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  • 12v vs 24v

    I have an off grid setup which is currently operating on 4 100ah batteries in parallel for 400ah. With a 12volt 1000w pure sign wave inverter. With a Classic 150 and 5 300w panels open circuit voltage of 46.12v Short circuit current of 9.56 amps, Optimum Operating Voltage 37.47v and Optimum Operating Current 8.01 amps. Running in to a midnight combiner box.

    From about 10:30am 4 Panels face south and are in the sun most of the day. 1 Panel faces west which gives me power till about 8pm before I start drawing from the batteries. I was wanting to add another panel facing East as it would give me power a couple hours earlier before the sun hits the south facing panels.

    If I convert the existing batteries to 2 - 24 volt batteries and then connect them in parallel for 200ah at 24 volts. I understand I would have to buy a 24volt inverter. We also never take our batteries below 85%. So with that said after the sun goes down would I have the same run time with the 24v 200ah battery setup vs the 12v 400ah current setup?

  • #2
    Any way you configure them it wont change the "watt hours" of the bank so theoretically, no change. The no load draw of the 24 volt inverter you buy may be different and could be less than your current inverter.
    You will need to series wire your panels 2 at a time for the higher voltage battery so you will need an even number of panels,....4, 6, etc. This will affect your East, South, West plan.
    Last edited by littleharbor; 06-03-2017, 10:07 AM.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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    • #3
      I wasn't sure if I would have to put the panels in series or not. Not a problem and Not a problem having an even number also. Going 24v I can add more panels to my existing setup 2 east 2 west.
      Last edited by Svencool; 06-05-2017, 06:05 PM.

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      • #4
        Originally posted by Svencool View Post
        I have an off grid setup which is currently operating on 4 100ah batteries in parallel for 400ah. With a 12volt 1000w pure sign wave inverter. With a Classic 150 and 5 300w panels open circuit voltage of 46.12v Short circuit current of 9.56 amps, Optimum Operating Voltage 37.47v and Optimum Operating Current 8.01 amps. Running in to a midnight combiner box.

        From about 10:30am 4 Panels face south and are in the sun most of the day. 1 Panel faces west which gives me power till about 8pm before I start drawing from the batteries. I was wanting to add another panel facing East as it would give me power a couple hours earlier before the sun hits the south facing panels.

        If I convert the existing batteries to 2 - 24 volt batteries and then connect them in parallel for 200ah at 24 volts. I understand I would have to buy a 24volt inverter. We also never take our batteries below 85%. So with that said after the sun goes down would I have the same run time with the 24v 200ah battery setup vs the 12v 400ah current setup?
        You tell me.

        Battery Watt Hour Capacity = Battery Voltage x Amp Hours

        12 volts x 400 AH = ???? AH
        24 volts x 200 AH = ???? AH
        48 volts x 100 AH = ???? AH

        That controler you have has a wattage input limit based on battery voltage?

        1380 watts @ 12 volt battery
        2700 watts @ 24 volt battery
        5022 watts @ 48 volt battery

        Never ever parallel Pb batteries greater than 2-volts
        MSEE, PE

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        • #5
          Ok so know matter the configuration I'm going to get 4800AH. and the symbol Pb stands for the element lead. The classic will take unto 96amps and 150volts I'm not sure I understand what your saying...

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          • #6
            Originally posted by Svencool View Post
            Ok so know matter the configuration I'm going to get 4800AH. and the symbol Pb stands for the element lead.
            Correct 4800 watt hours, but 48 volts is 1600% more efficient than 12 volts at a given power level. That is why utilities use high voltage. High voltage is more efficient and less expensive to implement.

            PB is a reference to lead acid battery

            Originally posted by Svencool View Post
            The classic will take unto 96 amps and 150volts I'm not sure I understand what your saying...
            Because you have the wrong assumption, and do not understand how a MPPT controller works. The 150 volts is the maximum voltage that can be applied to the [B]INPUT[/B], and 96 amps refers to the maximum [B]OUTPUT[/B], not Input as you stated.

            [B]Watts = Volts x Amps[/B].

