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  • MPPT recomendations

    Hi All,

    I'm rebuilding my system after complete battery failure. In my last post I got many recommendations to upgrade my charge controller from 80A PWM to MPPT. I have (9) 255 watt panels connected in parallel. I did the math 2295 watts / 24 Volts = 96 Amps, add 25% and I'm at 119 Amps... Well as you well know 120A CCs are expensive. I'm most likely only going to have (8) or (10) 12 Volt 110ah batteries so this seems excessive to me??? Do I have way too many watts on the roof? Should I sell 4-5 panels and buy batteries and a new controller? Can I run a smaller MPPT and still benefit over my PWM?

    Any advice is much appreciated.

  • #2
    If you haven't bought batteries yet you need to reconsider "8 - 10, 110 ah. 12 volt batteries. At 24 volts you will have 4 to 5 strings of batteries. Big no no. 6 volt batteries x 8 will give you just two relatively manageable 24 volt strings. OR one 48 volt string.
    Personally I would get a Midnite classic 150 , wire the panels 3s3p and get a 48 volt inverter.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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    • #3
      Thank you littleharbor but I'm confused. I'm in Mexico. My 2 battery options are 208ah 6V from Costo at $140 or 110ah 12V LTH brand at $110. So, the 12V are less $ /ah. Why are 4 to 5 strings of 12V such a No No... Sorry if thats a dumb question but it seem pretty common down here??

      I should add that my budget is about $1100
      Last edited by douglasjett; 05-22-2018, 01:34 PM.

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      • #4
        A few suggestions.

        Go to a 48V system; that will keep currents a lot more reasonable. (divide them all by 2)

        You can use more than one MPPT controller and connect them all to the same battery.

        Often putting 100 amps of panel (i.e. 100 amps at your battery voltage based on STC panel wattage) on an 80 amp controller is no problem. You will rarely see 80% of rated output under normal conditions.

        10 12V batteries (5 strings of 2) is a very bad thing to do to lead acid batteries. You want one string ideally. Two strings can work OK but you definitely don't want to go to 5. You would be much better off with 8 6V batteries, either all in series for 48 volts (ideal) or 4s2p 24 volts (not ideal but not too bad.)

        If you go with your original plan you'll end up with a 24V 550ah battery system. Max charge rate for flooded batteries is C/8 which would be about 70 amps. So you'd have to limit to about 70 amps charge current.


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        • #5
          Originally posted by douglasjett View Post
          Why are 4 to 5 strings of 12V such a No No... Sorry if thats a dumb question but it seem pretty common down here??
          Because lead acid batteries need to be charged with current, not voltage (to oversimplify the issue.) Once one string gets a little weaker than the others, it takes less current, gets less charged, so never reaches full charge, so gets weaker, which means it takes even less current etc etc.

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          • #6
            Originally posted by douglasjett View Post
            Thank you littleharbor but I'm confused. I'm in Mexico. My 2 battery options are 208ah 6V from Costo at $140 or 110ah 12V LTH brand at $110. So, the 12V are less $ /ah. Why are 4 to 5 strings of 12V such a No No... Sorry if thats a dumb question but it seem pretty common down here??

            I should add that my budget is about $1100
            Wow $140.00 per battery? In the states Costco charges like $87.00 for the same battery.
            It is nearly impossible to have balanced charging across multiple strings being that very small differences in resistance will adversely affect how much current each string accepts and gives up. All the connections required for parallel battery strings will result in differing resistance, plus it looks bad with all those cables. a single string @ 48 volts with 6 volt batteries requires only 7 interconnects. 4 strings @24 volts w/12 volt batteries will require 10 cables. That's not the bad part though. A single string cannot be unbalanced. 4 strings will virtually always be unbalanced and consequently not live as long as it should. Then you need to buy new batteries.
            2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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            • #7
              Due to my fixed peso income I'm stuck with 24v inverter and I'm stuck with 12V batteries... I guess I don't really have the money fix it correctly. So does a MPPT benefit me at all? If I'm going to abuse the 12V batteries no matter what I do, maybe I should just buy 4, wire them in parallel and run them till they die..

