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  • Magic switch?

    As background, I am a solar newbie, but with the help of this forum and its contributors, i.e. particularly Sun King, Mike and Sun Eagle and a few others, I put together a small system at my shop to run my beer fridge and game freezer, etc. The title of that thread is "Two Free Panels". Long story short, I have two 280W panels, a 45A Morningstar MPPT CC, a 1,740 Samlex inverter and 8 GC2 6V batteries wired in series/parallel for a 24V system. I also have a Samlex 24V 20A charger. I have been experimenting with the system and I can successfully run my loads off grid for 2-3 sunny days before I have to switch to the charger to top off the batts. I do this manually right now and what I'm looking for is a "magic switch" that will monitor the batteries and when they get to some low level (probably between 60-80% SOC, which will probably happen at night unless I get 2-3 cloudy days), kick out the inverter and transfer the load to grid power. Then, when the batts reach a charged state (probably 100% SOC) reverse the process to kick off the grid and switch to the inverter again.

    While I have searched the web, unless I get a very sophisticated inverter/charger which includes a transfer switch, I can't find a magic switch to accomplish what I want. I've gotten some tech advice from a tech who told me how I can do this with a battery guard/monitor which, when connected to a relay that is parallel wired with the on/off toggle switch on the inverter to kick out the inverter and then using a transfer switch, the grid power would be utilized. Additionally, the tech said if I wire in another relay that would be connected to the battery guard so that when the battery guard senses the batts are charged again, it would reverse the situation and transfer the load from the grid to the inverter. He even sent me a hand drawn schematic to assist me and recommended the two relays I could use. I ordered the two relays, which were less than $2 each, but after looking at them, I quickly realized that I am in over my head regarding the wiring. For that reason, I am again searching for a "magic switch" that will do what I want to make the entire process automatic. Does anyone know of such an item that would work and cost less than a new compact car?

    It has been suggested to me that I could just leave the 3 stage charger on constantly since it will bring the batts up until the float stage and then cycle down to minimum volts until it senses the batts are getting low and then it will kick in to bring the batts back up. Could something as simple as that work or would it end up cooking my batts to leave it on 24/7 or would I just use too much kWh to where it is not economically feasible?

    Last edited by Cajun Bill; 10-26-2016, 03:54 PM.

  • #2
    Well you could use something like this and program the inverter to shut down when the battery gets low...

    https://www.cdw.com/shop/products/Cy...6.aspx?pfm=srh
    Last edited by ButchDeal; 10-26-2016, 03:29 PM.
    OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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    • #3
      Originally posted by Cajun Bill View Post
      . . . I can do this with a battery guard/monitor which, when connected to a relay that is parallel wired with the on/off toggle switch on the inverter to kick out the inverter and then using a transfer switch, the grid power would be utilized. Additionally, the tech said if I wire in another relay that would be connected to the battery guard so that when the battery guard senses the batts are charged again, it would reverse the situation and transfer the load from the grid to the inverter. He even sent me a hand drawn schematic to assist me and recommended the two relays I could use. I ordered the two relays, which were less than $2 each, but after looking at them, I quickly realized that I am in over my head regarding the wiring. For that reason, I am again searching for a "magic switch" that will do what I want to make the entire process automatic. Does anyone know of such an item that would work and cost less than a new compact car?
      Many battery monitors (like the Flexnet DC) will have an output like that. Pair that with an inverter that will take an external enable switch (like the Outback FX series) and that should do what you want.

      If your inverter does not have an enable you will need a relay capable of switching full inverter power (200 amps for example) and they are both expensive and inefficient.

      You can also use the output of the Flexnet to turn your charger on. You'll need a relay again, but this time it will only have to switch ~15 amps or so, so it will be considerably smaller and more efficient. If you don't want to get a battery monitor you can make do with a voltage monitor to do the same thing; for example an old Xantrex/Trace C35 or C40 will do it. (PWM charge controllers are basically voltage controlled switches.) They have a load control mode that will do basically what you want.

