"You cant use a 36cell panel on a 12v mppt controller..." says one forum user

ApplesApples Solar Expert Posts: 39 ✭✭
zoneblue

You cant use a 36cell panel on a 12v mppt controller...
Post # 17 in thread titled My first off grip solar power system questions

The thing you have to realise is that mppt isnt magic. It introduces ineffciencys of its own, the internal buck converter is only ever going to be 90% effcient, and this combined with higher tare loss reduces the gain they get from your average panel to battery voltage differential. Also MPPT controllers need quite a bit of voltage headroom. You cant use a 36cell panel on a 12v mppt controller, you need something like twice the battery voltage to give it some room to down convert.

Since user zoneblue has not responded for two days, could someone else explain?

I ask, because I seem to be using, successfully, one or two (depending on cloud cover) 36-cell PV panels and a Morningstar SunSaver MPPT 15L to properly charge (according to the battery manufacturer's recommended charge profile) a 100 amp hour AGM battery.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    I am not sure I would say that MPPT controller won't work... But when all is said and done, there is probably no advantage to using an MPPT controller on a 36 cell (Vmp~18 volts) array unless you are trying to harvest power in sub freezing weather, where you might get a 10-15% improvement in havest over PWM type controllers.

    An issue with MPPT or PWM type controllers is in hot weather. The Vmp is reduced by quite a bit.

    Say your panel runs 30C over ambient in hot/sunny weather (probably near typical worst case). And it is 35C with a Vmp depression of -0.45% per degree C:
    • 17.5 Vmp * -0.0045 per C * (35C ambient + 30C rise - 25C standard temp) = -3.15 volts due to temperature Vmp
    • 17.5 Volts Vmp (STC) - 3.15 volts hot panel depression = 14.35 Volts [email protected] ambient (estimate)
    Depending on your battery, battery temperature, charging type (equalize, absorb, float), and battery type (flooded cell, AGM, etc.)... 14.35 Volts Vmp is pretty low for charging flooded cell batteries (typically 14.75 to 15+ volts)--Without taking wiring and charge controller voltage drop into account.

    Now, the "knee" on the IV/Power curve is not a sharp point, and you have 1-2 volts that you can be off and still receive a "reasonable" amount of power--But if you are trying to fast charge a cool flooded cell battery in the desert on a hot day--A Vmp~18 volt panel is probably not going to give you full power/current at that time.

    If you have an MPPT type charge controller (and it can accept higher input voltages), placing two Vmp~18 volt panels in series to charge a 12 volt battery bank is usually optimum for the typical MPPT charge controller. Your "low voltage/hot array" Vmp voltage problem goes away... And you can use much thinner cable from the array to the charge controller (high voltage, less current, less votlage drop, more voltage drop allowed by design, etc.).

    I don't know what a typical MPPT charge controller "needs" for voltage drop to work in MPPT/Charging mode--But I would not be surprised if 1 volt or greater is needed (to make buck mode regulation work as designed). A PWM controller may have a bit less voltage drop (depending on its switching electronics design).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CALLDCALLD Solar Expert Posts: 230 ✭✭
    When I moved from a 24v PWM charge controller to a 150/70 MPPT I did notice what you are talking about.

    MPPT charge controllers need a specific set of conditions to outperform a PWM charge controller. In some circumstances they will actually be worse!

    My Victron MPPT needs at least a 2 volt drop from the PV to the battery to work. A PWM doesn't need that voltage drop. So with a single 36cell panel in hot weather an MPPT will most likely not be able to get the batteries up to the absorb voltage even! However in very cold weather you will start noticing the advantage especially with a heavily discharged battery.

    The main advantage of MPPT is the ability to use a higher voltage solar array - like a 72cell panel with a 12v battery. This allows longer wire runs or thinner wire gauge.

    MPPT also gives better performance in cloudy weather where panels are cooler and produce higher voltages. MPPT also performed better when a heavy load is placed on the battery by responding with increasing the charge current as the battery voltage sags under the load.