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              • #8
                Originally posted by douglasjett View Post
                Due to my fixed peso income I'm stuck with 24v inverter and I'm stuck with 12V batteries... I guess I don't really have the money fix it correctly. So does a MPPT benefit me at all? If I'm going to abuse the 12V batteries no matter what I do, maybe I should just buy 4, wire them in parallel and run them till they die..
                If your stuck with the 24v inverter then buy 8 6v batteries. Wire them 4s2p as suggested by jflorey2.

                The 6v batteries may cost more upfront but they will last longer than the 12v. 2 parallel strings is manageable. 4-5 is not.

                (6v) 8 x $140 = $1120
                (12v) 10 x $110= 1100

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                • #9
                  Originally posted by douglasjett View Post
                  Due to my fixed peso income I'm stuck with 24v inverter and I'm stuck with 12V batteries... I guess I don't really have the money fix it correctly. So does a MPPT benefit me at all? If I'm going to abuse the 12V batteries no matter what I do, maybe I should just buy 4, wire them in parallel and run them till they die..




                  With 255 watt panels I'm going to wager a guess you have 60 cell panels and you already mentioned they are wired 9p, (all in parallel). Yes you actually NEED an MPPT controller. This is likely part of the reason your batteries failed. They aren't 24 volt nominal panels. They are actually considered 20 volt nominal.

                  Whatever you do battery wise try to avoid parallel strings. This would be the other reason your batteries failed.

                  So you've been informed the two reasons why you are replacing your batteries after your "Complete failure" wire your panels 3s3p into a 150 max voc. controller and avoid parallel strings.
                  Last edited by littleharbor; 05-22-2018, 05:40 PM.
                  2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                  • #10
                    Just did some reading.. I'm sure your right 'littleharbor' 20 volt nominal. So, the guy I bought all my stuff from sold me all grid tie stuff; wrong panels, wrong batteries, wrong controller... Might be an inexpensive fix tho; dual 40A MPPT online $207, and some new $110 batteries, wired in series.. Best I can do with my funds. Thanks all!!!

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                    • #11
                      Originally posted by douglasjett View Post
                      Just did some reading.. I'm sure your right 'littleharbor' 20 volt nominal. So, the guy I bought all my stuff from sold me all grid tie stuff; wrong panels, wrong batteries, wrong controller... Might be an inexpensive fix tho; dual 40A MPPT online $207, and some new $110 batteries, wired in series.. Best I can do with my funds. Thanks all!!!
                      To be clear, to fix this you will have to wire your panels in SERIES not PARALLEL. MPPT controllers do not boost voltage (at least, most don't.) 3s3p might work. To see if it will, take Voc x 3, multiply by 1.25 (to account for temperature) and make sure that voltage is less than the max voltage the MPPT will accept. For example - 40V OCV is pretty common for 60 cell panels, so you'd need a 150V input controller.

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                      • #12
                        Which MPPT controller are you looking at? Run all your components by us before you purchase anything. Lets prevent another failure.

                        And seriously get the 6V batteries. 12v batteries shouldnt even be on your radar.

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                        • #13
                          Ditto on the 6V 208ah golf cart batteries. 4 in series gives you 24V @ 208ah. paralleled with another batch of 4 in series, doubles the amp hr to 416ah @ 24V $ 8x140 = $1120 Ouch. So no 48V inverter this year. So you need at least 40amps to charge that string, what does your existing controller put out to the batteries ?
                          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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                          • #14
                            Another person stuck inside a 12 volt toy box. Why are you using 24 volts? At 48 volt only requires a 40 amp controller saving you big bucks and uses much smaller wire saving you more money.

                            At 2000 watts requires roughly 800 AH battery and at 48 volts 400 AH battery.
                            Last edited by Sunking; 05-23-2018, 09:38 AM.
                            MSEE, PE

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                            • #15
                              Because Sunking... Someone sold me all this garbage as a kit, a guy in Phoenix. I had no idea what I was getting and trusted the dude.. ooops

                              But, in his defense his name was Budget and the price was right. I just had no idea that meant the whole thing would be toast in 6 months... again ooops
                              Last edited by douglasjett; 05-23-2018, 11:28 AM.

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