Sixing MPPT Charge Controler

AkoAko Registered Users Posts: 162 ✭✭
Read a lot of articles but still cant get my head around how to work out what size MPPT charge controller i need to install or even if it will be possible considering the mixture of Panels i have . At the moment i have PWM and the system performs fine unless the weather is bad then its hopeless so i am hoping a MPPT would improve things however im confused by the methods given online to calculate everything .

I read that i need to add together things like Array total Volts or Watts but as i have both 12 and 24 volt panels and three different sizes from 75 watts to 140 watts and there's no indication what figures to use . If all the panels were the same it would be much simpler .

This is what i have at present all wired in Parallel either single or as pairs of 12 volts , all open circuits are similar , either around 17 volts for 12 volt panels or 35 for the 24 volt ones . I will not add or remove any panels so i have to make the best of what i have . I have divided the Array into 2 equal sets of 8 panels so i end uo with 2 cables coming from the array with exactly the same voltages and Amps , the 2 cables are later joined together , Positive to Positive and Negative to negative so i have juts 1 cable going into the charge controller .

I would be grateful to anyone who could explain in simple terms what figures i use to work out if its possible to change from my PWM to a MPPT and how to calculate what size unit i will need .

Nominal Panel Voltages Total Voltage Open Circuit Voltage



4 x 24 x 140 Watts 96 140
8 x 12 x 75 Watts 96 140
4 x 12 x 120 Watts 48 70



Totals 240 350




1050 Watt 24v Array . BP and Atersa 72 cell Panels 
Morningstar TS-60 Charge Controler
4 x 6v 400 Apm/Hr Trojan L16 AGM
Solener 1500w Pure Sinewave Inverter

Comments

  • mike95490mike95490 Solar Expert Posts: 9,582 ✭✭✭✭✭
    When weather is bad, I often get less than 10% of panel output.   It there are no photons, no wire arrangement
    will make any more.

    What voltage is your array wired for, and what voltage are your batteries configured to ?
    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 ,

  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    In my humble opinion, a properly designed and installed PWM system will produce about the same amount of energy per day as a properly designed and installed MPPT system.

    MPPT systems, for the most part, are "better" with larger arrays and longer distances from array to battery bank (higher Vmp-array, smaller AWG and/or longer wiring runs at a cost effective price). A "12 volt array" (Vmp~17.5 volts for 12 volt bank especially), needs crazy/heavy/expen$ive copper wiring for longer runs will acceptably (low) voltage drop.

    Also, arrays running into an MPPT charge controller can be "non-standard" (for example Vmp-panel=30 volt) solar panels--Typically high volume/low cost panels intended for Grid Tied Inverter power systems (1/2 the price or even less of Vmp=18 volt panels).

    And because you can run Vmp-array >> Vbatt with "higher end" MPPT charge controllers (Vpanel-max input in 100-300+ volt range for higher end MPPT controllers)--This works well for wide array voltage ranges (as well as smaller diameter copper wiring).

    Yes, you can get a bit more harvest (call it 10-15% or so) in cold climates (typically freezing or lower temperatures), the higher Vmp-array-cold can give you a bit larger harvest. In moderate to higher temperature environments, MPPT has little advantage over PWM system.

    MPPT controllers cost more. An array for MPPT controllers will typically cost 1/2 as much. Wiring will be easier (i.e., 1x 250 Watt "GT" panel vs 2x 125 Watt Vmp~18 volt panels)...

    Down sides--Shipping for large format panels (typically over ~140 Watts) must be packed on pallet and shipped by truck. And moving large format panels will typically need two people to avoid damage/mounting on racks.

    More or less, the Vmp-panel/array is based on temperature of the array. An array will have about the same output voltage on a clear sunny day vs a 1/2 overcast day. An MPPT controller will not really collect much more energy.

    Imp array is (more or less) simply based on Watts per SqMeter of sun hitting the array. Yes, an array on a cloudy day will be "cooler"--But the "extra" array voltage (Vmp rises as panels get cooler) is just not that much increase...

