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  • Voltage and Current in low light

    If a monocrystalline cell is rated at 0.6V and 8A, what happenes as light decreases?

    Do they both decrease at the same rate? Or does the volt stay the same and the current decreases? Or vice versa?

  • #2
    As light increases, the voltage quickly rises to normal. Amps are very dependent on the amount of light, and that's what changes as clouds go overhead. If you apply enough of a load (too much) you can eventually collapse the panel voltage, but you really have to work at doing that, any charge controller or GridTie inverter is programed to NOT do bad things.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      Have you looked at any IR curves of a solar panel? The curves answer your question in full detail.
      MSEE, PE

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      • #4
        Hi,
        I did - but they all showed curves with an assumed insolance of 1000w/m2.


        ... but I did find one with different insolance. I've attached it here - just in case someone was looking for one too.

        Am I correct to assume that the voltage of a solar panel is dictated by the battery's voltage and that the current is dictated primarily by the level of insolance battery (assuming a static voltage)?

        And because the voltage of a battery changes with it's "fullness", the amount of current changes drastically, especially towards the end of the curve.

        Which is why we use MPPTs? The MPPT detects the voltage of the battery, and changes the incoming solar panel's voltage so that the optimal charge can come in?

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        • #5
          Originally posted by solarnoobie View Post
          I did - but they all showed curves with an assumed insolance of 1000w/m2
          .Well then you need to look at another one like this that shows the curves for 1000 800, 600, 400, and 200. The voltage remains constant from 1000 to 200, it is the current that goes down as the light level is reduced

          Originally posted by solarnoobie View Post
          Am I correct to assume that the voltage of a solar panel is dictated by the battery's voltage and that the current is dictated primarily by the level of insolance battery (assuming a static voltage)?
          No and Yes. The voltage of the panel has nothing to do with the battery voltage because the charge controller isolates the two from each other. Now if you connected the solar panel directly to the battery terminals, then the panel voltage will equal the battery voltage.

          The current is dictated by the solar insolation factor, and the state of the charge of the battery. If the battery is fully charged and no load being demanded, then there is no current flowing (or power being used) in either the panel or battery. Basically dumping and wasting the panels whichis the downfall of battery systems.

          If the battery is in a discharged state, or a demand for power from load equipment, current will flow from the panel to supply the battery and loads.

          Originally posted by solarnoobie View Post
          Which is why we use MPPTs? The MPPT detects the voltage of the battery, and changes the incoming solar panel's voltage so that the optimal charge can come in?
          No not at all. It does not matter is the controller is a MPPT or Shunt PWM, its only function is to charge the battery and supply the load equipment with power from th esolar panels. If the batteries are charged and no load being taken no current flows period. Does not matter what type of controller it is.

          A MPPT controller is much more efficient as it is a DC-AC-DC switch mode device. It allows you to use high voltage panels up to 150 volts to charge a 12, 24, 36, 48, 60, 72, and 94 volt battery system. With a higher voltage panel means lower currents between the panels and controller, which means you can use smaller wire and have much less power loss on the wiring between the panel and controller.

          For example lets say you have a 1000 watt solar panel made for 12 volt battery system. The panel voltage is 18 volts and the current is 55 amps (1000 watts). The output of a shunt or PWM controller is 13 volts at 55 amps = 715 watts. See a problem? Where is the missing 285 watts leaving the solar panel? You just lost 28.5 of your power. Another huge downfall of battery systems

          Now with a MPPT controller we can now use a 1000 watt 100 volt solar panel at 10 amps. So at the input of the controller we have 100 volts at 10 amps = 1000 watts, and on the output we have 13 volts at 73 amps = 950 watts. See any difference? Only a 5% loss.
          MSEE, PE

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          • #6
            Originally posted by Sunking View Post
            .Now with a MPPT controller we can now use a 1000 watt 100 volt solar panel at 10 amps. So at the input of the controller we have 100 volts at 10 amps = 1000 watts, and on the output we have 13 volts at 73 amps = 950 watts. See any difference? Only a 5% loss.
            how did you get the 73 amps figure?

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            • #7
              MPPT controllers will step down the high voltage while at the same time the current will increase proportionately 13 volts (battery) x 73 amps = 949 watts.
              2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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              • #8
                Originally posted by littleharbor View Post
                MPPT controllers will step down the high voltage while at the same time the current will increase proportionately 13 volts (battery) x 73 amps = 949 watts.
                I understand the 950 Watts. what I don't understand is the 73. stepping down the voltage from 100 to 13 gives me a factor of 7.7 so proportionally I'd get 77 amps (7.7 x 10amps) and the whole 1000 Watts. how did Sunking come up with 73 amps?

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                • #9
                  95% efficiency. The losses may not be exactly 5%. This is a generalization. He chose 13 volts as the battery voltage, which could be anywhere from 12.x volts to 14.8 volts. . Bear in mind when battery voltage is in the 14's the current being accepted by the battery will be low as the battery is nearing a fully charged state. The numbers could one of many combinations but generally, while in bulk charging you will get something like 95% efficiency, depending on the model of MPPT charge controller being used.
                  Last edited by littleharbor; 10-16-2018, 09:47 AM.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                  • #10
                    Originally posted by manacabana View Post

                    how did you get the 73 amps figure?
                    Simple Ohms Law

                    MSEE, PE

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                    • #11
                      Originally posted by Sunking View Post
                      Simple Ohms Law
                      okay... current equals voltage divided by resistance. where did you get the resistance? in this case 1.37ohms. is it something the controller specs should give?
                      ​​​​

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                      • #12
                        Originally posted by manacabana View Post

                        okay... current equals voltage divided by resistance. where did you get the resistance? in this case 1.37ohms. is it something the controller specs should give?
                        ​​​​
                        Resistance does not enter the equation. Ohms Law is 12 equations. One of them is:

                        Amps = Power / Voltage.
                        MSEE, PE

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