    90% efficiency is poor though, mine does quite a lot better than that - 97% is what I typically see. Obviously this is dependent on having a high voltage array. Mine is 90v for a 24v battery. If I had a 36v array I might not even get 50% on a very hot day with a near fully charged battery...
  • ApplesApples Solar Expert Posts: 39 ✭✭
    Thank you very much, gentlemen, for your replies!
    If you have an MPPT type charge controller (and it can accept higher input voltages), placing two Vmp~18 volt panels in series to charge a 12 volt battery bank is usually optimum for the typical MPPT charge controller. Your "low voltage/hot array" Vmp voltage problem goes away... And you can use much thinner cable from the array to the charge controller (high voltage, less current, less votlage drop, more voltage drop allowed by design, etc.).


    Ah HAH! (and although my setup described in the OP seems to work and work well) I do like! this idea. So I can take my two 100W panels, connect them in series! and use just ONE of my 10ga copper extensions, increasing my systems efficiency. Again, this is a Morningstar SunSaver MPPT 15L... but how can I ascertain if it will "do" 24V in / 12V out? and not 24V in/24V out?
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Apples wrote: »
    but how can I ascertain if it will "do" 24V in / 12V out? and not 24V in/24V out?

    You connect the controller to the battery before you turn on the PV input to the controller. When the controller sees the battery it will configure itself to the battery's voltage.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    The Sunsaver MPPT 15 controller is very nice. Vpanel input is 75 Volts maximum (Vmp-array ~ 54 volts STC).

    One suggestion--Get the Remote Battery Temperature Sensor for that unit (if you do not already have one)--The on-board sensor tends to read "hot" and suppress the optimum battery charging voltage/set point.
    Apples wrote: »
    but how can I ascertain if it will "do" 24V in / 12V out? and not 24V in/24V out?

    There are DIP switches and a computer interface (option) to set the charging voltages.

    http://www.solar-electric.com/lib/wind-sun/SS-MPPT-15L.pdf
    http://www.solar-electric.com/lib/wind-sun/SSMPPT15-Owners.pdf

    Otherwise, I am not quite sure what your question is asking???

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ApplesApples Solar Expert Posts: 39 ✭✭

    Thanks Bill.

    I've already modified the SunSaver to my battery maker's charge voltage setpoint specifications via my laptop and Morningstar's MS View software interface. I'm not experienced enough to write a tutorial; far from it! but the system works very well now.

    My question is once I input 24 volts (my two 12V panels in series), will the controller remain outputting 12V (one 12V battery source)? ...for lack of a better way of asking it.

    Yes, have already added the RTS and the RM-1 meter. Great little system.
  • ApplesApples Solar Expert Posts: 39 ✭✭
    After browsing the paper booklet Operator's Manual for this SS MPPT 15L, I *gather* that, yes, although PV input is limited to 75V max, there's an example on Page 8 depicting three 12V panels wired in series effecting a 36V input...

    So it would SEEM, based on my limited! but hands-on experience, one could choose either 12V, 24V or 36V (or even 48V etc etc etc up to 75V) for input into the controller, while the controller determines battery voltage (within a specified range) on it's own, and will continue to output for whatever voltage battery (or bank) is connected to it. Does that make sense?

    I am not very willing (but I am certainly excited!) to connect 24V PV input just yet, until I hear a second and/or third opinion. Being this is Saturday, I can't call Morningstar tech support. Thanks again for your time.

  • ApplesApples Solar Expert Posts: 39 ✭✭

    MY APOLOGIES TO vtmaps! I missed his post because my eye went straight to Bill's reply.... my bad.

    I believe he had already answered my question!

    Thank you very much, sir!
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    For modern MPPT controllers, the input array voltage is automatically controlled by the charge controller. Only the output voltage to the battery bank is programmable. (Of course, within limits)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ApplesApples Solar Expert Posts: 39 ✭✭
    Gotcha.

    It is programmed (I now remember a spread sheet) for a 12V output, and that was one of the items that was not adjusted in an edit then reprogrammed to the device.

    Thanks again for all your help, everyone. I'll update this thread when I reconnect the PV in this system. It is a very small system within a camp trailer and when not in use, the battery is connected to "shore" or, the grid and is floated.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Apples wrote: »
    MY APOLOGIES TO vtmaps! I missed his post because my eye went straight to Bill's reply.... my bad.

