Series Panels Better in Low Light for a 12V system?

bdosborn
bdosborn Registered Users Posts: 11 ✭✭✭
I ran across this in another forum and I'd never heard of an arrangement like this. These are two 12V solar modules mounted on an RV.

"I placed two solar modules on my roof (partly for space requirements and since buying two modules was a good deal money-wise)
There is a toggleswitch wired for series to parallel operation. Normally the switch stays in the parallel mode, But when the sunlight is less than optimal I can switch to series and still receive charging. Being too much a cheapskate to buy a MPPT type controller its a good compromise. Often I am camping in winter when it rains for days on end, or sometimes at altitude with snow when the module output is 9-11volts or such. When the input to the controller is lower than the batteries its supposed to be charging at of course the results to capture is zero. At least in series I receive something of a charging vs nothing, but I never have followed up in details to measure exactly the current available."

My thoughts are that a 12V panel probably isn't putting much current when the output voltage is at 9-11 volts so the series output doesn't help. Has anyone played around with this kind of an arrangement and can speak towards their experience?
Bruce

Comments

  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?
    bdosborn wrote: »
    I ran across this in another forum and I'd never heard of an arrangement like this. These are two 12V solar modules mounted on an RV.

    "I placed two solar modules on my roof (partly for space requirements and since buying two modules was a good deal money-wise)
    There is a toggleswitch wired for series to parallel operation. Normally the switch stays in the parallel mode, But when the sunlight is less than optimal I can switch to series and still receive charging. Being too much a cheapskate to buy a MPPT type controller its a good compromise. Often I am camping in winter when it rains for days on end, or sometimes at altitude with snow when the module output is 9-11volts or such. When the input to the controller is lower than the batteries its supposed to be charging at of course the results to capture is zero. At least in series I receive something of a charging vs nothing, but I never have followed up in details to measure exactly the current available."

    My thoughts are that a 12V panel probably isn't putting much current when the output voltage is at 9-11 volts so the series output doesn't help. Has anyone played around with this kind of an arrangement and can speak towards their experience?
    Bruce

    From the theoretical side, this does not make a lot of sense since, for a mono or polycrystaline panel anyway, the voltage output is not going to change drastically as the light input is reduced unless it is driving a load at the same time. And if one panel drops below 12 volts with no current coming from it, it should not be able to deliver much current either when placed in series with another panel at the same voltage. I agree with you there.
    Maybe this is a particular kind of solar panel which does not behave in that same way?
    SMA SB 3000, old BP panels.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?
    bdosborn wrote: »
    My thoughts are that a 12V panel probably isn't putting much current when the output voltage is at 9-11 volts so the series output doesn't help. Has anyone played around with this kind of an arrangement and can speak towards their experience?
    Bruce

    You are correct that in low light conditions panels do not produce much current. However they may produce some, but at a Voltage level too low to pass current to the battery. Therefore placing the panels in series would bring the Voltage up, and thus send current to the battery. (It would halve the current available from the panels in parallel.) That is his theory.

    Here is a key sentence: "but I never have followed up in details to measure exactly the current available."

    It's is doubtful that this series/parallel switching arrangement would produce any significant results in real world conditions.He might even be losing power through the extra wiring necessary to make this switching possible.

    I tried a "three panels in series" array on a 12 Volt system once, with an MPPT controller. The main result I found was the ability to send the array Voc soaring above the input max of the controller, causing it to shut down. There was no noticeable improvement in charge power on dull days.
  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?

    Most likely the people who do this, THINK it's helping, but in reality, as "inet" said, it's the AMPERAGE that drops off as light decreases, and the voltage is the last thing to drop off. Once the voltage starts to drop, any useful amperage is pretty much long gone.
    So no, what you read about on the other form, will not work in actual practice. The idea mostly makes sense only to those who don't understand how solar electric panels behave in low light.
    As Caribeecoot said, he tried it once, but with an MPPT controller, which is a whole different kettle of fish.
    Oh! And Welcome to this form! You'll find it to be possibly the most professional, truly informative and useful forma of it's kind to be found anywhere. No BS here.
  • bdosborn
    bdosborn Registered Users Posts: 11 ✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?

    Thanks for the responses. I found a John Wiles article that summarizes panel voltage and current in low light conditions nicely:

    "Open-circuit voltage is only slightly influenced by
    irradiance. Obviously, in total darkness, the voltage output is
    zero. However, even in dim light (dusk, dawn, heavy clouds)
    the open-circuit voltage is very nearly the STC rated value.
    Direct sunlight does not have to be shining on the module
    for voltage to be on the output terminals. Current may be
    extremely low, but nearly full voltage can be expected in dim
    light. Thin film modules may have different characteristics,
    and the module manufacturer should be contacted for
    details."

    Here's the entire article:
    Solar Linky

    So I guess I won't be rewiring my panels from parallel to series after all. ;)

    Bruce
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?
    bdosborn wrote: »
    Thanks for the responses. I found a John Wiles article that summarizes panel voltage and current in low light conditions nicely:
    "... nearly full voltage can be expected in dim
    light. Thin film modules may have different characteristics,
    and the module manufacturer should be contacted for
    details."

    Here's the entire article:
    Solar Linky

    So I guess I won't be rewiring my panels from parallel to series after all. ;)

    Bruce

    Do you have any way to determine whether the RV owner telling the story was using thin film panels? Some RV and trailer owners are attracted to flexible thin film panels for mounting on a curved roof surface.
    SMA SB 3000, old BP panels.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    Re: Series Panels Better in Low Light for a 12V system?

