I'm currently working on a project where I have to charge batteries by solar. For this reason I chose the CN3722 IC. Since I want to create my own PCB with all the necessary parts I had to read the datasheet in order to wire everything right.
However, I stumbled upon something that confuses me.
The CN3722 IC has one pin labeled "MPPT". One can set the MPP voltage of the solar panel by altering the resistor divider (R8/R3) (see page 2 of the datasheet).
The datasheet also states that a charging cycle only begins if the following three criterias are met (the first two are irrelevant for this question):
- The voltage at VCC pin rises above the UVLO level
- The voltage at VCC pin is greater than the battery voltage by sleep mode release threshold VSLPR
- The voltage at VCC pin is no less than the maximum power point voltage set by the external resistors
The last one heavily confuses me. I don't understand why they designed it that way. I know that every solar cell has a different MPP which also depends on other environment factors, like temperature and sunlight intensity.
- Why doesn't a charging cycle begin when the MPP voltage isn't reached? Sure, the charger won't operate as efficiently, but it would still generate power, wouldn't it? Why did they design the IC this way? Is there any logic behind that decision?
- I also have a question regarding solar panels. For instance, I bought these cheap solar panels. How can I obtain its MPP voltage (at room temperature I suppose)? Sadly there was no datasheet or similar given.