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Wiring Lower Amp Into Higher Amp Controller

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  • Wiring Lower Amp Into Higher Amp Controller

    Hi all. I'm looking to wire a single panel into a pwm controller. The panel outputs 8.3 amps. I'd like to purchase a 20-30a controller just for future panel needs if required. I know MPPT is the way to go but for now its not logical until i'm at higher amps. Anyways from what I gather PWM input is the same as output. 8.3a in is 8.3a out no matter the size of the controller. Correct? Thanks.

  • #2
    You are correct. For a PWM CC the amps that go in are what can come out to charge the battery.

    Starting small with a single panel and a cheap PWM CC will work but if you start to enlarge your pv wattage over 200w you tend to not get the full output of those panels through the CC to charge your battery.

    So starting off with a 30amp PWM CC might makes sense if you plan on enlarging your system, the problem is it will also steal watts from your panels because of the amps in = amps out principal.

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    • #3
      Thank you for confirming. I understand that i'm losing watts. The issue currently is that i don't have the battery capacity to with stand higher charging voltage. Starting very small. 2 12v 50ah batteries. I have 4 300w panels just gotta invest in batteries but in the mean time i'd like to put a small system to use. Thanks again!!!

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      • #4
        higher charging amps i meant***

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        • #5
          Originally posted by Shane.R13 View Post
          Thank you for confirming. I understand that i'm losing watts. The issue currently is that i don't have the battery capacity to with stand higher charging voltage. Starting very small. 2 12v 50ah batteries. I have 4 300w panels just gotta invest in batteries but in the mean time i'd like to put a small system to use. Thanks again!!!
          I understand your position. I just wanted to explain the pit falls of going with a PWM CC.

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          • #6
            Originally posted by Shane.R13 View Post
            Anyways from what I gather PWM input is the same as output. 8.3a in is 8.3a out no matter the size of the controller. Correct? Thanks.
            Yes but your logic and economics are flawed.

            PWM Output Current = Input Current
            MPPT Output Current = Panel Wattage / Nominal Battery Voltage

            One more formula and this is where your logic and economics are flawed.

            Power = Volts x Amps.

            So now look at your Vmp Voltage. Example if it is say a 72 cell 300 watt panel will have a Vmp = 36 volts and Imp = 8.3 Amps into a 12 volt battery. 8.3 amps x 12 volts = 100 watts. So much for your 300 watt panel.

            Same panel, same battery voltage with a MPPT Controller. 300 watts / 12 volts = 25 amps.

            Is 25 amps greater than 8.3 amps? Is 300 watts greater than 100 watts?

            Sure get yourself a 30 amp MPPT controller today. When you upgrade to a proper sized battery, you will have to wire all your 1200 watts so of panels in parallel requiring a lot more larger wire, gives your 30 amps x 24 volts = 720 watts. So much for your 1200 watt panels. You effectively throw one panel away in addition to battery expense. Not logical or economical. Smart money is a 50 to 60 Amp MPPT controller.
            Last edited by Sunking; 04-18-2018, 04:09 PM.
            MSEE, PE

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            • #7
              ok, well my solar situation changed today. i've acquired 2 235ah 6volts wiring into 12v. so from what i gather if i use one 300watt panel at 36volts and 8.3 amps with an MPPT controller i'll push 25 amps max to the batteries, which is just about ideal i think. if i were to use two panels id be pushing 50 amps with an MPPT controller, which is to much. Am i correct?

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              • #8
                Originally posted by Shane.R13 View Post
                ok, well my solar situation changed today. i've acquired 2 235ah 6volts wiring into 12v. so from what i gather if i use one 300watt panel at 36volts and 8.3 amps with an MPPT controller i'll push 25 amps max to the batteries, which is just about ideal i think. if i were to use two panels id be pushing 50 amps with an MPPT controller, which is to much. Am i correct?
                Fantastic. Yep 600 watts / 12 volt = 50 amps on paper, slightly less in practice. If those are golf cart batteries you can get away wiht 2 panels, just wire them in series assuming the controller Voc is 100 volts or more input. Ideally you want 150 vol Voc controller.

                MSEE, PE

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                • #9
                  To be honest I don't know if they a specifically golf cart batteries. I've attached a photo of label. How come if they are specifically golf cart I can wire 2 in series and feed higher amps? Thanks for helping and clarifying all this by the way.

                  30715105_1607813079332904_2232289787304214528_n.jpg

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                  • #10
                    They are Golf Cart batteries. You can wire any battery in series, wire 4 in series and you have 24 volts @ 235 AH.

                    Gold Cart batteries are hybrids which is a trade-off between a SLI (starting, lighting, & ignition aka cranking battery) and a True Deep Cycle. They can handle higher charge/discharge amps than a deep cycle, but not the every high rates SLI batteries can handle. The trade off is cycle life. You cannot deep cycle a SLI Battery, if you do will only have 50 or so cycles.

                    Those are used batteries, so save your coins for replacements. Look at Trojan Golf Cart batteries for you new replacement. Trojan is a good battery at good value.

                    Good luck.
                    MSEE, PE

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