Outback MX60 question

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hunter44102
hunter44102 Registered Users Posts: 24 ✭
in Solar Beginners Corner #1
For an MX60, can I use 34V solar panels but have 12V batteries?

I saw this in the specs "Can charge a 12 or 24 V DC battery from a 48 V nominal PV", so it appears that you can have different voltage PV's and batteries.

However, isn't there an efficiency hit? Or does the controller make use of the extra voltage and turn it into usable current?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #2
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    Re: Outback MX60 question

    Yes, with an MX 60 controller (and pretty much any good quality MPPT charge controller) you can have a much higher PV voltage than battery bank voltage and still efficiently charge your battery bank...

    An MPPT charge controller is a "switch mode" DC to DC power supply--very similar to what is used in many computer power supplies.

    A DC to DC power supply is roughly the equivalent to an AC transformer. Both can "transform" "high voltage" and "low current" into low voltage and high current following the equation of P=V*I... For example, if V is transformed by 1/2, then current will be doubled.

    There are the details which can cause issues (a DC to DC switching power supply does waste some power to run the electronics and, in the case of the common MPPT controller can only drop voltage to the battery--so the solar panels have to be, at least, a couple volts higher than the maximum
    battery charging voltage required (temperature and equalization effects on battery voltage must be taken into account--and solar panel Vmp must be looked at on a hot day, and Voc must be checked for cold days to make sure the system will work reliably in all your local conditions).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • hunter44102
    hunter44102 Registered Users Posts: 24 ✭
    #3
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    Re: Outback MX60 question

    Thanks BB! So I can have the PV at a higher voltage, but do I still need to be consistent with the PV voltage? Do I need all 34V panels on the input, or can I have a 24V in there also? I'm guessing you have to tell the MX60 what you will be supplying.

    Where do I find more info on the VOC and VMP in relation to climate and my particular system?

    If a panel has a Voc of 43.5V, how would I ever get into trouble when the MX60 says it can handle 150V.

    I will read the MX60 manual and hopefully some of this is answered, but I think I need to read more on theory
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #4
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    Re: Outback MX60 question

    Solar panels are like batteries... Parallel like voltages and add in series like currents (the farther the voltages are apart--or currents--the more power you will probably loose with the combination). And you can put in series two 12 volt panels in parallel with one 24 volt panel, etc... However, if you can stay with all one model--it will make your life more sane.

    Try reading the manual first--then people here can answer your questions more specifically...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #5
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    Re: Outback MX60 question

    As I understand it, different panels on one MPPT charger will cause inefficiencies because the different panels will have different maximum power points and the controller can only pick one power point at a time. Correct?
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
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    Re: Outback MX60 question
    Sage wrote: »
    As I understand it, different panels on one MPPT charger will cause inefficiencies because the different panels will have different maximum power points and the controller can only pick one power point at a time. Correct?

    Almost.

    Paralleling different panel voltages, you end up with voltage closer to the lowest voltage panel. That's going to waste the difference between the 24V panel and the 36V panel
    12V *6A = 72 watts lost, no matter what you do.

    If you ran a separate controller for each array voltage, that would harvest the most.
    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 ,

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
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    Re: Outback MX60 question
    Sage wrote: »
    As I understand it, different panels on one MPPT charger will cause inefficiencies because the different panels will have different maximum power points and the controller can only pick one power point at a time. Correct?

    Sort of--How much the error/loss in power really is depends on how far apart in voltage the panels are....

    For example, assume that one panel is Vmp=16 volts and a second is Vmp=17.5 volts at Imp=10 amps each (just round numbers)...

    Two different MPPT controllers (such as the new MorningStar MPPT)

    P=IV=16v*10a + 17.5v*10a=160w+175w=335 watts max available

    Assume that the MPPT settles on Vmp=16 volts (worst case):

    P=16v*(2*10a)=320 watts...

    320/335=95.5% or a loss of 4.5% of maximum power...

    A large difference in panel voltage will make a much larger issue (say Vmp=17v vs 34v)--and, I would guess, that, depending on MPPT controllers design (and local sun/weather conditions)--it is possible that the MPPT controller may "track" the 34 Volt panel's output and the 17 volt panel will add zero energy... But even if the MPPT tracks the 17volt panel--then you would loose ~1/2 of the Panel wattage rating of the 34 volt panel because of the lower MPPT voltage.

    You can run the numbers out (buying a matching panel, getting two smaller controllers instead of one larger controller, etc.) and see how much this impacts your end design.

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