Generally, when the solar panel voltage reaches 17.5V, the 12V battery starts charging (the maximum open circuit voltage of the solar panel in 12V system is 21V). the voltage will drop to 13.5-14 V. The rest of the voltage drop adds to itself. This is the characteristic of solar panels. Panel voltage must be higher than battery voltage to charge. I hope this helps.
This is normal. According to relevant information, the output voltage of solar panels is not fixed, and fluctuates up and down when it is higher than 5.5V or more, it is easy to cause permanent damage to mobile phones and other products. range. A solar panel is a panel that uses solar energy to produce electricity.