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  • System Voltage question

    I purchased 18 used solar panels from Children of the Promise that have finally arrived here in Haiti. The panels are 34VDC 5A nominal which I understand means they are meant for a 24VDC system. I would prefer to run them in two banks of 9, each with their own combiner through two 60A charge controllers and two 12VDC battery banks and then through my 3000W 12VDC - 120 VAC Inverter which is set up for two inputs of 12VDC only. My question is; Can I use two PWM charge controllers or must I use two MPPT controllers?

  • #2
    Originally posted by Craig Breckenridge View Post
    I purchased 18 used solar panels from Children of the Promise that have finally arrived here in Haiti. The panels are 34VDC 5A nominal which I understand means they are meant for a 24VDC system. I would prefer to run them in two banks of 9, each with their own combiner through two 60A charge controllers and two 12VDC battery banks and then through my 3000W 12VDC - 120 VAC Inverter which is set up for two inputs of 12VDC only. My question is; Can I use two PWM charge controllers or must I use two MPPT controllers?
    That is a very simple question to answer.
    With a PWM charger, the current out is exactly equal to the current in. As a result using a 34VDC panel to charge a nominal 12V battery will waste at least 50% of the potential panel power. If you run one of the 34V 5A batteries into an MPPT charge controller (CC), you will get 10A or more out to charge the battery.

    But that does leave you needing a CC that can handle more than 80A output, or else having to use two CCs per bank and split the panels up into groups of 5 and 4. Once you go to an MPPT controller, you also have the option of putting the panels in series to reduce the size of wiring necessary. (Wire for 80A at 12V is pretty hefty and expensive per foot. With a series circuit the wire from the combiners to the CCs will be much more practical, and the CCs can be mounted very close to the batteries.

    The other thing that concerns me, though, is your description of the inverter. Does it really need two electrically separate 12V supplies? More likely they just provided two sets of input terminals so that you can use two runs of smaller wire instead of one run of very thick and hard to handle wire. Check that out before you go any further with your planning.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Craig if it is possible run 48 volts and forget 12 volts.

      At 12 volts you are going to need 3 very expensive MPPT controllers with 3000 watt of panel input. You will also need very large and expensive copper conductors, and big expensive hydraulic tools to terminate huge cables.

      Use 48 volts and just 1 of those expensive MPPT controllers and much smaller copper wire.
      MSEE, PE

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      • #4
        Originally posted by inetdog View Post
        That is a very simple question to answer.
        With a PWM charger, the current out is exactly equal to the current in. As a result using a 34VDC panel to charge a nominal 12V battery will waste at least 50% of the potential panel power. If you run one of the 34V 5A batteries into an MPPT charge controller (CC), you will get 10A or more out to charge the battery.

        But that does leave you needing a CC that can handle more than 80A output, or else having to use two CCs per bank and split the panels up into groups of 5 and 4. Once you go to an MPPT controller, you also have the option of putting the panels in series to reduce the size of wiring necessary. (Wire for 80A at 12V is pretty hefty and expensive per foot. With a series circuit the wire from the combiners to the CCs will be much more practical, and the CCs can be mounted very close to the batteries.

        The other thing that concerns me, though, is your description of the inverter. Does it really need two electrically separate 12V supplies? More likely they just provided two sets of input terminals so that you can use two runs of smaller wire instead of one run of very thick and hard to handle wire. Check that out before you go any further with your planning.
        A couple of things you say raise more questions. As far as I have gleaned from several solar web sites the amperage for the charge controllers is simply the nominal amperage x the number of panels when they are wired in parallel. 9 x 5 = 45. That's why 60A controllers as they are 25% larger than required. With respect to the inverter, it will only function at the full 3000W capacity if both sets of terminals are used with equal battery banks otherwise it's rating drops to 2500 W. My choice here is coming down to whether I need to purchase a different inverter or not.

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        • #5
          Originally posted by Craig Breckenridge View Post
          A couple of things you say raise more questions. As far as I have gleaned from several solar web sites the amperage for the charge controllers is simply the nominal amperage x the number of panels when they are wired in parallel. 9 x 5 = 45. That's why 60A controllers as they are 25% larger than required. With respect to the inverter, it will only function at the full 3000W capacity if both sets of terminals are used with equal battery banks otherwise it's rating drops to 2500 W. My choice here is coming down to whether I need to purchase a different inverter or not.
          I forgot to divide by two to account for only 9 panels each. .
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

          Comment


          • #6
            Originally posted by Craig Breckenridge View Post
            A couple of things you say raise more questions. As far as I have gleaned from several solar web sites the amperage for the charge controllers is simply the nominal amperage x the number of panels when they are wired in parallel. 9 x 5 = 45. That's why 60A controllers as they are 25% larger than re
            That is only true for a PWM controller. With PWM: INPUT CURRENT = OUTPUT CURRENT. Using a PWM controller with a panel wattage of less than 200 watts is IGNORANCE, and you will soon discover this. So in your example putting 9 panels in parallel on a 12 volt battery will yeild you 45 amps at the input (1530 watts) and that is a net power output is 12 volts x 45 amps = 540 watts. You just lost 65% of your power from Ignorance.

            Those same 9 panel with a MPPT controller will output 130 amps @ 12 volts. Which is better 130 amps or 45 amps? Use a PWM controller in your application you just as well as throw 12 of then away because that is effectively what you are doing. With MPPT in your situation yields 283% more power.

            Now think about that.
            MSEE, PE

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            • #7
              Originally posted by Sunking View Post
              That is only true for a PWM controller. With PWM: INPUT CURRENT = OUTPUT CURRENT. Using a PWM controller with a panel wattage of less than 200 watts is IGNORANCE, and you will soon discover this. So in your example putting 9 panels in parallel on a 12 volt battery will yeild you 45 amps at the input (1530 watts) and that is a net power output is 12 volts x 45 amps = 540 watts. You just lost 65% of your power from Ignorance.

              Those same 9 panel with a MPPT controller will output 130 amps @ 12 volts. Which is better 130 amps or 45 amps? Use a PWM controller in your application you just as well as throw 12 of then away because that is effectively what you are doing. With MPPT in your situation yields 283% more power.

              Now think about that.
              Thanks for being so polite. I'm not ignorant.

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