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Thread: What voltage do i use for calculation

  1. #1
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    Default What voltage do i use for calculation

    Hi all,
    I am in the process of designing my wiring diagram for a number of solar arrays, i have 16 x 200w panels, ratings on panels are 8.27amp and Voc max 32.51. My charge controller is mppt 40amp max input voltage 140v and battery bank 48v.
    I was planning on 4 strings with four panels in series, (Voc max 130.04 still at 8.27a) and then connected to combiner in parallel, (Voc max still 130.04 but 33.08a) now, if i use the ratings on the panels to ascertain my totol amps that is 33.08amps, however i want to be sure, i know i have 3200w from the panels, so what voltage do i use for the division of the total watts to be sure i do not overload the amps that are going to my controller ??
    Many thanks Mike.

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  3. #3
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    Thanks Naptown,
    your answer was a bit short and i believe you mean the VMP of each panel x 4 of the first string ? This again seems to be another big variable, my reason for posting this question was because of the variables, unfortunatly, this doe's not apply to a very expensive charge controller if i get it wrong!!

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    have a look at the link attached,

    note that plasmatronic controllers are PWM but it does explain the caluculation using VMP



    http://www.plasmatronics.com.au/down...zing_V1.10.pdf

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    Thanks Bala,
    Ok, i have got my head around the link you sent, it seems that the bottom line is, forget any calculations for the whole array and just accept what the Imp figure is on each panel, it is great that you have that link but, (and this is a big one!!).
    Why should anyone who simply wants (or needs) an alternative power supply have to try and understand this stuff ? as i found and i am sure that every NEWBIE has found when looking into solar for the first time you get a mountain of technicalities and a bucket load of jargon.
    When you have some understanding you still end up with loads of variables. Everyone who starts with wanting solar are instructed to compile a list of your power needs!! This is the biggest variable of them all, leave a light on, open the fridge ten times instead of five, add a new dvd player, even changing a bulb for a different type screws all that up.
    Two of most expensive things in solar are, panels and batteries, both have fixed outputs and capacities, the only one variable is the SUN, so what if you buy a different fridge, or need to add a security system within two months of doing your consumption list and buying your array, these types of variables are far more likley to happen and cost big money to rectify or as most do, just carry on and then the whole system suffers.
    So my BIG question to all of you is, why not start with the fixed values we know, if we forget the only one variable for now (sun light), and look at the constants, the panels have a fixed output (Watts) and the batteries have a fixed capacity (Amp hours), we must be able to link x amount of panels or watts, to x amount of batteries or amphours and then bring the sun back into it for where you live, (we have charts to show average sun hours for almost all areas of the globe). This then show's how much power you have, not what you need.
    I am sure we all agree, we have to make do with what our systems provide, and we would all use more if our budgets allowed, this is the main problem with the first premiss of solar, we make our consumption list as instructed and then USE MORE, why start there??
    Mike.

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    Why should anyone who simply wants (or needs) an alternative power supply have to try and understand this stuff
    Its like anything, if you dont understand it you dont know what you are being sold, or how to really get the most from your system.

    if you dont understand it then you have to rely on the salesman at the solar shop and that is risky.

    I am sure we all agree, we have to make do with what our systems provide, and we would all use more if our budgets allowed, this is the main problem with the first premiss of solar, we make our consumption list as instructed and then USE MORE, why start there??
    A lot of people dont do the hard work and get a real consumption figure, most cases they will understate the consumption or have no idea about their real consumption, they buy on price not on what is really needed,then the system does not supply their needs, then the batteries fail quickly.

    I live on a stand alone solar system with genset back up, bottom line is stand alone if expensive and a pain in the butt.

    if you dont understand how your system works then you wont manage it properly and you will have problems. You have to learn your OWN system, your weather patterns etc, when there will be excess power to utilize and when to cut back.

    To get good battery life there is no set and forget unless you have an auto start genset and are not concerned about fuel consumption.

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    I got real bad news for you my friend. Your charge controller is not going to work period. With a 40 amp MPPT controller the maximum panel power input vs battery voltage is:

    500 watts @ 12 volts
    1000 watts @ 24 volts
    2000 watts @ 48 volts.

    You have 3200 watts. Back to the drawing board for you. FWIW the voltage you use is the nominal battery voltage. For example 500 watts / 12.5 volts = 40 amps. You are screwed.

    Your option is to buy another 40 amp controller and run a battery voltage at 48 volts. You make 2 1600 watt panel arrays serving a common 48 volt battery bank of around 600 AH.

    The error you made is assuming the MPPT controller current rating is at the input. It is not, it is the output. That 100 Vmps @ 33 amps on the input (3200 watts) gets down converted to 48 volts @ 67 amps. You really need a 80 amp MPPT charge controller, or another 40 amp controller.
    MSEE, PE

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    You could place 3 panels in series (about 95V) and parallel several "strings of three" your your charge controller.
    And you have to buy another charge controller, and maybe 2 more panels to really finish the system, as Sunking said so graciously, you need another controller, the one you have is too small.

