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Hello and question- 24 Volt panel into a 12 volt bank?

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  • Hello and question- 24 Volt panel into a 12 volt bank?

    Hello everyone, I am kind of new was experimenting with solar 10-12 years ago and dropped out for last 10 years or so and now starting a system for our 30' motorhome we plan to fulltime in.

    Question revolves around a guy I ran into running 2 24 volt 250 watt panels into a bank or 3 telcom batteries wired for 12 volts through a charge controller, didn't catch info on controller.

    My Question goes like this. If you have a solar panel (Amerisolar 250 W Watt 24Volt UL Polycrystalline PV Solar Panel) putting out approx. 250 watts of POWER which would be approx. 10 amps at 24 volts and you ran that through a decent charge controller feeding a 12 volt 400-500 bank what would the output be? I believe the input to the bank would be approx. 14 volts charging voltage at 17 amps approx. I believe a good controller will handle it with no problem but not sure on output.

    Sorry if this is a stupid question just not sure I am right. Thanks for any help and sorry if I am totally wrong. I understand power just not how the charge controller will handle it and bugging the snot out me. Thanks again

  • #2
    Pulse Width Modulating (PWM) controller - At most the batteries will see 250W / 24V = 10.4A during bulk charging until the battery voltage reaches the absorption setpoint. Usually that is 14.X V. After that it will start tapering amps in order to hold the battery voltage at the 14.X setpoint to complete the absorption charge phase, then it drops into float. A PWM controller will be wasting the extra voltage offered by the panels.

    Maximum Power Point Tracking (MPPT) controller - it will work as you describe. Controller will take everything the panels can dish out (250 watts) and convert that into 250/14.XV = 17ish amps to the batteries until they reach the absorption setpoint. Then the controller will behave like the PWM and taper the amps in order to maintain the voltage at the 14.XV absorption setpoint, then drop to float once that charge cycle is complete. MPPT optimizes the power provide by the panel and converts it all into amps to the batteries during bulk charging.
    I'm an RV camper with 470 watts of solar

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    • #3
      You use a MPPT Charge Controller which is a Power Converter. 250 watts / 12 volts = 20 amps and would require a 20 amp controller.

      If you use a PWM Controller with that panel on a 12 volt battery is only 120 watts.

      PWM Output Current = Input Current.

      MPPT Output Current = Panel Wattage / Battery Voltage

      Huge difference.
      Last edited by Sunking; 08-24-2016, 05:27 PM.
      MSEE, PE

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      • #4
        His explanation was clearer than mine. MPPT controllers are more expensive than comparably rated PWM ones, but you can user higher voltage panels which are less $/watt to buy. A PWM controller means excess voltage is wasted so it only makes sense to use 12V nominal panels which cost more on a $/watt basis. For very small systems, PWM is often adequate and cheaper but as you increase the size of the system MPPT becomes the clear way to go. When I built my camper system, this threshold was said to be in the 500 watt range, but as panels have gotten cheaper that threshold may have gone down. I'm running PWM with 280 watts of 12V panels.
        I'm an RV camper with 470 watts of solar

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        • #5
          Thank you for the explanation, I was positive that a MPPT controller would handle the conversion with minimal loss. I never had a MPPT but a Outback flexmax 80 is top of my list right now. Thank you again I just felt like I was missing something. Sorry for double post admin can combine if needed

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          • #6
            Morningstar & Midnight Solar have several smaller MPPT controllers that may be a better fit for a small array, if you don't need 80A and programming gear
            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
              The plan as of right now is to have 4-5 of the 250 watt 24volt panels mounted on the passenger side roof with tilt. We also may have a fold up set of 2 that can be deployed if we need them on the ground with tiltable rack if we needed a little more power for long term boon docking in the future.

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              • ewarnerusa
                ewarnerusa commented
                Editing a comment
                Wow, that's a lot of solar. What is the battery setup?

            • #8
              Hopefully 500ah lithium. The problem is we will be camping in shaded spots hopefully a couple summers in Alaska rainy days etc and feel we would rarely see any where near full output from the panels. We are constantly tweaking load calculations but this is and estimate for now. The expansion set of panels would be something we could do later for like Alaska if we needed or omit is we didn't. I just don't want to skimp on CC or inverter knowing I may have bigger plans for future.

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