    Well, apparently my reply was wrong... I didn't realize which controller you were asking about. Many controllers do self-configure, but Bill points out that the SunSaver uses dip switches.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    It may self configure too. I did not read the manual.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ApplesApples Solar Expert Posts: 39 ✭✭
    The paper manual included with the product does not touch upon those exact details. It does give instruction and settings to choose different switch combinations, but no where on paper does it mentioned how output is configured. It does, however, once connected with MS View. I'll have to go in and have another look. I only asked here first to save myself the effort...

    Thanks again.
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    This controller automatically selects 12/24 volt, and the dip switches set the "fine charging" parameters. From the manual page 9:
    Before connecting the battery, measure the battery
    voltage. it must be over 7 volts to power the controller.
    For 24 volt systems, the battery voltage must be greater
    than 15.5 volts to properly detect a 24V battery. the 12/24
    volt battery detection is automatic and the check is only
    performed at start-up.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ApplesApples Solar Expert Posts: 39 ✭✭
    Thanks, Bill. That must be from a manual read online. Only mention of it on paper is at the bottom of Page 7 - "Minimum Battery Voltages: 12 V Battery 7 V ... 24 V Battery 15.5 V ... DO NOT INSERT A FUSE AT THIS TIME"

    I appreciate you looking further into this. I'm now confident I can go ahead and connect my PV in 24V. This is interesting stuff. What happens now to amperage going in (wiring two 100W panels in series to input 24V rather than parallel for 200 watts @ 12V)? Then what goes to the battery? I'll find out later this morning! Thanks again.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    The amperage going in from series panels would be the common Imp of the panels, assuming no shading.
    The MPPT CC changes the voltage and the current to maintain the same power going out as coming in. It is analogous to what happens with AC going through a transformer. Cutting the voltage in half allows it to double the current.
    SMA SB 3000, old BP panels.
  • ApplesApples Solar Expert Posts: 39 ✭✭

    Thank you inetD.

    When connected in parallel in southern California's June sun, these two 100W Renogy monocrystalline 36-cell panels through this SS MPPT 15L CC have sent 11.56 amps to this single, 100Ah AGM battery. That's well over 5.5 amps per panel, and producing, too, an absorb voltage of 14.7V. Equallization has been turned off. Float is 13.6.


    vtmaps

    Well, apparently my reply was wrong...

    Apparently, you are mistaken. Your reply was correct, my good man.

    Thanks again everyone for your help. Coming up, on the hour, I'm going to connect these two panels in series. The battery is enjoying a one-hour rest from an 8 amp discharge of one hour which followed a 1 amp draw per hour overnight for 14 hours from a resting 12.9+ V.

    Again, I'm a no0b at this so please pardon my tech jargon errors...
  • ApplesApples Solar Expert Posts: 39 ✭✭
    After an hour's rest the battery no-load volt reading was 12.66.

    In full, high sun (11 a.m. PDT), these two panels connected in series initially produce 34V and over 11.5 amps, with voltage dropping slightly as the panels warm in the sun. Charger is in it's bulk phase, with the amp reading varying only slightly.

    At 11:15, panel input voltage read (on this Morningstar RM-1) 31.93, battery 13.53, amps to the battery 10.96. CC still in bulk phase.


    Presently the benefits from a 24V input appear to be that I'm able to use just one 10 ga copper extension instead of two. Amps to the battery from the controller remain similar, as expected. Overall, it's nice to have the capability to "switch" back and forth depending on this camp trailer's local surroundings and weather conditions. Pretty neat stuff. I suppose the bottom line here is that I'll never again need two PV cables to the controller location. Again, this system does appear to work well when only one 36-cell panel is used to power this Morningstar SunSaver MPPT 15L. I can still use this single-panel configuration when the trailer is running down the road, en route, to continue to charge.



    Edit:

    At 11.30 a.m., 31.39V and 11.00 amps continue to the battery. Battery V reads 13.71 CC remains in bulk phase.

    At 11:45 a.m., 31.53V and 10.96 amps continue to the battery. Battery V reads 14.07. Controller in bulk.

    At 12:00 p.m., 34.52V and 10.0+ amps but tapering. Battery is nearing the programmed 14.7V showing 14.63. Contoller now in absorption phase, which will last 3 hours (180 mins is the setting).


    OK, that's enough out of me for now. Enjoy your Sunday, everyone.
  • SkippySkippy Solar Expert Posts: 301 ✭✭
    Sounds like you are talking about what I just picked up for my system . . ss-mppt-15L charge controller....

    I currently have mine connected to my 3 - 110 watt panels. . putting out about 60 volts at the solar terminals . . connected too a 24 Volt battery bank . . at the battery terminals, I believe it was reading around 26 - 29 volts . . I can check the readings if you want, but it is working great ! Right now, the batteries are old and worn out, but I can still run my solar water heater pumps off this set up - kill - a -watt meter is telling me I am using 1 KWH a day for running the pumps, as well as running an air purifier during daylight hours (say 9am to 6 pm) and that uses 50 watts per hour . . keeping the battery meter at full - 100 % reading . . Attachment not found.


    I LOVE IT ! I have solar power that I can actually use now :) I figure, that when I am ready to expand, I may get a large 300 watt panel and one more of these units - or can you guys suggest a cheaper alternative ? right now I am looking at 300 $ for the panel and 400 for the inverter - so - about 700 $ for 300 watts - Any comments ?

    I am looking forward to the cooler weather.
    2 - 255W PV - Tristar 45 MPPT CC / 3 - 110W PV -wired for 36V- 24V Sunsaver MPPT CC / midnite bat. monitor.
    1 KW PSW inverter 24V / 2.5 KW MSW inverter-24V ~ 105 AHR battery.
    3 ton GSHP.- 100 gallon warm water storage / house heat - radiant floor / rad
    Apricus solar water heater with Tempra 12 on demand for backup.
    9 -220W PV - net meter - Enphase inverters and internet reporting system.
    420 Gallon rain water system for laundry.***  6" Rocket Mass Heater with 10' bed for workshop heat.
    Current project is drawing up plans for a below grade Hobbit / underground home.
  • ApplesApples Solar Expert Posts: 39 ✭✭
    or can you guys suggest a cheaper alternative


    I am not qualified to recommend anything I haven't used... but I'm glad to hear you like your SunSaver MPPT 15L. Mine's working well!

  • ApplesApples Solar Expert Posts: 39 ✭✭

    I should add that my MPPT charge controller works well too with just one 36-cell panel. Almost 6 amps in full sun, and when the controller enters it's absorption phase, one panel is all that's needed. Again, I only added the second panel for use during days of coastal cloud cover, for the added amperage to speed bulk charging. This second panel is not absolutely needed.

    skippy

    I may get a large 300 watt panel and one more of these units


    Have a look at the SunSaver MPPT 15L Operators Manual. It states "Nominal input Power [for] 12V is 200 watts, 24V - 400 watts"
  • Belmont SolarBelmont Solar Registered Users Posts: 26 ✭✭
    You can still use a 300 watt module. The controller will just limit it to 15 amps which is about 215 watts. This only comes into play when there's lots of sunshine.
  • ApplesApples Solar Expert Posts: 39 ✭✭

    Indeed! Funny, I was looking at this PDF last night... Belmont is 100% correctomundo.

    It states, "Input power can exceed Nominal Maximum Operating Power, but controller will limit and provide it's rated continuous maximum ouput current into batteries. This will not harm the controller (reminder: do not exceed Voc [75 volts])."


    http://www.morningstarcorp.com/wp-content/uploads/2014/02/SSMPPT_ENG10_1111.pdf
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    My "cost effective" MPPT max panel equation:
    • 15 amps 8 14.5 volts * 1/0.77 panel+controller derating = 282 Watt array "cost effective" maximum
    More or less, unless you live in very cold winter climate, very seldom on an average day (~9 months of the year in warm weather), will you see the controller current limit its output.

    Specifically, the little MorningStar MPPT controller has been designed and used for down converting/battery charging.. .I.e. connect the panel input to a 24 or 48 volt battery bank and have the controller charge a 12 volt battery bank. Works very nice for the job (and add a breaker/fuse to the panel input to prevent a short circuit drawing excessive current from the higher voltage battery banks).

