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  • Battery cable interconnects sizing

    Hi Everyone,
    I am upgrading my solar system (24 vDC) and had a question in terms of what size wire interconnects would be appropriate for my system. I have 2 lines of thought. I can go with either a 24 or 48 vDC system. I am planning to purchase either a Magnum Energy MS 4024 PAE (max 5 second draw is 5800w) or MS 4448 PAE inverter (8500w).My batteries that I going to purchase are the Trojan T-105 (225 AH). I will be purchasing 24 of these for a battery bank size of either 1350 AH (24 vDC) or 675 AH (48 vDC). I am trying to determine what would be the best most appropriate size wire interconnect to maintain 2% or less loss for the battery bank. I have been looking at different information on the web and feel like the products out there tend to run toward the thicker wire size and I'm not sure why because the wire size charts don't seem to indicate that is what is needed. If anyone could help me find a way through all the information that would be great. I determined that my minimum daily usage is 3800w AC = 31.66 AH/day (no loss factored in and this has a fair amount of fudge factored (actual was 3488 w AC because may add some grow lights for greenhouse and would max out at 5400 w/day but staying conservative for now) so that I have room to grow (ha ha).

    Information overload
    Walter

  • #2
    First thing is tap the brakes and reconsider batteries. Last thing you want to do is use parallel batteries and Golf Cart Batteries. With 24 T-105's configured for 24 volts gives you 900 AH or 450 AH at 48 volts. If you need 900 AH buy 900 AH batteries or 450 AH batteries. T-105's are hybrid batteries and will not last as long as a true deep cycle battery. Trojan also make a T-105RE which is a true deep cycle battery with 5 pounds more lead with double the warranty and cycle life. However you need either 450 or 900 AH depending on your voltage. That pretty much eliminates Trojan product line except for the L-16RE lineup in 2 volt cells.

    You don't want to start off on the wrong foot to learn this lesson and 2 years from now replacing batteries again. You want to get at least 5 years out of your investment before having to replace them. Cable size will depend on maximum inverter current and we will cross that bridge when you figure out the batteries and inverter voltages and current requirements.
    MSEE, PE

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    • #3
      Hi Sunking,

      First, thank you for your response. The reason I thought to go the route of the T-105 is because that is what I have had and they lasted about 10 years, but my electrical use is in the process of going up somewhat because of wanting to add the grow lights and because I am getting married. I am not sure how you come by 900/450 AH. The T-105s are 225 AH and times 6 (in a 24 vdc system) or 3 times 225 (3x8 for a 48 vdc) would give the 1350/675 AH wouldn't it? My current system has been 16 t-105s and an array size of 1500w (20 astropower 75w module panels although for the last year it's down to 18 because I moved the panels from an upper roof to a lower roof where I could keep them free of snow better, but in the process cause one of the panels to shatter its glass front and have since taken its pair offline so as not to create the shadow effect and pull down the rest of the charging current). Forgive me if I am not understanding the 900/450 unless you are figuring in the maximum depth of discharge into those numbers.

      Sincerely,
      Walter

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      • #4
        This link
        http://www.solar-guppy.com/download/...calculator.zip
        is for a mildly complicated but very complete spreadsheet for calculating voltage drop vs wire size.

        This link http://www.smartgauge.co.uk/batt_con.html
        walks you through the minefield of parallel batteries. Best avoided totally. If you have a
        debugged working system (and have killed your first set of batteries) then you ARE ready to
        invest in a professional battery bank, and leave the golf cart batteries (which I recommend to neophytes) behind.

        Mike
        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

        Comment


        • #5
          Originally posted by Wallymtn View Post
          Forgive me if I am not understanding the 900/450 unless you are figuring in the maximum depth of discharge into those numbers.
          Well Walter lets run the numbers. Would not be the first time I screwed up, I do it in my head, but lets do it the hard way on paper with calculator.

          You are buying 24 batteries correct? 24 T-105's right? A T-105 is 6 volts @ 225 AH right? If you configure for 24 volts you will have 4 batteries in series to make a string (already see my mistake) which makes 24 volts at 225 AH. So 6 strings in parallel is my bad, 6 x 225 or 1350 AH @ 24 volts or 675 AH at 48 volts. So thanks for the catch. That just makes things worse. You do not want 6 or 3 parallel strings. At 48 volts in a Trojan product you really want to look at the Industrial or RE lineups.

          48 volt candidate is:

          Ind13-6V 695 AH battery

          24 volt candidate is:

          IND23 4V 1270 AH

          The IND line is top of line and comes with a 36/96 warranty. RE line warranty is 24/60, and golf cart line (T-105 included) 12/24. If you want the best longest lasting then Rolls 5000 series is as good as it gets. If you insist on T-105's at least get the T-105RE. But I and others will strongly suggest you stay away from parallel batteries.

          Having all that said with the battery capacity you are looking at minimum panel wattage requirement is 3000 watts and that immediately rules out a 24 volt system because it would require 2 60 amp MPPT charge controllers. At 48 volts just a single minimum 60 amp MPPT controller. Not sure but does not sound like you do not have enough panel wattage. Either way takes a minimum of 3000 watts and a max of 4400 watts. With that said you are looking at a 48 volt battery system and a 80 amp MPPT charge controller as that will allow you to grow up to 4000 watts with 675 AH battery and maximum of 5200 watts for future growth.

          So I suggest you rethink things a bit and figure out where to go. Right now some of your numbers do not work. 24 volts is out. With 24 volts you are limited to around 2000 watts on a 80 amp mppt controller depending on which manufacture you go with.
          MSEE, PE

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