    Just using made up numbers. Vmp rises ~0.0045% per degree C. Say array under 1/4 sun is 20 C cooler, the power increase based on cool temperatures would be:
    • 20C drop * 0.0045% per C power increase =  0.09 = ~9% increase in harvest
    • but under cloudy sky at 25% sun * 0.09 increase = 2.25% increase (or 27.25% actual MPPT harvest vs 25% for PWM harvest vs 100% for full sun day)
    You could not accurately measure that small amount of energy increase without laboratory instruments and a solar intensity meter.

    Regarding your panels:

    4 x 24 x 140 Watts

    8 x 12 x 75 Watts

    4 x 12 x 120 Watts


    4x 140 Watt panels==560 Watts total with Vmp~30 volts or Vmp~36 volts ?
    8x 75 Watt panels==300 Watts total  with Vmp~18 volts?
    4x 120 Watt panels== 480 Watts total with Vmp~18 volts?
    ============================
    1,340 Watt array total...

    If you could (made up numbers--Don't know pricing in Spain--Including shipping/delivery) buy 4x 350 Watt panels or 1,600 Watt array for ~USD $800 -- Would you do it and sell/donate your other panels?

    And what is your battery bank voltage (12/24/48 volts)? What is your battery bank Amp*Hour capacity?

    Do you have an MPPT controller you "like" (link/specs)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AkoAko Registered Users Posts: 162 ✭✭
    edited April 18 #4
    " What voltage is your array wired for, and what voltage are your batteries configured to ? "

    Checked now and all my panels are vmp 18 or 36  and battery bank is 400 amp/hrs AGM . I dont have a specific size controller in mind other than i though of buying a Victron MPPT one 

    Array either 24 volt panels or 12 volts wired as pairs then all wired in Parallel as 24 volts . There are 2 heavy cables leaving the array  , each cable is 24 volts and around 24 amps , they then meet up just before the charge controller and connected in parallel and enter the Controller as 24 volt 48 amps when the suns shinning . Battery bank also 24 volts .

    Summer is never a problem , winter is when we occasionally get the cloudy conditione with around 10% orf our normal input , its also much colder through the season , thats when i am looking for ways to increase the meager inputs without adding panels if at al possible . Rewiring Panels and changing equipment is all i wish to do . MPPT was considered as raising the voltage to 48 or more would allow the possibility of using Lithium batteries which would not require a regular full charge and can be cycled much lower . I don't cycle our AGM below 50% ever , i start the generator and Battery charger when it gets close .

    My Array is in the open and well away from anything that could cast a shadows over any part of it .
    1050 Watt 24v Array . BP and Atersa 72 cell Panels 
    Morningstar TS-60 Charge Controler
    4 x 6v 400 Apm/Hr Trojan L16 AGM
    Solener 1500w Pure Sinewave Inverter
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    Solar panels are, more or less, current sources... They have a relatively stable Vmp regardless of the amount of sun. Vmp is (mostly) affected by temperature.

    Remember that Power = Voltage * Current. And Vpanel approaches Voc (voltage open circuit) as current drops. And approaches zero volts as current approaches Isc (short circuit current).

    Here is a thread with some discussion of mixing solar panels:

    https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

    And a post with the solar panel output graphs (IV Curves):

    https://forum.solar-electric.com/discussion/comment/359660#Comment_359660

    Running X number of panels in series does not increase harvest in poor light--Panel output current (Imp) is pretty much based on the amount of solar energy received at the moment.

    A PWM controller is "just as efficient" as a MPPT charge controller when you "match" the array Vmp-array to battery bank voltage through PWM controller.to battery bank voltage. (PWM or Pulse Width Modulation is an "on/off" switch from Array to Battery Bank--No voltage/current conversion takes place)

    An MPPT charge controller is a switching (typically Buck Mode or voltage reducing) digital power supply. Designed to efficiently down convert high input voltage to lower output voltage.

    A MPPT controller is much more efficient when you have a Vmp-array "miss match" to battery bank voltage. For example there are Vmp=30 volt solar panels that are much cheaper (and larger panels in wattage and physical size) vs the "12 volt" solar panels. For example, Vmp=30 volts is not high enough voltage to charge a 24 volt battery bank and 2x in series for Vmp-array=60 volts cannot use a PWM controller to charge the battery bank--Where as an MPPT charge controller can take the higher voltage/lower current array output and efficiently down convert to the lower voltage/higher current needed to charge the battery bank. MPPT is sort of like the DC equivalent of using an AC Transformer.

