the best way to connect 30v panels to 12v batteries?

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karl
karl Registered Users Posts: 6 ✭
in Off Grid Solar & Battery Systems #1
Hi all,

i've enjoyed reading the posts on this forum for sometime but this is my first post so apologies upfront for asking anything daft!
i have been living off-gridd in the south of france for about a year now and am looking to upgrade my panels to a much bigger system but wanted to chack it will work before i go ahead and spend.
my current system consists of: 680ah 12v lead-acid battery bank (to be doubled), morningstar tristar 60A mppt controller, 2x 100w 12v panels and 2x 130w 12v panels.
i have the possiblity to sell existing panels and buy 6x 250w panels of this spec: max power voltage 30.96v
max power current 8.07A, open current voltage 37.92v, short circuit current 8.62A, cell efficiency 17.90%.
these panels are really for grid tie but are available cheap so i would like to make it work, also i need to keep battery bank at 12v as my sine wave inverter is 12v input and they cost an arm and a leg.
so i guess what i want to know is what is the best way and highest voltage i can wire the panels? my charge controller docs say that it will take 150voc but only mentions 12, 24, or 48v as possible configerations. any help would be much appreciated. thanks, karl.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: the best way to connect 30v panels to 12v batteries?

    Welcome to the forum Karl.

    South of France, off grid--Hmm, sounds like you have an interesting life there.

    Regarding maximum array voltage--The issues:

    There is the maximum input voltage of the charge controller. 140-150 VDC is very common for the larger/higher end MPPT charge controllers, and when you take into account cold temperatures and the fact that solar panel Vmp/Voc (voltage maximum power and voltage open circuit) rise in cold weather, rough Vmp~100 volts (STC--standard test conditions--spec. sheet numbers), around 100 VDC is usually about the highest voltage you can wire an array (many vendors have a tool for "sizing" the solar array--so you need to check the numbers for your setup).

    However, there are now more MPPT charge controllers that have higher maximum input voltages... Midnite Solar is one. Their maximum input voltage is 250 VDC plus battery bank voltage (12+250=262v max; 48+250=298v max). Note, typically the controllers are a bit less efficient (a percentage point or two--not a huge amount) with very high input voltages vs low battery bank voltage (most efficient is Vmp~2x Vbatt).

    And there is the Xantrex/Schneider 600 VDC max input voltage MPPT Charge Controller--But very expensive and "mixed" customer support issues (should be a pretty good controller though).

    Is your array pretty far from your Battery Bank/Charge controller (10-30+ meters) and you want high array voltage so you can use smaller gauge wiring--Or are you just looking for Matching your array (GT panels) to your system?

    Personally, if you can sell your existing panels and replace the whole a array with a set of larger panels--Probably a good thing. Less wiring issues and mix/match issues with old and new panels (although, it is perfectly OK to put two or more charge controllers in parallel to a battery bank).

    Another issue--A very large/high amperage 12 volt battery bank. Ideally, you should be charging the battery bank at 5% to 13% (rough rule of thumb) rate of charge. 10% of 680 AH battery bank would be 68 amps--Just about the maximum charging current for most MPPT charge controllers--So any more current (larger battery bank), you would need another charge controller to feed the bank.

    Vs going to a 48 volt battery bank... Which would still have ~60-90 amp maximum, but because of the 4x higher voltage, that can manage a 4x larger array (power=volts*amps; 4x the voltage, 4x the power).

    Also, I am not a big fan of lots of parallel batteries... More cells to check for water, more wiring to go bad (should have a fuse/breaker per string for fire protection--added expense), and when you get high power inverters/charge controllers, you need a lot of copper to manage the current and keep the voltage drop low (especially critical for 12 volt systems where you can have about a 0.5 to 1.0 volt maximum). I like to recommend 100 amps * bank voltage as the rough maximum recommended power (12 volts * 100 amps = 1,200 watts, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
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    Re: the best way to connect 30v panels to 12v batteries?

    Karl, welcome to the forum. Many folks realize after a year or more that they want or need to expand their system. Many of them started with 12 volts and feel that they have 'painted themselves into a corner' because of their 12 volt investment. Now would be a good time for you to sell your 12 volt inverter and start again with a 24 or 48 volt system. You can keep your present battery bank and just reconfigure it to be 24 or 48 volts. (I am assuming that you have some batteries in parallel that could be reconfigured to 24 or 48 volts). Do not mix old with new batteries! When your present batteries die, that is the time to upgrade to new larger batteries. Also, you can keep your present panels and controller and add new panels and new controller to the same battery bank.

