(1) Load impedance: When the load impedance matches the output characteristics of the battery assembly (I-U curve) well, the battery assembly can output the highest power and produce the greatest efficiency. When the load impedance is large or increases due to some factors, the battery assembly will operate at a voltage higher than the maximum power point, and the efficiency of the assembly and the output current will decrease at this time. When the load impedance is small or becomes smaller due to some factors, the output current of the battery assembly will increase, the battery assembly will operate at a voltage lower than the maximum power point, and the operating efficiency of the assembly will also decrease.
(2) Intensity of sunlight: The output power of the battery module is proportional to the intensity of solar radiation, and the output power of the module will increase as the sunlight increases. The influence of the change of sunlight intensity on the I-U curve of the module is shown in Figure 1. It can be seen from the figure that when the ambient temperature is the same and the shape of the I-U curve remains the same, with the change of sunlight intensity, the output voltage of the battery assembly does not change much, but the output current rises greatly, and the maximum power point also rises.
(3) Component temperature: the higher the temperature of the battery component, the lower the working efficiency of the component. As the component temperature rises, the operating voltage will drop, and the maximum power point will also drop. Every time the ambient temperature rises by 1℃, the output voltage of each cell in the battery assembly will drop by about 5mV, and the output voltage of the entire battery assembly will drop by about 0.18V (36 cells) or 0.36V (72 cells). The relation curve between component temperature change and output voltage is shown in Figure 2.
(4) Hot spot effect: In the battery module or square array, if there is a shadow (such as leaves, bird droppings, dirt, etc.) covering a certain part of the battery module, or when a battery piece inside the battery module is damaged, the partially covered or damaged cell will be provided with the power required by the uncovered cell. The shaded or damaged cell is equivalent to a reverse-working diode in the assembly, and its resistance and voltage drop are large. It not only consumes power but also generates high temperature heat. This phenomenon is called the hot spot effect. In the high-voltage and high-current battery array, the hot spot effect can cause the battery to break, the solder ribbon will fall off, the packaging material will burn out and even cause a fire.
Read more: What are the main performance parameters of battery modules?