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  • Off-Grid for Well Pump

    I have been a long time reader, but never had a question (hence why I never registered) until now. I have a degree in EE, so electrical concepts/math are second nature to me, but solar is something that just isn't taught in school . I am hoping you guys can help answer what will probably seem like basic knowledge to some of you veterans!

    My brother in law lives in upstate NY and wanted to supplement his propane generator with a small solar system for just his well pump. Even though he has a 500 gallon propane tank, there are times during the winter when rationing propane is appropriate when the power is out. Since water is essential to living, he wanted to focus on providing solar power for that first.

    He purchased the following:

    6: 100 watt, 12 volt solar panels (currently wired in parallel)
    1: 60 amp MPPT charge controller
    4: AGM 12 volt, 100 AH batteries (currently wired in parallel)
    1: 3000 watt continuous/12,000 watt peak low-frequency split-phase inverter (12 volt input)
    His well pump is 1/2 HP, 220 volt. I don't have the make/model yet, but I have seen specs online where units of this type surge 35 amps on start (@220) and drop to 9 amp continuous.

    I already realize he should change the way the system is configured to take advantage of the MPPT charge controller, but I want to focus on figuring out why it is not working properly first.

    Now the bad news. When the pump starts, the inverter immediately goes into fault mode.
    I gave him my volt meter and had him check everything, so I know its wired correctly.
    I've checked all his wire gauges/lengths vs current draw/voltage drops and it all seems within spec.

    Let's do a little math and you guys tell me if I am correct.

    Surge watts for this pump would be 220 x 35 = 7,700
    Since the input voltage to the inverter is 12 volts, the inrush current would be 7,700 / 12 = 642 amps

    - Are those assumptions correct so far?
    - Regular car batteries have CCA (cold cranking amps) specs that would apply to this situation, but does this apply for AGM batteries for solar systems?
    - Is the inrush current exceeding what these batteries can supply and that is why it is failing? If so, can he swap the inverter to one that supports 48 volt input, wire the 4 batteries in series, which would drop the current from 642 amps to 160 amps?
    - If all he is using the inverter for is the 220vac well pump, does he really need split phase?

    Thanks in advance and I apologize if these are noob questions!!

  • #2
    The batteries and cables cannot supply the starting surge. A reasonable limit for a 12V system, would be 1,000 w of inverter

    200w of PV panels is not going to recharge the batteries, it will maintain them, but would take days to recharge. In that time, the batteries will be damaged a bit from sulfation. 600W of panels will be a good start, to pump a little bit of water in the winter.

    So changing up some things may help.
    More PV, to get to 70V, maybe 500 -800W and run that to the MPPT controller
    Wire batteries in series to get 48V. then 7,700w of starting power is only 160A @ 48VDC Still going to need fat cables and good hydraulic crimps for that to work

    Inverter needs to be a quality 48V pure sine inverter. Mod sine inverters cause motors to consume even more (20% more roughly) power than they would on grid/sine power.

    Sadly, that's the story
    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

    Comment


    • #3
      Originally posted by Mike90250 View Post
      The batteries and cables cannot supply the starting surge. A reasonable limit for a 12V system, would be 1,000 w of inverter

      200w of PV panels is not going to recharge the batteries, it will maintain them, but would take days to recharge. In that time, the batteries will be damaged a bit from sulfation. 600W of panels will be a good start, to pump a little bit of water in the winter.

      So changing up some things may help.
      More PV, to get to 70V, maybe 500 -800W and run that to the MPPT controller
      Wire batteries in series to get 48V. then 7,700w of starting power is only 160A @ 48VDC Still going to need fat cables and good hydraulic crimps for that to work

      Inverter needs to be a quality 48V pure sine inverter. Mod sine inverters cause motors to consume even more (20% more roughly) power than they would on grid/sine power.

      Sadly, that's the story
      Thanks for the feedback. Since his charge controller only supports 12/24 volt battery arrays and the inverter only supports 12 volt input, he is going to need to replace equipment. It's probably not worth the expense and money better spent elsewhere. At a minimum, he needs to rewire the solar panels to charge controller, so that he has > 12 volts coming in.

      Comment


      • #4
        you can configure the PV panels any way you want. But the 12v batteries, wires and inverter will still NOT start the load.
        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

        Comment


        • #5
          Originally posted by Mike90250 View Post
          you can configure the PV panels any way you want. But the 12v batteries, wires and inverter will still NOT start the load.
          I should have been more clear. I am not expecting that re-wiring the panels would start the load. I was merely stating that in order to take advantage of an MPPT controller, you should be feeding it higher voltage than your battery array.

          Comment

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