    Another way to deal with your old system is to keep it and build a 2nd system at 24 or 48 volts.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #4
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    Re: the best way to connect 30v panels to 12v batteries?
    karl wrote: »
    ...680ah 12v lead-acid battery bank (to be doubled),...
    unless the battery is young I wouldn't add batteries
    karl wrote: »
    ... morningstar tristar 60A mppt controller,
    a good controller for higher voltage panels, since the MPPT type of controller will down convert to output you choose.
    karl wrote: »
    ... buy 6x 250w panels of this spec: max power voltage 30.96v
    max power current 8.07A, open current voltage 37.92v, short circuit current 8.62A, cell efficiency 17.90%. These panels are really for grid tie but are available cheap so i would like to make it work,...
    They will work with your MPPT charge controller, but you controller is rated for 60Amps at out put you have in theory the potential of 1500watt array/12volt minimum out put amperage of 125 Amps (not gonna happen, but well above the capacity of the Charge controller, so You would need to buy another of these some what exspensive charge controllers
    karl wrote: »
    ...i need to keep battery bank at 12v as my sine wave inverter is 12v input and they cost an arm and a leg.
    so i guess what i want to know is what is the best way and highest voltage i can wire the panels? my charge controller docs say that it will take 150voc but only mentions 12, 24, or 48v as possible configerations....
    These are output nominal voltages...

    Suggestions; use the batteries you have an reconfigure to a 24V system if possible? and save the money you would need to spend on a new charge controller and use the one you have at 24 Volts it should handle the 1500 watt array, and put the money toward a new inverter.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • CATraveler
    CATraveler Solar Expert Posts: 98 ✭✭✭✭
    #5
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    Re: the best way to connect 30v panels to 12v batteries?

    Morningstar has a calculator where you enter the panels, contoller, temperature and they calculate array configurations. I'd guess for 12V you would use 2(parallel)x3(series) or 3x2. A wiring calculator will help to determine the wire size requirements.
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  • karl
    karl Registered Users Posts: 6 ✭
    #6
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    Re: the best way to connect 30v panels to 12v batteries?

    thanks for the quick reply everyone.
    well this has thrown up some problems! i didn't realise that my controllers 60A was measured on the output, i assumed it only related to relay size! now cableing is not an issue as everything is within 2 metres but the battery bank is made up of 6 2v cells each 680ah and as they are less than a year old i thought i should be able to get away with adding another bank of the same type/capacity. i hope they won't need replacing for a long time as they are new "forklift" batteries so the best i could do would be to wire them and the new string to give me 24v and buy a new inverter as suggested this way i up my relay size and also the capacity so i am covered for 2 or 3 cloudy days. i do also have a spare stecca 12/24v 30A pwm controller so i could perhaps run my old panels through that and buy less of the others enabling me to buy a new pure sine wave inverter in 24v... wish i had had more knowledge when i started down this rd in 2008! my aim when i bought a ruined barn over here was to live off-grid but with a normal level of comfort as in laptop, fridge/freezer, internet and electric lighting with all heating and cooking on wood or gass, but although in summer i managed for months with only a 460w array at this time of year i need a good bit more. if money was no object then i would start over in 48v but with the renovation still not finnished (i am doing everything myself as well as working to pay for it) that is not possible.

    just as a quick addition, i also have a 500w 220v turbine powered by a stream that runs for about 5 months in winter and was thinking of running a 10A charger from that to trickle charge the bank/cover some drain 24 7, any thoughts on whether this would be worthwile or not?

    thanks again, karl
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  • CATraveler
    CATraveler Solar Expert Posts: 98 ✭✭✭✭
    #7
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    Re: the best way to connect 30v panels to 12v batteries?

    How about staging for the future? With a good panel price buy 6, use the max allowed by the conroller. Then second battery to run in parallel until you can upgrade inverter to 24V.
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  • karl
    karl Registered Users Posts: 6 ✭
    #8
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    Re: the best way to connect 30v panels to 12v batteries?
    CATraveler wrote: »
    How about staging for the future? With a good panel price buy 6, use the max allowed by the conroller. Then second battery to run in parallel until you can upgrade inverter to 24V.

    think that is my best option! thanks for all the help
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #9
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    Re: the best way to connect 30v panels to 12v batteries?

    HI Karl

    Yes i would recommend switching to 24V if possible- it's better for everything. As for the inverter... what size inverter are you looking at and what makes? In my experience they're not that expensive, unless you are looking at a very high kW. I could give you some indications on the prices you should be looking at so you have a point of reference if you like

    Cheers
    Larry

    ps just to be nosey, what part of France are you in, I have a "ruin" in the Borgogne that I hope to make renewable one day (and habitable :))?
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  • karl
    karl Registered Users Posts: 6 ✭
    #10
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    Re: the best way to connect 30v panels to 12v batteries?