    Thin Film panels would behave in a similar way.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Series Panels Better in Low Light for a 12V system?

    Short version:

    No light = no current. No current = no power.
    Voltage doesn't matter.
    :cool:
  • gopaul
    gopaul Registered Users Posts: 2
    I have just found this discussion. This is exactly the question I have. I have a bus with 1200w of panels. The whole project has been done on lowest possible cost basis. I have four 250w flexible panels that I purchased new of ebay and two 100w flexibles purchased elsewhere second hand. feeding an epever 100 amp mppt into 630AH of AGM storage. At the time I did the hookup I formed the opinion that series would work best in low light as it would give the controller more voltage overhead to work with in low light. I have the four 250w panels in series and the two 100w in parallel then series with the 250s. This in a spread sheet using the numbers off the panels will give me 1100w max from my 1200w of label ratings. I have just redone the numbers with exactly the reverse connections and on paper I would get a bit over 1250w. right now we are heading into winter here and with a very low sun angle and almost full overcast I am getting 8 to 12 amps at 14v into the batteries. the opinions here seem to tell me to leave things the way they are. Input welcome.Paul
  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    Welcome to the forum Gopaul,

    To connect the solar panels in series, you should match Imp-panels to within 10% or better.

    If you are connecting panels (or strings of panels) in parallel, you should match Vmp-panel (strings) within 10% or better.

    In solar, details matter. What are the Vmp/Imp of the 250 Watt panels, the Vmp/Imp of the 100 Watt panels, what is the battery bus voltage (12 volts?), roughly, near what major city is the system installed, the high and low temperatures, and do you have a link or model number of the 100 Amp EpEver controller?

    Regarding series or parallel connections for panels in Low light... It really does not matter. The solar panel voltage, once there is weak (morning sun as an example), the panels will pretty much reach Vmp. The panel available current will pretty much match the amount of sunlight (more sun, more available current).

    If you have shading, there are lots of arguments if series or parallel connections are better. I am a believer that paralleling panels is better for environments with shade (sail boats are a common application).

    If any panel (or even a single cell) has shading, that panel will typically lose 50% or more of its output power (a single line or electrical wire across a panel does the same thing). There are internal bypass diodes to allow the rest of the string to generate current, but the Vmp of that shaded panel will drop by ~1/2 or more.

    Where MPPT controller are very nice--You can run higher voltage solar arrays (On a 100 Volt input controller, upwards of 65 Volts (more or less, ambient temperatures and exact panel/array Vmp matter too). So, 65 volts into the controller, and ~15 volts out (charging batteries)--Means that you have much less current in the array--Lower current, smaller wire can be used--6 AWG or 1/8th the mm^2 of the copper wire is usually very easy--Great for long wire runs (10's of meters or more).

    Another thing MPPT controllers can do... As the cell temperatures drop (very cold, and sub freezing weather), Vmp of the panels rise, and so does power output (Pmp=Vmp*Imp)... In cold climates, MPPT controller can get another 10-20% more harvest...

    Please feel free to start a new discussion/thread with your details. I suggest it is usually easier to have these detailed discussions in "your own discussion"--Avoids confusion with the original poster's information and questions.

    -Bill "moderator" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Sadly, this whole theory falls apart.
     With almost any amount of light, the panels produce their nominal voltage.   The AMOUNT of light determines how many amps the panels produce - more light = more amps

    If things are extremely dim, there is just not much power no matter how they are wired
    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 ||
    || 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 ,

  • gopaul
    gopaul Registered Users Posts: 2
    Bill Thank you very much for your info. The system is on a converted Bus RV I think in your terms, so perhaps the sail boat analogy is the best one. Batteries are 12v at the moment in overcast with very low sun angles I am getting up to 100v and 2 amps and the meter on the battery bank suggests that within the second decimal of that is getting to the batteries which is in the too good to be true range. I will collect a bit more history and make up some temporary cables to try an parallel option. Again thanks for your info. It is a pleasure to get straight answers. Stay safe Paul New Zealand
  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    Our pleasure Paul,

    Feel free to start another discussion with your setup. Details matter (each panel's Vmp/Imp, how they are wired, what charge controller, what battery bank voltage, location, array angle, etc.).

    The various solar calculators usually let you set the array tilt, and you can see how much (or how little) optimizing tilt for your season/location can help.

    The basic math is:

    Panel rating & amount of sun * Cosine of angle from sun
    Cosine 0 degrees (pointing directly at sun) = 1.00
    Cosine 10 degrees = 0.984
    Cosine 45 degrees = 0.707
    Cosine 60 degrees = 0.5

    Mounting flat to roof "away from the equator" is always going to cut on your harvest. Play with the solar calculator and see what is worth your time and money to "fix".
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Tauranga
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:

    JanFebMarAprMayJun
    6.65
     
    5.88
     
    4.82
     
    3.57
     
    2.50
     
    2.01
     
    JulAugSepOctNovDec
    2.22
     
    2.91
     
    4.03
     
    5.03
     
    5.93
     
    6.58
     

    Tauranga
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 37° angle from vertical:
    (Optimal winter settings)

    JanFebMarAprMayJun
    5.23
     
    5.24
     
    5.21
     
    4.86
     
    4.12
     
    3.66
     
    JulAugSepOctNovDec
    3.85
     
    4.16
     
    4.67
     
    4.75
     
    4.83
     
    5.03
     

    If you can tilt your panels--Can really help your winter harvest...

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