity, time and location, etc. Therefore, batteries typically are used as energy storage elements. They can be charged when extra power is available from the solar panel, as well as to power the system when the available power from the solar panel is insufficient. How do we design a Li-Ion battery charger to get the most out of the solar cells and efficiently charge the Li-Ion battery? First, we'll discuss the operating principle and electrical output characteristics of a solar cell. Then, we'll cover battery charging system requirements and system solutions for matching the solar cell characteristics to get the maximum power from the solar cells.
Solar cell I-V characteristics
Basically, a solar cell comprises a p-n junction in which light energy (photons) causes electrons and holes to recombine, generating an electric current. Because the characteristics of a p-n junction are similar to those of a diode, the electrical circuit shown in Figure 1 often is used as a simplified model of the solar cell's characteristics.
Figure 1: Simplified Circuit Model of a Solar Cell
The current source IPH generates a current proportional to the amount of light falling upon the cell. With no load connected, nearly all the current generated flows through diode D, whose forward voltage determines the solar cell's open circuit voltage (VOC. This voltage varies somewhat with the exact properties of each type of solar cell. But for most silicon cells, it is in the range between 0.5V and 0.6V which is the normal forward voltage of a p-n junction diode.
The parallel resistor (RP) represents a small leakage current that occurs in practical cells, while Rs represents the connection losses. As the load current increases, more of the current generated by the solar cell is diverted away from the diode and into the load. For most values of load current, this has only a small effect on the output voltage.