      If you have AC available I would go for the voltage monitor plus the switched charger.
      Last edited by jflorey2; 10-26-2016, 05:37 PM.

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      • #4
        Can you scan and post the schematic that was drawn for you? Only way I can think of to do this is very clumsy.
        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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        • #5
          Sorry Mike, I just saw your message. I'll try to scan it soon and send it.

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          • #6
            Mike, I have an email that contains the schematic if I can figure out how to get it to you.

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            • #7
              Howdy Cajun bill just email it to me and I will pass it on. Just hit the contact us button, cheers.

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              • #8
                This is what the basic controller is called on Amazon for $13, Low Voltage Disconnect Module LVD, 24V

                That will turn off the inverter. A DPDT line powered relay can then be connected to the inverters output. When the inverter turns off the relay will drop out and then switch to the grid.
                Pull in and Disconnect voltages are selected by switch.es

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                • #9
                  Originally posted by PNPmacnab View Post
                  This is what the basic controller is called on Amazon for $13, Low Voltage Disconnect Module LVD, 24V

                  That will turn off the inverter. A DPDT line powered relay can then be connected to the inverters output. When the inverter turns off the relay will drop out and then switch to the grid.
                  Pull in and Disconnect voltages are selected by switch.es
                  Interesting PNP. Although I had searched Amazon before, I did not see that item, probably because I did not use specific enough language in my search. This module also allows one to preset the reconnect voltage. Now, if I can figure out how to wire in the relay so it switches to the grid, this might work. Just not sure what will happen after the batts recharge back up to the reconnect voltage. But then, what will cause the grid to drop off, the relay because it will sense the voltage thru the LVD when it is back in the circuit?

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                  • #10
                    Another thought: If the relay works to switch to the grid when the LVD drops off the batts/inverter, it would seem that I run the risk of the two events happening quick enough to run the risk of "voltage fighting voltage" aka DC fighting AC for a brief moment and causing an unwanted result. Then, when it reverses, even if I'm successful in getting the relay to drop the grid off once the batts get up to the reconnect voltage, the same thing may happen. I'm thinking I might need a transfer switch of some sort to enable a built-in time delay when it switches back and forth.

                    By the way, there is no way I can use the $13 module because it won't handle the amps but there is an 80 amp version for $50 that should do it. Am I correct when I compute my maximum amperage coming from the batts as being the maximum inverter wattage of 1,750 divided by the 24V to be about 73 amps?

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                    • #11
                      They do have a 30A version, but my fridge takes 123A @12V on startup . That would be 60+ amps for you as you calculated. Many inverters have a remote on switch tat handles very low current, just enough to run the control electronics < 0.1A. On my inverters that didn't I just connected to the slide switch. One side has 12V all the time. It is the other side that should be soldered to and run that wire to the load side of the voltage relay. Relays consume a lot of power that can switch high amps and I would avoid doing it that way. Everyone says, I just want it to do this. Most operations are more complex. I have written several programs for the NANO 328. These little micros are only $2.50 but quite powerful for jobs like this. It included dropout delays. and transfer timers that prevent fridge hot starts. However. just powering an AC relay from inverter AC should work sufficiently if proper motor relay is used with enough spacing of the relay contacts. Choose one that is rated for at least 480V and that will prevent arcing between grid and inverter. Remember the normal withstand voltage has to be at least twice the supply voltage and then inductive spikes have to be added in. RC surge arestors and/or MOV's need to be included. Those inverters that have momentary push buttons require a micro.

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                      • #12
                        PNP, I stayed with you until you started talking about the NANO 328, from that point on, everything was over my head. I'm not savvy enough to do what your are suggesting so I'm thinking I might get the LVD module with the 80A capability and just use it to cutoff the batts to prevent damage and then I'll manually switch it to the grid and then back when the batts are sufficiently recharged. I could use the LVD built-in the inverter, but that doesn't operate until it senses about 20V which would damage my batts according to everything I read.

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