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  • Solar dump load after batteries charged

    Familys solar system get's a full charge at mid-day pretty often during the summer months. Is there a way to automatically or manually take the additional energy and load dump to an electric water heater.

    There is probably 1200W of extra power that is being lost after the MPPT controller tapers off to float. Thus, a pretty reasonable amount to help the hot water system...

  • #2
    Yes, but not simple.
    Search for threads where the subject has been discussed.
    The first problem is to choose a voltage (battery SOC) at which the heater should turn on.
    To avoid resistance matching problems and diverting current without dropping the battery voltage or confusing an MPPT CC you may be best off sending AC to the heater element from an inverter.

    You want to avoid micro cycling the battery and reducing its life. That will cost you more than the saved energy is worth.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      The Midnight Classic charge controllers have 2 programmable Aux. relay contacts. They can be set to switch on a load when the battery reaches FLOAT. Read the manuals for them, I don't use the feature and don't know exactly what the limits are.
      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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      • #4
        The CC is a Outback MPPT, thus idono if it has the same options as the Midnight. Part of the system also has a 30amp Trace PWM CC that controls some older panels that don't match with the new panels that run the Outback CC.

        I do have two old Trace modified sine 2400w inverters that I could hook up to dedicate to a hot water tank. I just need to figure out how to pull off the CC without disrupting the batteries SOC.

        I have tried searching, but most of the set-ups are for constant load for wind, or solar hot water. It's hard to weed through it and find load dumping, solar electric hot water.

        Thanks for the replies guys!

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        • #5
          It's actually simple to do.

          Xantrex C series controllers can be used as dump load controllers. The way you describe you want is the same way charge controllers are used with wind gennies.

          Xantrex C series controllers are PWM controllers when used as a standard charge controller. But work backwards when used as a dump load controller. They turn on when a specific voltage is reached. To use them with an MPPT controller on the PV side you set all the float and absorption voltage the same as your bulk voltage then set the dump load controller to a couple tenths below that.

          WWW

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          • #6
            Originally posted by keaton85 View Post
            ...I have tried searching, but most of the set-ups are for constant load for wind, or solar hot water. It's hard to weed through it and find load dumping, solar electric hot water...
            They use standard electric heaters water heaters and replace the elements with DC units.

            WWW

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            • #7
              Since the Xantrex CC is PWM, how would I install that into the MPPT system since line in from the panels is 80-100Vdc (not 100% on voltage from panels).

              If I can hook up a C60 into the system, run that to one of the old Trace inverters, and power up a hot water heater with normal elements. Would that work? since I have the inverters and normal hot water heaters are a dime a dozen.

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              • #8
                Originally posted by keaton85 View Post
                Since the Xantrex CC is PWM, how would I install that into the MPPT system since line in from the panels is 80-100Vdc (not 100% on voltage from panels).

                If I can hook up a C60 into the system, run that to one of the old Trace inverters, and power up a hot water heater with normal elements. Would that work? since I have the inverters and normal hot water heaters are a dime a dozen.
                The MPPT is installed just as it normally would be wit the adjusted programming. Dump load controllers are installed between the battery bank and the dump load. They work simply as a voltage controlled on/off switch for the load. With wind the dump loads need to be larger than the genny could possible produce to keep the battery bank from overcharging. With PV they could be smaller.

                You can not use them with an inverter. They are set under a narrow on/off cycle (usually only 0.1V or less). That narrow range causes them to flutter on/off quickly. An inverter would not be able tolerate that.

                WWW

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                • #9
                  I guess I'm getting a little confused. How would the PWM CC interact with the MPPT controller so that it will output full amperage? This is a 24VDC system....

                  As in the MPPT CC ramps up to 50-60amps, once charged it tappers down to 5-10amps. If I put a load on the system the MPPT CC doesn't even notice and keeps the float.

                  Wouldn't the PWM CC just act like any other load on the system and not use the pull output of the panels? Sorry, just not getting it I guess.

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                  • #10
                    Originally posted by keaton85 View Post
                    I guess I'm getting a little confused. How would the PWM CC interact with the MPPT controller so that it will output full amperage? This is a 24VDC system....
                    As in the MPPT CC ramps up to 50-60amps, once charged it tappers down to 5-10amps. If I put a load on the system the MPPT CC doesn't even notice and keeps the float.
                    Wouldn't the PWM CC just act like any other load on the system and not use the pull output of the panels? Sorry, just not getting it I guess.
                    You choose a DC load that is equal or greater than maximum panel output.

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                    • #11
                      Wind gennies don't have charge controllers on the production side like PV has so a lot of this is theory as you don't see dump load controllers used much with PV.

                      The theory is that your MPPT will never finish the bulk cycle. If your bulk is set to end at 14.4V and the dump load comes on at 14.2 and off at 14.1 then voltage will never get high enough for bulk charging to finish. Your bank will still get fully charged as it is "Floating/absorbing" above 14.1V. That is one of the reasons for having the dump load larger than the PV output. If it were smaller your voltage could possibly reach the end of bulk charge.

                      Having the absorption and float charges set to the same as bulk is more of a safety to keep the array fully producing.

                      Smaller loads may not kick your MPPT back to bulk. But larger loads on long enough should.

                      WWW

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                      • #12
                        The only issue I see with that is the system also has a small Trace PWM that might interfere with having a dump load from the MPPT..

                        So:
                        The battery bank is 24V T-105 cell, thus bulk is 29.6Vdc

                        Load dump controller set to 29.6 ON and 29.5 OFF. Wouldn't this start cooking the batteries keeping voltage at 29.6Vdc for a few hours a day? And what happens if say the house water pump comes on at 40amps putting a load on the system which in turn will drop the Bulk voltage way below 29.5Vdc?

                        Am I making any sense of this? or have I confused the situation even more?

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                        • #13
                          Originally posted by keaton85 View Post
                          The only issue I see with that is the system also has a small Trace PWM that might interfere with having a dump load from the MPPT..

                          So:
                          The battery bank is 24V T-105 cell, thus bulk is 29.6Vdc

                          Load dump controller set to 29.6 ON and 29.5 OFF. Wouldn't this start cooking the batteries keeping voltage at 29.6Vdc for a few hours a day? And what happens if say the house water pump comes on at 40amps putting a load on the system which in turn will drop the Bulk voltage way below 29.5Vdc?

                          Am I making any sense of this? or have I confused the situation even more?
                          Higher voltage might mean a little more water usage. If you see too much then set the voltages a little lower. I would expect 29.6 to be a little high. I would expect 28.8 to be closer. You just need to tailor it to your system.

                          If the voltage drops below the set off voltage then the charge controller turns the dump load off until voltage rises above the turn on point. Nothing to worry about as that is what it should do. What currently happens if the water pump turns on just before reaching bulk voltage?

                          I think your trying to overthink this. A dump load works just like any other load. Only voltage regulated to keep your batteries from overcharging. Wind gennies run these without any type of charge controller between the genny and batteries, Your MPPT would simply be there for the voltage conversion to power and a safety device should the dump load fail.

                          WWW

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                          • #14
                            We have a 2kW solar panel system and once the 24 volt battery bank is charge generally around 10am the SOC is 100% and the aux relay in the CC closes which operates a relay and this switches a solid state relay which connects the batteries to a 24 volt 900 watt element in the water cylinder. The relay is also controlled by the hot watt cylinder thermostat. This works very well. I have a delay time and hold time set in the CC which is a Midnite Classic.
                            Cheers,

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                            • #15
                              How many solar panels of 150w and inverter require to run an ac refrigrater of approx 170watt

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