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PV/MPPT/battery bank/inverter interaction with varying circumstances?

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  • PV/MPPT/battery bank/inverter interaction with varying circumstances?

    As a newbie with no equipment, I am having trouble imagining how power is managed on the DC side of the system. I tried to find a solar system "simulator" or description, and have found nothing.

    Here are a couple of scenarios.

    1. Full sun @ noon, batteries have been in absorb for 2 hours, and are consuming about 1/2 the available PV power. No AC load on the inverter. Does the charge controller move off the MPP of the PV array as the the power to the battery bank decreases, so the array is putting out 1/2 capacity? Since power can't vaporize, it seems this must be the case. There is no load diversion in this example.

    2. Same as 1) (absorb) a) A microwave oven drawing less than half the unused PV capacity is turned on for 5 minutes. Does the charge controller respond by moving towards the MPP to deliver the extra needed power, the power is drawn from the battery bank, some of both? b) A microwave oven drawing more than half of the unused PV power is turned on. Any difference than in a)?

    3. Batteries are fully charged and not under charge, and it is still noon on a sunny day. a) There is no load on the inverter. Does the charge controller go open circuit to the array? b) Now, the microwave oven is turned on. Does the charge controller load the array to get the needed power, is power delivered by the battery bank, both?

    If someone could answer these very basic questions or point me to a resource, I'd be most appreciative. I did try to find the answers elsewhere. Thank you!

  • #2
    Originally posted by Living Large View Post
    1. Full sun @ noon, batteries have been in absorb for 2 hours, and are consuming about 1/2 the available PV power. No AC load on the inverter. Does the charge controller move off the MPP of the PV array as the the power to the battery bank decreases, so the array is putting out 1/2 capacity? Since power can't vaporize, it seems this must be the case. There is no load diversion in this example.
    Pretty much it. Solar panels are current sources from MPP down to Isc. They are voltage sources from MPP to Voc. So in this example they behave like any voltage source.

    Answer is the same for all three questions. When batteries are fully charged up, you are in float mode and the controller is on PWM mode. Turn a load on and the PWM modulation increases to maintain voltage and power comes from the panels. Exceed what the panels can supply and batteries make up the shortage.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Pretty much it. Solar panels are current sources from MPP down to Isc. They are voltage sources from MPP to Voc. So in this example they behave like any voltage source.
      OK, that is an interesting fact.

      Originally posted by Sunking View Post
      Answer is the same for all three questions. When batteries are fully charged up, you are in float mode and the controller is on PWM mode. Turn a load on and the PWM modulation increases to maintain voltage and power comes from the panels. Exceed what the panels can supply and batteries make up the shortage.
      So an MPPT controller operates in PWM when the batteries are charged? That's interesting as well. But not while it is in charge mode - when the MPPT algorithm is used.

      Thanks!

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      • #4
        Originally posted by Living Large View Post
        So an MPPT controller operates in PWM when the batteries are charged? That's interesting as well. But not while it is in charge mode - when the MPPT algorithm is used.

        Thanks!
        The CC will only operate in full MPPT mode during those times when the battery can absorb the full output wattage of the panels. Any lower power requirement and the CC will move to operate at a voltage above the MPP and a correspondingly lower current. I do not think that a typical MPPT controller will ever operate in pure PWM mode with a series switching element and no DC to DC conversion. But they will not deliver any better performance than a PWM controller when the required battery current is less than or equal to the panel current.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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