The output characteristic curve of the solar cell module is shown as in Figure 1. It can be seen from Figure 1 that the volt-ampere characteristics of solar cells have strong nonlinearity, that is, when the sunlight intensity changes, the open circuit voltage will not change much, but the maximum current generated will change considerably. so its output power and maximum power point will change accordingly. However, when the light intensity is constant, the current output by the solar cell module is also constant, which can be regarded as a constant current source.
What is concerned from the application point of view is the external characteristics of solar cells. First of all, for a monolithic solar cell, it is a PN junction. In addition to generating electricity when sunlight shines on it, it also has all the characteristics of a PN junction. Its rated output voltage is 0.48V under standard lighting conditions. In the solar LED lighting system, the solar cell components used are composed of multiple solar cells in series and parallel. It has a negative temperature coefficient. When the temperature rises by one degree, the voltage drops by 2mV. For solar cell modules composed of multiple solar cells, this is a problem that cannot be ignored.
In Figure 1, ISC is the short-circuit current; Im is the peak current; VOC is the open circuit voltage; Vm is the peak voltage; Pm is the peak power. Another important parameter of solar cells is the fill factor FF
FF=(VOC×ISC) /(Vm×Im） （1-30）
The fill factor FF indicates the quality of the solar cell. The larger the value of FF, the better the output characteristics of the solar cell and the higher the efficiency. It is an important indicator of the quality of the solar cell. For the application of solar cells, the understanding of the output characteristic curve is very important. This is because it not only shows the main technical indicators of solar cells, but also in the design of solar cell applications, a lot of work is carried out around the output characteristic curve. When the light conditions are constant, the shape of the output characteristic curve is also constant. With the change of the load, the operating point moves on the output characteristic curve. When the load resistance is zero, the operating point is at ISC. Although the short-circuit current is very large at this time, since the solar cell terminal voltage is zero, the output power is zero; when the load resistance is infinite, although the terminal voltage of the solar cell is at the maximum value VOC at this time, since the current of the solar cell is zero, the output power is still zero.
The actual state is not as extreme as the above, the operating point is moving on the output characteristic curve. For example, in the solar LED lighting system, the solar battery charges the battery. At the beginning, because the battery voltage is relatively low, the operating point is above the output characteristic curve, and the charging current is relatively large. As shown in Figure 2, the operating point is Pa. As the charging time increases, the terminal voltage of the battery gradually increases, the charging current gradually decreases, and the operating point gradually moves from Pa to Pb.
During this movement, the working point passes through Pm, which is the maximum power output point of the solar cell. If the solar LED lighting system always works at this operating point, the load will get the maximum power, and the control method of tracking the maximum power output point of the solar cell through the control circuit is called MPPT.
If the solar LED lighting system uses 40W solar battery modules to charge a battery of 150~180Ah, if the battery is seriously short of electricity after several consecutive cloudy and rainy days, the battery voltage will be relatively low, the system will deviate from the maximum power output point, and it will not be able to recover for a long time, and the efficiency of the system will be relatively low. However, at this time, it is most hoped that the battery can obtain more energy from the solar cell to store energy for the next consecutive cloudy and rainy days. It can be seen that it is wrong to blindly increase the capacity of the battery in order to increase the lighting demand for multiple consecutive cloudy and rainy days. Because the utilization efficiency of solar cells is very low at this time, if the system tracks the maximum power output point through the control circuit, the battery can obtain the maximum charging energy.
In use, solar battery as a kind of power source, unlike other power sources, open circuit or short circuit will not cause damage. In fact, it is precisely this feature that controls the charging and discharging of the battery in the system.