            150 volts x 96 amps = 14,440 watts. If you forced that into the input would blow up in your face. As stated the 96 Amps refers to the maximum [B]Output Current[/B]. For a MPPT Controller:

            [B]OUTPUT CURRENT = PANEL WATTAGE / BATTERY WATTAGE[/B]

            So answer these equations to enlightenment

            12 volts x 96 amps = ???? Watts
            24 volts x 96 amps = ???? Watts
            48 volts x 96 amps = ???? Watts

            Does that help enlighten you? With a MPPT controller we can input say 120 volts DC @ 9.6 Amps and output 12 volts @ 96 Amps. Look at the equation both Input and output are equal.

            120 volts x 9.6 amps = 1152 watts
            12 volts x 96 amps = 1152 watts

            So what happens if you used a 96 volt battery and 96 amps of charge current. What is the panel wattage? 96 amps x 96 volts = 9216 watts .

            Now this statement I made earlier should make sense to you now.

            [B]
            That controler you have has a wattage input limit based on battery voltage?
            [/B]

            1380 watts @ 12 volt battery
            2700 watts @ 24 volt battery
            5022 watts @ 48 volt battery
            If you run 12 volts you are limited to 1380 watts power input.
            Last edited by Sunking; 06-03-2017, 08:33 PM.
            MSEE, PE

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            • #7
              Yes I can see how it would way more efficient to run a higher voltage. If your going to get 96amps out of your controller higher voltages are better! So yes you have as you would call it enlighten me!!!! Thank You for that!!!! After seeing your second equazion 120v vs 12v has enlighten me as well.... Its makes allot more sense and as always I appreciate you taking the time to enlighten us!!!! or me lol....
              Last edited by Svencool; 06-04-2017, 05:06 PM.

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              • #8
                Originally posted by Svencool View Post
                Yes I can see how it would way more efficient to run a higher voltage. If your going to get 96amps out of your controller higher voltages are better!.
                OK sounds like you may not quite have your noodle wrapped around the fork yet. You have to get Input and Output in your head separated. 96 Amps is on the output side, not Input.

                One of the benefits or using MPPT Controllers is to allow you to use much less expensive higher voltage Grid Tied panels. Example at 12 volts the Classic works best at 90 volts for 12 volt batteries. So if you wired 1200 watts of panels on the input would be 90 volts @ 13.5 amps. That allows you to use very small inexpensive wire between the panels and controller. On the output is 96 amps @ 12 colts. 96 amps at any voltage will require a minimum 2 AWG wire, and at 12 volts is only good for a few feet. See what is going on? 13 amps in, 96 amps out.

                New controllers will allow you up to 600 volts input. That same 1220 watts would be less than 3 amps with 96 amps out. Really small 14 AWG wire on the input

                Originally posted by Svencool View Post
                Thank You for that!!!! After seeing your second equazion 120v vs 12v has enlighten me as well.... Its makes allot more sense and as always I appreciate you taking the time to enlighten us!!!! or me lol....
                You are welcome, happy to help.

                MSEE, PE

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                • #9
                  Originally posted by littleharbor View Post
                  You will need to series wire your panels 2 at a time for the higher voltage battery so you will need an even number of panels,....4, 6, etc. This will affect your East, South, West plan.
                  If I just want to convert my battery bank to 24v and get a new inverter. My panels are already 24v panels do I still need to wire them in series?

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                  • #10
                    Yes
                    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                    • #11
                      I was talking with one of the Techs at Midnight Solar and he suggested that I run 3 strings in parallel. 2 panels each in parallel that way I still could have 4 facing South and 1 could be facing east and the other could be facing west. Each string would be about 37 volts at 16amps. Is he right or wrong?

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                      • #12
                        You have your terminology wrong. A string IS series wired panels. If they are all in parallel you have 6 "strings" of 1
                        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                        • #13
                          Haha well one day maybe I will get it right! But I guess what the guy at Midnight said is right and will work as a 24v setup and do the job?

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                          • #14
                            Well what another guy was telling me today confirmed what you guys were saying. I can run it 2 way Series and Parallel. But in Parallel VOC of approx 37v in lab conditions more likely 30v and battery bank 24v there is not much of a spread. So in Series I would be getting around 60-65v @ 8amps so there is a bigger spread and would more likely start charging earlier than the parallel setup. I ended up going with a Samlex 24v Inverter....

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                            • #15
                              Ok I have all panels put in series and my batteries are now wired for 24volts, my question is, I use to have the BULK and FLOAT setting in the classic set to the manufactures 12v specs. So now that they are wired in series do I basically double those settings? BULK was 14.5v and Float was 13.5v. So now would I do 29v on BULK and 27 on FLOAT?

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