    Hi Larry,
    i've done a bit of research and they are not as bad as i at first thought. £250-500 for pure sine wave 1500-3000w. i don't need anything too big as the only large draw items are my power tools and for them i run the genny. i've managed with a 1500w for a year now and hardly use more than 150w at one time.
    i am situated in herault near st pons de thomieres 34220, about an hour directly west from beziers on the med coast. i traveled a lot before finally settling here but new the area as a child as it is my mothers birth place (i'm from cambridge england originaly).
    good luck with your project, let me know if you have any questions regarding it i would be happy to help if poss. i have learnt that you need 2 things above all, a strong back and the ability to accept that once started there can be no walking away until the end!
    that said it is a very character building excercise.
    all the best, Karl
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  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
    #11
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    Re: the best way to connect 30v panels to 12v batteries?

    HI Karl,

    Sounds great, I think i passed through there once, lovely area... I spent a very meromable summer in Cap d'Agde when i was 22.. oh those were the days. I think the sunshine is better than the west coast of France... but i suppose it's a lot less than we have down here in Andalusia, which means more panels. We'll see what happens with my project, ta.

    As regards to the inverter, my advice is not to skimp on quality. We see so many failed installations for cheapo inverters to save a couple of hundred quid, and it can provoke short circuits and other dangers, damage equipment and is generally a false economy. Always go pure sinewave, and the lower wattage of a good make, will cope with much higher demand than a higher wattage cheapo make.

    We use the make Victron, good quality vs costs. However, a bit more expensive than you may have been looking at. For example, a 1600W/24 inverter will cost you around 800€ mark. There are cheaper 1200W and 800W versions that deal with peak currents of up to 2000W. The decision is yours, but I would definitely look into all the options before choosing.

    Good luck with it all!

    Larry
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  • karl
    karl Registered Users Posts: 6 ✭
    #12
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    Re: the best way to connect 30v panels to 12v batteries?

    Hi Larry,
    cap d'agde eh... i can imagine! thanks for the info i will be looking at better quality and lower wattage as you say. as in most things in life you get what you pay for.
    enjoy the andalusian sun!
    Karl
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #13
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    Re: the best way to connect 30v panels to 12v batteries?

    If you do not need a lot of power--You could stay with 12 volts and use a MorningStar 300 watt (600 watt for 10 minute) 12 volt TSW inverter. Has a couple of low power configuration options which are great too--It can shut down if there are no AC loads >6 watts, and you can wire up a small switch for remote on/off.

    This is the 120 VAC 60 Hz version, but they do have a 230 VAC 50 Hz version too. There are very few smaller inverters (especially with 12 volt input) that have these functions at this low of price. And so far, the inverters have been reliable for the folks here (as always, plan your finances on replacing your "electronics" (inverters, charge controllers, etc.) every ~10+ years).

    Use this for your small but critical loads, and get a big/cheap TSW or MSW inverter for the power tools, etc. (note, on MSW, motors can draw upwards of 20% more power--So continuous loads like refrigerators/compressors/and small transformers/bump in the cord power supplies can overheat and/or fail on MSW).

    All About Inverters
    Choosing an Inverter - Home Power Magazine

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • karl
    karl Registered Users Posts: 6 ✭
    #14
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    Re: the best way to connect 30v panels to 12v batteries?

    thanks Bill, after looking at all the options and costs and also reading a lot of off-grid related stuff on this forum i am going to stick with my system as is but add 3x 250w panels to what i have already giving me about 1kw through 2 controllers. the battery capacity i have is not perfect but when is it? the most important thing is to be able to charge them up from 50% to full in 1 short winters day as it is unusual to have more than 3 or 4 bad days in a row whithout at least 1 good day here. i also intend to get a good silent diesel genny in the future as funds allow and set up with an auto start then i wont have to worry about the batterys dropping below 50% when i am not looking! though setting one of those up to cut in and out at the right time might have me pestering you guys again some time ;)
    once again thanks for all your help. ps love the pics of your place i found here on a thread about off-grid, looks peacefull.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #15
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    Re: the best way to connect 30v panels to 12v batteries?

    There are mixed views on auto-start gensets vs manual start (electric or hand start)... Things can go wrong with autostart too.

    If you power needs are near zero when you are not there (say you can turn off refrigerator/etc. when you are not there)--You probably can manage a manual operation genset just fine.

    And--I would suggest putting your money into a Battery Monitor (Victron out of Europe is very good too) of some sort--Voltage is important--But a Battery Monitor tends to be more accurate and easier to explain to guests/spouse/kids when "the battery bank goes below 50%, start the genset until it is >80% state of charge--And call me if you cannot start the genset or battery SOC continues to fall"...

    And--those were pictures by other folks... I have a home in a suburban ara south of San Francisco CA and two block from (probably the first) shopping malls/etc.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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