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I've been reading about the theory of solar cells, and doing some circuit simulations using the idealized model of solar cells. But I still feel like I have some problems understanding how the solar cell behaves and would like to check some things.

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My understanding is that the current source in the solar cell model produces a variable current that depends primarily on irradiance, ranging from zero amps to the short-circuit current. Depending on the voltage applied to the solar cell, a portion of this current will flow back uselessly through the diode while the rest will flow out of the solar cell into the load. The voltage at which maximum power is extracted from the solar cell is the maximum power point.

So then my question is, if I have a battery, with a proper undervoltage/overvoltage circuit, and I connect it directly to a solar panel (ignoring any blocking diode for now), then the voltage across the battery will dictate the voltage across the solar panel, and some current will flow from the solar cell to the battery as long as the battery voltage is below the panel's VOC. The amount of current flowing in the battery will be given by the solar panel's I-V curve at any given battery voltage.

If I select the solar panel such that its maximum power point is achieved at a voltage near the nominal battery voltage, then the battery will extract reasonable power (near the maximum power point) from the panel, without needing an MPPT controller.

As I understand it the main downside of this approach is that the VOC will drop at low irradiances, moving the panel's maximum power point, such that the battery may charge inefficiently (or not at all if VOC falls too much) in low lighting conditions. The VOC also depends on the cell temperature.

And what an MPPT controller does is to apply a specific voltage across the solar panel to reach the maximum power point, by presenting a varying load to the panel, and then regulates that voltage to whatever voltage the battery needs, so that the solar panel can work efficiently across more lighting conditions and more diverse battery chemistries.

Is my understanding correct?

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    \$\begingroup\$ Depending on the voltage applied to the solar cell - what are you saying here? Are you suggesting that a voltage external to the photocell is used? You also make the same assertion further down at least two more times??? \$\endgroup\$ – Andy aka Jan 15 at 10:09
  • \$\begingroup\$ You are pretty much right about everything, but a couple of point I think you may not appreciate fully. First Voc and Vmpp change very little with irradiance. It is primarily the current that changes. Also, if you connect a solar panel to a battery, and the battery voltage is somewhat well matched, you still need a charge controller to prevent over-charge of the battery. The early off-grid solar systems typically paired 36 cell panels with 12V lead acid batteries (or maintained that ratio of cells per volt for 6 or 24 or 48 volt batteries). MPPT only provides mild benefits to such systems. \$\endgroup\$ – mkeith Jan 15 at 10:30
  • \$\begingroup\$ MPPT allows you to match panels to batteries even when the battery voltage is not close to Vmpp. That is its principal benefit. \$\endgroup\$ – mkeith Jan 15 at 10:32
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    \$\begingroup\$ Others are calling you out over the terminology of "applying voltage" to a solar cell. I noted that, but I don't think it is that big of a deal. You apply a voltage to the cell, and it supplies current into that voltage. If I connect a battery to a resistor I am applying a voltage. But if I connect two power sources together, which one is applying the voltage and which one is the load? But that is just terminology. Your understanding is pretty much correct. \$\endgroup\$ – mkeith Jan 15 at 10:34
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    \$\begingroup\$ Pretty close. But a simple PWM controller (not MPPT) between them can accomplish 2 things : allow a higher Voc panel, using PWM to bring the output voltage down to the battery, and allowing charge control to avoid battery damage through overcharge. \$\endgroup\$ – user_1818839 Jan 15 at 14:30
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It seems like your understanding is pretty much correct. There is just one clarification. The Voc and Vmpp are only slightly sensitive to irradiance. See attached graphs which are real DC data from one string in my grid-tie solar array. The grid-tie inverter logs data at 5 minute intervals.

Solar Output Graphs

As you can see, Vmpp does not vary much during the day. And its cloudy-day and sunny-day values are pretty close, all things considered. The depression in Vmpp on the sunny day is probably a reflection of panel temperature.

So it is possible to have an OK installation without using MPPT in some cases. But, in order to maximimize power over the full range of battery voltages, or when the battery voltage is not close to the solar panel Vmpp, MMPT charge controllers are required.

In general, some kind of charge controller is always required if for no other reason than to avoid over-charging the battery. But it doesn't always have to be an MPPT charge controller.

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  • \$\begingroup\$ You can imagine that if I had a battery bank consisting of 75 lithium ion cells in series, it might be possible to charge it with this solar string without MPPT (as long as max charge current and voltage was respected). \$\endgroup\$ – mkeith Jan 16 at 5:10

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