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.
My questions:
- 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 logical reasoning 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…
Best Answer
Best guess: A solar panel is a current source over most of its characteristic; the output voltage is set by the input impedance of whatever charger is connected to it. MPPT in this IC probably works by reducing the current it delivers on the output until the MPP is reached. If it can't reduce the current enough to get to the MPP by changing its input impedance, then there isn't enough current for charging.
Oversimplified, but good enough, probably: You could measure the open voltage and shorted current of the panel under full sun, giving you two points on its I/V characteristic curve. Then draw in a characteristic with the same shape (not values) as done here:
(Image source - my website: Using a solar panel for USB charging)
You can then make an educated guess of the MPP from the curve; the surface of the rectangle should be roughly the 1W mentioned in the spec of the panel. Note that in the image, the open voltage is 21 V and the shorted current is 0.62A; not the values you will see.
Alternatively, you could do more complex measurements, or you could mail the supplier and ask for more specs 8-).