MPPT and VOC limits

solardad
solardad Registered Users Posts: 23 ✭✭
Are MPPT controllers / chargers designed to run efficiently at their upper limits? Is it a good idea or ok to design a system where the array voltage is runs near the max of a controller? Anyone have issues with a similar design?

Designing a system where by the overall array should ideally be broken up into two sub arrays (shading issues), 6 panels and 6 panels that feed into dedicated MPPT chargers. The 3s2p setup would be 144v20a for each. The control that I am looking at has a max of 145 and absolute cold max of 150 (Victron 150/60). 

I could jump to the next model, 250/100, which would provide over 100v of headroom but is that necessary? Expectation that a panel would go beyond the stated VOC limits? 

I was hoping by choosing the smaller model I could cost effectively handle both sub arrays separately - main reason is to deal with shading issues (1~2hr at either end of the day). The150 series is roughly 1/2 the cost of the 250. 

Thanks

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    The question is, what nominal voltage is the system designed for, if charging batteries the voltage should be 50-100% above battery nominal, using higher will be slightly less efficient, generally speaking. 

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    edited November 2018 #3
    Way too close for comfort. The first really chilly, clear, winter morning you may find you've ruined your controller and will have ended up spending double the money to get back to square one. And still have lower Voc. limits.
     What is your system voltage? Can you wire 2s3p?

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • solardad
    solardad Registered Users Posts: 23 ✭✭
    Thanks for feedback


    @mcgivor 48v s the plan

    @littleharbor2 I have distance to deal with. Array will be in the backyard, roughly 150’ from the house. So trying to avoid massive cables by boosting the volts. I was planning on combining at the array which would require a 6awg to keep my vdrop under 2%. Going down in volts I guess would work also.. just move the combinderbox to the house and have a short run to the mppt... just more wires to run. I think I Iike this idea... smaller wires (but more of them) and safer headroom.
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    Not sure of your location/availability, but there are other brands that do high voltage. Midnite Classic 200, Magnum PT 100, Morningstar TS-MPPT-60-600V-48 . While a bit pricy the Morningstar will take up to 600 volts, eliminating the need for combiner boxes and fusing in many cases.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • solardad
    solardad Registered Users Posts: 23 ✭✭
    edited November 2018 #6
    @littleharbor2 I have thought about those also. Related to my scenario I just came across an old post related to the idea that you want to limit the upper end of the voltage the MPPT has to handle for stepping it down to the system voltage, in my case planning 48v. 

    Not sure if this is still the case, I was unable to find any cautions from Victron, but if I run with a 96v I"ll be at 100% or 144v I'll be at 150%. Is there still a need to be wary of that % over the system voltage OR have controllers improved in performance that this is a non-issue stepping down from these high voltages? Related to @mcgivor warning earlier. 

  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited November 2018 #7
    Controller design varies, so it's not good to generalize.     Specs suggest that the Victron needs only 5V above Vbat-max to work - not 50%.  Generally (:-)), DC-DC converter efficiency drops with increasing voltage differential.

    I am available for custom hardware/firmware development

  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    You will lose a negligible amount of efficiency and so be generating a bit more heat. These controllers have been designed to operate within the voltage ranges listed. In your case with long wire runs higher voltage is warranted so that's why you would use a high Voc. controller.
     
     You're not specifying what voltage you're talking about. When you say 96 or 144, is this the Voc. listed on the panels? You still need to factor in the temperature coefficient for anything other than 25 degrees Celsius, or 77 degrees Fahrenheit. This is why you need to be careful on those cold, clear mornings. It's not raw power (current) that kills your controller. it's high voltage, which will likely come with very little current on those early mornings when your controller first wakes up.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    There is still a bit of a trade-off going with higher voltage controllers.  Midnite has output curves on their website that you can use to get a sense of this.

    With most controllers, you want to look up the lowest temperature ever recorded near your location, and make sure the array is configured so max temp adjusted output voltage is at least 15v or more under to allow some leeway for temp variance from local terrain etc.  If, for example, you have a 150v controller, and input voltage hits 151v, most contollers will flip an overvoltage bit (voiding warranty), and may (or may not) be damaged.  Midnite contollers have what they call "hypervoc", which as I understand it, will instead take the array offline at 150v, and not "flip the bit" until voltage reaches 150v plus bank voltage (eg 150v + 48v =198v).

    IMHO, you could relax the 2% voltage drop criteria a bit in sizing wire.  Panels are cheap enough vs wire these days that (within reason) it may make more sense to add a bit more pv if need be to offset a bit of wire loss.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • solardad
    solardad Registered Users Posts: 23 ✭✭
    @littleharbor2 96 & 144 is based on the panels Voc, 48.21.  2s3p and 3s2p respectfully 

    @Estragon thanks for the info 
  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    Definitely calculate and use the worst case, low temperature Voc.   For example (only!), this can be 20% higher than the 25C Voc.

    I am available for custom hardware/firmware development

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    The PT 100 is 187V operating, 200VDC death, and 100A output
    http://www.magnum-dimensions.com/pt-100-mppt-charge-controller
    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
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  • WaterWheel
    WaterWheel Registered Users Posts: 375 ✭✭✭
    When choosing a controller brand it's nice if it is the same brand as the inverter so they can communicate.      This makes programming the controller easier.       I've never used Midnight Solar but I understand it may be very programmable without communicating with the inverter control panel.

    Conext XW6848 with PDP, SCP, 80/600 controller, 60/150 controller and Conext battery monitor

    21 SW280 panels on Schletter ground mount

    48v Rolls 6CS 27P

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    Yes, Midnite classic controllers can be monitored and controlled through their "LocalApp" software.  With appropriate network setup, it could also be used remotely.

    I've found no need to access inverter data or settings remotely, but it would be simpler to do if same brand.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter