I wanna build a simple MPPT that keeps voltage on output of the solar panel constant. According to documentation, maximum power is drawn, when output voltage is about 4.2V. My way of reasoning is like this:

  • the more current I draw, the lower voltage will be on output
  • the less current I draw, the higher voltage will be on output

Does it make any sense? Or MPPT that uses variable, mcu controlled buck regulator will be much more efficient?

  • \$\begingroup\$ What does this mean? Output of what? "when output voltage is about 4.2V" \$\endgroup\$ – Misunderstood May 14 '19 at 20:28

The voltage produced by a solar panel specified in the datasheet is at 25°C and decreases as temperature rises. I have observed a decrease of around 8% in the maximum power point voltage on hot, sunny days.

If you are designing a constant-voltage MPPT, a good approach would be to take 5% off of room-temperature maximum power point voltage and use that. If you use the room-temperature voltage, the power will drop off quickly as the solar panel voltage decreases due to heating.

If you look at https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve you will see that power drops off rapidly as voltage increases past the maximum power point. Due to temperature changes, it is best to err a little bit to the left of that point to optimize efficiency throughout the day as the panel heats up.

If you want to get every last bit of efficiency out of your panels in all sunlight and temperature conditions, a MCU-controlled regulator is the way to do it. A common algorithm used is "Perturb and Observe". The MCU regularly makes small changes to the voltage setpoint and observes the effect. If the power goes up, it keeps going in that direction. If the power goes down, it goes in the other direction.

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