    But details matter here... "12 volt and 24 volt" panels are not specific enough. A 12 volt panel is typically around Vmp= 17.5 to 18.0 volts or so... That means Vmp-array-hot is high enough to charge a lead acid battery at ~14.75 volts (Vmp falls as panels get hot, voltage drop in wiring and charge controller, etc.).

    For 24 volt panels... There are typical panels with Vmp=30 volts, and Vmp=35-36 volts, and some other voltages too. For your panels we need the actual Vmp rating of the panels (30/35 Vmp or other?).

    Just changing out from PWM to MPPT and rewiring your present array to higher Vmp-array will not increase your harvest (assuming your "24 volt panels" are Vmp~35-36 volts). And if your 24 volt panels are Vmp~30 volts--They are not a good mix with "12 volt/17.5 volt) panels.

    If you choose to get a new MPPT controller because it has Li Ion charge settings, possible other features (better networking/logging of data, etc.), then I would also rewire your Vmp~35 volt Vmp array to ~70 Volt Vmp array. It will be "better" (more efficient) for use with an MPPT controller.

    Note--A Vmp-array set to 70 volts under standard temperature can over voltage a 100 Volt panel input rated MPPT controller if you get very cold weather (Voc-array~85 volts, get below freezing, Voc-array-cold can be over 100 Volts)... You would want a 140+ rated charge controller (roughly).

    Details matter here... Your exact panel Voc/Vmp/Isc/Imp ratings, your min/max temperatures, and exact brand/model/ratings of the charge controller.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AkoAko Registered Users Posts: 162 ✭✭
    Temperatures are from 8c min in Winter to 38c Max in Summer . Panels are good quality , all are Atersa except for 4 x75 watt 12 volt ones which are BP . A the moment i have a Morningstar TS60 PWM but thinking about changing to a Victron MPPT .
    1050 Watt 24v Array . BP and Atersa 72 cell Panels 
    Morningstar TS-60 Charge Controler
    4 x 6v 400 Apm/Hr Trojan L16 AGM
    Solener 1500w Pure Sinewave Inverter
  • BB.BB. Super Moderators, Administrators Posts: 32,642 admin
    If I use the typical -0.33% per C Voc change vs temperature...
    • [85 Voc-array * (8C - 25C Std) * -0.0033 per C 85 Voc-std-series] + 85 Volts Voc-std = 89.8 Volts [email protected]
    So, you do look OK. (assuming ~42.5 volts Voc for 24 volt panels, or 21.7 Volts Voc for 12 volt panels).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • UpNorthManUpNorthMan Registered Users Posts: 18 ✭✭
    Hello Ako
    From the information provided, I only see one  combination of your existing panels that will work. If new panels are out and  you want to switch to a mppt charge controller, see the string below.

    2s 140w = 70voc
    2s 140w = 70voc
    4s 75w = 70voc
    4s 75w = 70voc
    4s 120w = 70voc

    Fuse each string in your  combiner. Circuit breakers work nicer in my  opinion. If you need to troubleshoot your system, you can isolate your strings.

    More panel information is needed for more a accurate answer.

    I  made the assumption that you have a 24v  system.  70voc will work.

    Ed

    1000W panels Kid CC 184 ah battery bank @ 48v
    Weekend cabin, 250w inverter.
    Looking to upgrade inverter!
  • AkoAko Registered Users Posts: 162 ✭✭
    This is getting very confusing , after weeks of asking the dealer has finally got back to me to say i need a 100/80 Victron MPPT Charge controller for the panels given above , the cost of the unit would be $756 . Very odd considering i believed i had a small system compared to many so im wondering what the others use and how small the system that use the smaller Charge Controller's might be .
    1050 Watt 24v Array . BP and Atersa 72 cell Panels 
    Morningstar TS-60 Charge Controler
    4 x 6v 400 Apm/Hr Trojan L16 AGM
    Solener 1500w Pure Sinewave Inverter
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