    Your site location "record cold temperature" is used to calculate the panel voltage for a cold frosty morning, if you are sure of never getting too cold (panels produce higher voltages when cold) you could place 4 in series, and risk the 130V.
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    Powerfab top of pole PV mount | 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 || || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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    Thank you for your replys,
    Bala, you are right, i would be a very foolish man to have only basic understanding for the project i am doing but, reading this forum many people who wish to have an alternative energy supply do so with a simple idea, "I want to run a fridge" or "I want to power my lights" should they really have to understand 'Plasmatronic controllers' or how to calculate what the VMP of there array is. As for doing the hard work, that may be true but as a support group like this forum can we not make things simpler, when i built my first array in 1998 i quiclky learnt that with all the variables of an off grid system, one of the biggest was MY CONSUMPTION list, which took two weeks to compile, and as you have said most people get it wrong so why is everyone instructed to start there? (instant plan for failer)
    It seems to make sense to me to start with the less variables, X amount of panels, produce X amount of power (factoring in location) and cost X amount of money, with that info i know what i can run and if its not enough try to increase my budget BEFORE i buy the wrong equipment, two 40a controllers are far more expensive than one 80a true!! The same applies to panels and batteries even more so if what you bought in the first place cannot be use to give you what you want, many,many post on this forum has, "not big enough" "get a new one" "cant run that"

    Sunking,
    Thanks for your reply, this was the reason for my original post, i want to be sure that the very expensive equipment i have purchased will not be distroyed by a calculational mistake, looking at the spec for my controller PCM5048,
    max pv input volatage 140v Voc
    max pv input current 40a
    max pv input power 3400w
    Nominal batt voltage 12/24/36/48v
    max Charge current 50a
    My gut feeling is the same as you, the controller is not big enough, but looking at the spec they support my wiring diagram of S4P4. What have i missed?? I am by know means an expert and most of the lessons i have learnt are through trial and error, plus a vast amount of time in front of this pc. I NEED HELP. I know my string voltage is close @ 130v but the array is in Gambia (heat is a big factor) batteries are configured 24 x 2v 440a = 48v, i am happy with the 440a capacity as every system i have seen in Gambia suffer with reduced charge ability (in 14 years no one knows why?)
    Mike,
    Not sure what you mean by "several strings" my thoughts are the same with the voltage (Gambia not cold). This is only one array of six arrays of the same size i have to construct in the comming months (I leave on 7th Feb and very limited internet there), i do have 4 additional controllers PCM 6048 for a number of smaller domestic arrays i also have to construct.
    164 x panels, 160 x 2v traction batts, 48 x 6v gel batts 10 x charge controllers and 8 x inverters, all left last week, the only replacements out there if i get things wrong are very cheep chinese units, this also brings me back to one of the 'bees in my bonnet' forget the consumption i WANT, (it will never be enough) i get what i get and have to make do, we have to adapt to make our systems successful, likewise the more success the members of this forum get the more solar systems are sold and the more we all win with price reductions.
    Is it possible that you guys with the brains and long term experience can come up with a simple formula and multiples of that formula in order to help people use solar. As i have said before, start with the best of the uncertainties. X amount of watts (panels) will give X amount of Amp hours (charge Batteries) then check your location for sun hours and you get X amount of power to use, if thats enough for lights and that's all they want great, if they want to run a fridge or pump then multiply there systems.
    Unfortunately the many people i talk to who have solar, have already made the biggest mistake of all, and that is after making there consumption list and spending sometimes, big money there systems, they do not work for them, and quite often that has nothing to do with there list, it is just that their are too many variables with starting that way, our domestic lives can, and do change sometimes on a daily basis but our systems cannot.
    Any help is very much welcome Mike.

  10. #10
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    Quote Originally Posted by Gambia river lodge View Post
    Sunking,
    Thanks for your reply, this was the reason for my original post, i want to be sure that the very expensive equipment i have purchased will not be distroyed by a calculational mistake, looking at the spec for my controller PCM5048,
    max pv input volatage 140v Voc
    max pv input current 40a
    max pv input power 3400w
    Nominal batt voltage 12/24/36/48v
    max Charge current 50a
    I see said the Blind Man.

    OK I see we had a communication problem. I thought you had a MPPT controller. As it turns out it is a PWM controller and yeah it will work, but you will not be happy with what follows. It is the nature of the PWM controller.

    Only way to use all your panels is to configure it as you say. But here is the problem. It will turn your 3200 watt panel into 1500 watts...

    Go read this and you might understand.

    Short story you need to trade your controller in and get a true MPPT 80 amp controller.
    MSEE, PE

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