    People that need 12 volts for HAM, DC water pumps, other 12 volt appliances have done the down converter before without issue (there was an old firmware bug that the green LED did not work correctly if the Vpanel input received power 24x7--New firmware fixed that years ago, as I recall).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    BB. wrote: »
    the little MorningStar MPPT controller has been designed and used for down converting/battery charging.. i.e. connect the panel input to a 24 or 48 volt battery bank and have the controller charge a 12 volt battery bank.

    boB has warned (in other threads) that batteries are too "stiff" to down convert with an MPPT controller. When the controller tries to sweep, it can't load the battery enough to get a voltage drop. boB has suggested putting a power resister on the output of the higher voltage battery. Are you saying that the Morning Star can do this down conversion without a resister?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 29,489 admin
    Solar guppy, old time poster, said that the MS 15amp MPPT controller was fine to connect directly to the battery bank on the vpanel input (with fuse/breaker).

    Don't know about others, but since Bob designed many of then, I would follow his suggestion.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ApplesApples Solar Expert Posts: 39 ✭✭
    Specifically, the little MorningStar MPPT controller has been designed and used for down converting/battery charging.. .I.e. connect the panel input to a 24 or 48 volt battery bank and have the controller charge a 12 volt battery bank. Works very nice for the job (and add a breaker/fuse to the panel input to prevent a short circuit drawing excessive current from the higher voltage battery banks).


    Interesting stuff!
  • dropkickdropkick Registered Users Posts: 23
    So another scenario of MPPT and PWM: I want to put up 2 identical 12V 100W panels. One will get good sun but the other will have very little tilt and mostly westward sun so consider it 'shaded' at times. Since the panels are matched and would be wired parallel is there any benefit with MPPT? If I went with parallel 36cell panels (200W+) but still 12V battery obviously MPPT comes into play but would it handle the shading? Or should that be 2 PWM CC's knowing of the amperage mismatch?

    Ideally I could put 2 or 3 250W in series, albeit on the flat roof, and go MPPT and come out ahead but there are some long term mounting considerations and they may end up 'just sitting up the there,' so the other two locations are better for now. This is just an experimental backup setup.
    6 250W Renogy panels / Morningstar TriStar MPPT 60 charge controller / 8 Costco CG-2 batteries @ 24V / Samlex PST-1000-24 inverter / Samlex SDC-23 24/12V converter and BG-60 LVD / Midnite Solar boxes, breakers, etc.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    dropkick wrote: »
    So another scenario of MPPT and PWM: I want to put up 2 identical 12V 100W panels. One will get good sun but the other will have very little tilt and mostly westward sun so consider it 'shaded' at times. Since the panels are matched and would be wired parallel is there any benefit with MPPT? If I went with parallel 36cell panels (200W+) but still 12V battery obviously MPPT comes into play but would it handle the shading? Or should that be 2 PWM CC's knowing of the amperage mismatch?

    Ideally I could put 2 or 3 250W in series, albeit on the flat roof, and go MPPT and come out ahead but there are some long term mounting considerations and they may end up 'just sitting up the there,' so the other two locations are better for now. This is just an experimental backup setup.

    If your panels are 12 volt (nominal) panels in parallel, there will be very little (if any) advantage to using an MPPT controller over a PWM controller. If the panels are in parallel, it doesn't matter if one panel is 'shaded' at times.

    If the panels are in series you need an MPPT controller to down convert the higher voltage 24 volts nominal) to 12 volts. Neither type of controller will give satisfactory performance if one of the panels in a series string is shaded.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • dropkickdropkick Registered Users Posts: 23
    K Thanks, that's kinda what I figured... Was curious if an MPPT would have issues trying to find the best point in a parallel setup with one shaded. I'd imagine it would pretty much ignore the bad panel and just work with the peak on the good one. If I end up doing a series string all the panels would be on the flat part of the roof with no shade other than the less than optimal tilt. At least there are no trees!
    6 250W Renogy panels / Morningstar TriStar MPPT 60 charge controller / 8 Costco CG-2 batteries @ 24V / Samlex PST-1000-24 inverter / Samlex SDC-23 24/12V converter and BG-60 LVD / Midnite Solar boxes, breakers, etc.
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