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I would like to connect a 9 V solar panel to a charging controller. The input range according to its data sheet is 7.5 V to 28 V.

The output of the solar panel can vary and its value can go below 7.5 V.

I noticed that most of the charging controllers have an input range, so the minimum value is not that small (I also saw 4.5 V).

I guess that on voltages below 7.5 V it will stop charging; isn't that inefficient? I mean, I can be sure that the output voltage of the solar panel will drop under 7.5 V during the day.

Am I right or am I missing something?

This is the 9 V, 460 mA solar panel I was considering.

This is the controller that was suggested by Marko Buršič (input range: 5 V to 28 V).

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2 Answers 2

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A solar panel is a current source over most of its characteristic, and its voltage depends on the input impedance of the charger connected to it. MPPT chargers vary this impedance to keep a solar panel at its MPP.

The voltage at the MPP doesn't vary all that much across irradiances and temperatures, and will be around 9 V for your panel. This allows for a simpler algorithm that is less efficient than a "true" MPPT implementation, but still quite good. This simpler algorithm is what the charger in your question uses.

What the IC used in that charger does, when needed, is reduce its input current to keep the panel at or above the (fixed) voltage you have set as the (approximate) MPP voltage.

Only when this is no longer possible will the input voltage drop to below the voltage you set. When that happens, available current will be insufficient for charging anyway.

So, the 7.5 V limit isn't as bad as you think, as the IC itself will do its best to keep its input, the output of the solar panel, at 9 V (or whatever voltage for the MPP you set), and even under very low irradiance the panel's MPP voltage will be over 7.5 V.

Having said that, I would use a 12 V panel and/or an IC that will go down to 4.5 V, for flexibility, and also because my possibly misguided instinct tells me bigger margins are better.

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  • \$\begingroup\$ Thank you for the answer. so you are saying that once I set the MPPT voltage, the controller will change (increase \ decrease) the panel current in order to maximize the panel output. how do you know what value of MPPT voltage one should set? given that 9v panel for example.. \$\endgroup\$
    – user1673206
    Commented Sep 17, 2020 at 20:50
  • \$\begingroup\$ The MPP voltage should be in the specs of the panel. Most manufacturers call a panel with a 9V MPP voltage a 9V panel, but no guarantees there. \$\endgroup\$
    – ocrdu
    Commented Sep 17, 2020 at 21:01
  • \$\begingroup\$ if you edit your question and furnish working links to both your panel and your controller, or even pics, we can better answer you. \$\endgroup\$
    – MicroservicesOnDDD
    Commented Sep 17, 2020 at 23:26
  • \$\begingroup\$ @MicroservicesOnDDD I edited the question- put links to solar panel and controller \$\endgroup\$
    – user1673206
    Commented Sep 18, 2020 at 6:42
  • \$\begingroup\$ @user1673206 -- Thank you. Here is the datasheet for the BQ24650. Sorry. We really still need more information. Temperature shifts the curve, so max/min temp matters. Also, how much average wattage are you expecting to have to deliver, that is, what is your specific application, and how critical is it to operate in the winter, when the sun is lowest on average. If you are in Alaska, the sun doesn't even rise for months at a time. All that said, my gut says 12 volts minimum for a panel, though you can put two of these in series for a nominal 18 volts... \$\endgroup\$
    – MicroservicesOnDDD
    Commented Sep 20, 2020 at 19:02
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Your link can't be opened, but I think it relies on CN3722, a Chinese version of BQ24650. It is a buck converter, therefore Vpanel > Vbat. You have to set the MPPT point with resistors according to your panel - note that this is not a true MPPT converter.

If the voltage falls below the Vbat voltage + the voltage drop across the switching element, let's say 1.5 V, then it won't charge.

The panel voltage can be lower, even under bright sunlight, if one of the cell is shaded.

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  • \$\begingroup\$ thank you for he answer. so you are saying that a real MPPT will adjust its voltage by itself? by measuring the input current and voltage? \$\endgroup\$
    – user1673206
    Commented Sep 17, 2020 at 20:34
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    \$\begingroup\$ @user1673206 Not this type IC, you have to define MPPT voltage with resistors and it's fixed. You may look for LT8490/LT8491, LTC4015, SM72441 these are true MPPT controllers. Analog Devices would call your IC and similar MPPC: analog.com/en/technical-articles/… \$\endgroup\$
    – Marko Buršič
    Commented Sep 18, 2020 at 7:00
  • \$\begingroup\$ A real MPPT will find the MPP voltage by itself, usually by continually changing the input a bit and then measuring if the output power increases. If it does, it will then change the input some more in the same direction, if it doesn't, it will change the input in the other direction. In this way it will find the MPP where output power is maximalised. \$\endgroup\$
    – ocrdu
    Commented Sep 18, 2020 at 8:51

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