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  • Taking the power to charge the battery from one panel in a system

    I appreciate your input/advice here.
    We don't have hydro available on our property, so last year I built a system to run the well pump. The system is using 6x225W connected 3x in series and then both strings in parallel as at the end I can run directly the pump with max 90VDC. The well pump is very flexible in term of power; it has a generous power range 30-300VDC or 90-240VAC.

    Because I'm planning to build an automated irrigation system in spring, I'll need 12V battery to run the valves and timer. In order to charge the battery I'm planning to connect the charger to one of the panels from system. The charger is PWM type LMS2430 which accept max 40V input. For baterry I'll use 2x 6V Trojan T105-RE in series.

    Do you see any problem taking the power to charge the batteries from one panel in the system?
    Thank you.
    Attached Files

  • #2
    Two problems come to mind.

    1. 225 watt panels tells me they are grid tied panels meaning Vmp is likely around 25 to 30 volts, and Imp is around 7.5 to 9 amps. You will have to check your panel specs but what you want to know is Imp.

    PWM is basically a switch and Output Current = Input Current. MPPT on the other hand is: Output Current = Panel Wattage / Nominal Battery Voltage

    So here is what happens your 225 watt panel supplies the battery Imp of say 7.5 amps or Imp of the panel. Do the math Current x Voltage = Power, or 7.5 Amps x 12 volts = 90 Watts from your 225 watt panel . No where near enough to maintain a pair of Trojan T105'w which need 18 minimum and up to 30 amps

    MPPT output current = 225 watts / 12 volts = 18.75 amps or roughly 225 watts.

    2. The other issue is you now are left with a Prime number of panels (5 is prime) which means you only have two configuration options of all in Parallel or all in Series.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Two problems come to mind.

      1. 225 watt panels tells me they are grid tied panels meaning Vmp is likely around 25 to 30 volts, and Imp is around 7.5 to 9 amps. You will have to check your panel specs but what you want to know is Imp.

      PWM is basically a switch and Output Current = Input Current. MPPT on the other hand is: Output Current = Panel Wattage / Nominal Battery Voltage

      So here is what happens your 225 watt panel supplies the battery Imp of say 7.5 amps or Imp of the panel. Do the math Current x Voltage = Power, or 7.5 Amps x 12 volts = 90 Watts from your 225 watt panel . No where near enough to maintain a pair of Trojan T105'w which need 18 minimum and up to 30 amps

      MPPT output current = 225 watts / 12 volts = 18.75 amps or roughly 225 watts.

      2. The other issue is you now are left with a Prime number of panels (5 is prime) which means you only have two configuration options of all in Parallel or all in Series.
      Thanks Sunking for explanation. If the valves will use just 12watts per day, theoretically 8A from one panel will be plenty enough to recharge that batteries, correct? or 18-30A is still required? Sorry, I'm quite green with solar.

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      • #4
        Originally posted by rogerius View Post
        Thanks Sunking for explanation. If the valves will use just 12watts per day, theoretically 8A from one panel will be plenty enough to recharge that batteries, correct?
        12 watts is a rate at which power is being used. You need to know the WATT HOURS or how much energy the valves use in a day. Watt Hours = Watts x Hours. So for example if the valves use 12 watts for 24 hours they use 12 watts x 24 hours = 288 watt hours.

        No 8 amps is not enough for the T-105's to prevent Stratification. However even if those valves do use 288 watt hours, you do not need that large of battery. You only need 12 volts @ 100 AH, and 8 amps is enough for a 100 AH battery. That will save you some coins and a better fit.

        One warning here you need to check the panels Voc rating. If it exceeds the controllers voltage, it will damage the controller and let the magic smoke out.

        Here is what I suggest. A pair of T-105-RE is going to cost roughly $320 to $350 bucks. So consider this: Get a 20 amp MPPT Controller. You can pick one up for $150 to $190. Then get a decent 12 volt 100 to 120 AH battery for another $100 to $140. End the end it cost roughly the same, and gives you twice as much energy and most bang for your buck. It is a better option and smart money. Then when that battery needs replaced down the road, you can replace them with T-105- RE's and will be the perfect setup for them.

        MSEE, PE

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        • #5
          Originally posted by Sunking View Post
          12 watts is a rate at which power is being used. You need to know the WATT HOURS or how much energy the valves use in a day. Watt Hours = Watts x Hours. So for example if the valves use 12 watts for 24 hours they use 12 watts x 24 hours = 288 watt hours.

          No 8 amps is not enough for the T-105's to prevent Stratification. However even if those valves do use 288 watt hours, you do not need that large of battery. You only need 12 volts @ 100 AH, and 8 amps is enough for a 100 AH battery. That will save you some coins and a better fit.

          One warning here you need to check the panels Voc rating. If it exceeds the controllers voltage, it will damage the controller and let the magic smoke out.

          Here is what I suggest. A pair of T-105-RE is going to cost roughly $320 to $350 bucks. So consider this: Get a 20 amp MPPT Controller. You can pick one up for $150 to $190. Then get a decent 12 volt 100 to 120 AH battery for another $100 to $140. End the end it cost roughly the same, and gives you twice as much energy and most bang for your buck. It is a better option and smart money. Then when that battery needs replaced down the road, you can replace them with T-105- RE's and will be the perfect setup for them.

          The valve spec shows 2W and I'll use 6 of them for just 1 hour so 12W per day. The max input for charger is 48V, so I'm safe here. I've got the batteries for "free", sorry nothing is free; helping the neighbor, he gave me these, so now I have to use it somehow. Do you think buying 2 panels connected in parallel dedicated just to recharge the battery will prevent stratification? Or, is there a charge controller which I can connected to the existing system of 90V and charge the batteries?

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          • #6
            If you want to NOT kill the batteries, you need to have a charger that will hit them with enough amps to stir them up.

            ideally, you could use a MPPT controller and parallel it to the array (about 130Voc by my guess) That may confuse the pump for 30 or 40 minutes as the battery charges, but once it's full, no problem. The Midnight KID may be the best option for this. Also, with such shallow cycles, you will have to program a short Absorb time, and use a weekly EQ to stir the cells
            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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            • #7
              Originally posted by Mike90250 View Post
              If you want to NOT kill the batteries, you need to have a charger that will hit them with enough amps to stir them up.

              ideally, you could use a MPPT controller and parallel it to the array (about 130Voc by my guess) That may confuse the pump for 30 or 40 minutes as the battery charges, but once it's full, no problem. The Midnight KID may be the best option for this. Also, with such shallow cycles, you will have to program a short Absorb time, and use a weekly EQ to stir the cells
              Thanks Mike, you saved my day.

              I ran the sizing tool from midnite website and seems if I'll use the entire system(3 in series, 2 in parallel) will be too much for 12V battery system, but if I will take just one string from my system(3 in series) will be fine. In this way, the well pump will not be disturbed at all when the battery is charged, just a lower gpm if I need water.

              I'm planning to use Classic 200 instead of Kid, only because I can reuse it next year when I'll build a larger system for the new house and I'll switch everything on 110V off grid.

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              • #8
                Originally posted by rogerius View Post
                The valve spec shows 2W and I'll use 6 of them for just 1 hour so 12W per day.
                You need to get your terminology right. 2 watts x 6 units x 1 hour is 12 WATT HOURS not 12 watts.

                That only requires a very small 5 to 10 AH battery and a 10 watt panel.
                MSEE, PE

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