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I have 2 x 125 amp hour batteries and 2 x 100w solar panels. More panels required?

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  • I have 2 x 125 amp hour batteries and 2 x 100w solar panels. More panels required?

    Hi, I am building this system for camping. I meant to only purchase one 125 amp/h deep solar battery, but I bought 2 by accident (online shopping cart misclick). My math tells me that my two 100w solar panels will not be enough to charge both batts in a day (Northern New York). Should I go ahead and get another 2 x 100w solar panels? Thanks, I'm new to this and reading a lot, but I thought I would just ask this question. Thanks,
    Rob (first post, be gentle)

  • #2
    Panel wattage and battery capacity is calculated by daily use, not guessing.

    What kind of controller are you using? MPPT or PWM?

    200 watts of panel using PWM controller will generate roughly 400 watt hours daily and will support a 12 volt 150 AH battery.

    200 watts of panel using MPPT controller will generate roughly 550 watt hours daily and will support a 12 volt 225 AH battery.
    MSEE, PE

    Comment


    • #3
      Hi, thanks for the reply. No guessing, got it. But, what do you mean by 'daily use'? Are you advancing the notion that I need to test out the equipment to see what kind of results I get? Or were you trying to say something else?

      I am using a 30A PWM Charge Controller (which allows system expandable up to 400W(12volts) and 800W(24volts))

      Here is my exact equipment (with 2 panels instead of 1): http://www.amazon.com/Renogy-100W-Mo...ds=solar+panel
      I also have two of these batteries currently: http://www.amazon.com/Vmaxtanks-Vmax...=solar+battery

      Thanks, Rob

      Originally posted by Sunking View Post
      Panel wattage and battery capacity is calculated by daily use, not guessing.

      What kind of controller are you using? MPPT or PWM?

      200 watts of panel using PWM controller will generate roughly 400 watt hours daily and will support a 12 volt 150 AH battery.

      200 watts of panel using MPPT controller will generate roughly 550 watt hours daily and will support a 12 volt 225 AH battery.

      Comment


      • #4
        I mean how many watt hours per day will you use? If you do not know that, then you are only guessing and hoping what you have will work and not leave you in the dark. Solar systems are designed based on daily watt hour usage, time of year use, location, tilt angles, and orientation. .

        As for MPPT vs PWM a 30 amp MPPT has a maximum input power of 400 watts @ 12 volts and 800 watts at 24 volts. But the difference is huge. 400 watts input on a PWM controller at very best only puts out 260 watts vs 400 for MPPT.

        With MPPT controllers allows you to use very inexpensive grid tied panels say a 250 watt panel that cost just under $250 dollars ($1/watt). To equal that with PWM forces you to use expensive battery panels. To equal 250 watts would require a 375 watt battery panel or roughly two more of the panels you already have. . Much less expensive using MPPT once you hit the 200 watt panel wattage. Just changing out your PWM to MPPT with 200 watts input will give you 17 amps of charge current (200 watts @ 12 volts). Right now your 200 watt panels on a PWM controller the best you can hope for is 11 amps charge curremt (130 watts @ 12 volts) . Essentially the same thing as you adding another 100 watt battery panel to your PWM controller.
        MSEE, PE

        Comment


        • #5
          Ahhh yes, I understand now--in fact, I have seen this thought represented previously and I had calculated (prior to purchasing) what my potential power draw might be under different situations. The truth is that I calculated 200w panel plus a 125amp/h battery would suffice for my 'baseline' needs (while camping). After that, it was a situation of adding non-necessities as capacity allowed. However, like I mentioned, I screwed up the order and bought 2 batteries instead of the one. Now that I have it (mail order), it is too much trouble and $ to return it (freakin thing weighs a tonne). So my original question was more about, "hey, I have 2 of these batteries. Assuming I draw about as much as is practically and optimally possible per day, I wanted enough solar panels to have them near fully charged by the end of the day. Obviously if I am using next to no power, one panel would charge 100 batteries I'd never draw from. However, I expect to 'find' uses for the excess battery power, assuming the panels are there to drive them.

          On the second note about the controller, I am intrigued. I had read that notion in several other posts around. Assuming I grab 2 more panels (or perhaps just one more and a controller??) any recommendations on which controller? Thanks again for the help.
          Rob (still reading)


          Originally posted by Sunking View Post
          I mean how many watt hours per day will you use? If you do not know that, then you are only guessing and hoping what you have will work and not leave you in the dark. Solar systems are designed based on daily watt hour usage, time of year use, location, tilt angles, and orientation. .

          As for MPPT vs PWM a 30 amp MPPT has a maximum input power of 400 watts @ 12 volts and 800 watts at 24 volts. But the difference is huge. 400 watts input on a PWM controller at very best only puts out 260 watts vs 400 for MPPT.

          With MPPT controllers allows you to use very inexpensive grid tied panels say a 250 watt panel that cost just under $250 dollars ($1/watt). To equal that with PWM forces you to use expensive battery panels. To equal 250 watts would require a 375 watt battery panel or roughly two more of the panels you already have. . Much less expensive using MPPT once you hit the 200 watt panel wattage. Just changing out your PWM to MPPT with 200 watts input will give you 17 amps of charge current (200 watts @ 12 volts). Right now your 200 watt panels on a PWM controller the best you can hope for is 11 amps charge curremt (130 watts @ 12 volts) . Essentially the same thing as you adding another 100 watt battery panel to your PWM controller.

          Comment


          • #6
            Bert - stop burning through cash.

            Until you know your actual power draw over time, we are just throwing money out the window and crossing our fingers.

            That said, if one wants to wing it, the ONLY thing you can do is go by worst-case scenarios and battery specs.

            What we know:
            A single 125ah agm like the vmaxtanks will only accept a max of 0.3C inrush current. That would be 37.5A.
            You don't want to take a battery below 50% DOD, so you really only have 62.5A to play with (750 watt-hours) with a single battery.
            In your area, you only have 4 hours of insolation in the summer, and just TWO hours in the winter. These are different from sunrise-to-sunset hours.
            In the winter, you won't make a recovery in a single 2-hour solar day with 4 100w panels from one of those taken down to 50% DOD, and your batteries will sulfate and die early.

            Then again, if all you are doing is charging laptops and running a few led lights on a weekend trip, then you are humping around much more lead than you need to. In fact, depending on what you are actually doing, you may not need solar at all, and just a full charge before you take off for the weekend.

            Since we don't know how much power you are pulling, and for how long each day, this is just a crapshoot.

            Comment

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