Selecting panels

garynappi
garynappi Registered Users Posts: 120 ✭✭
in General Solar Power Topics #1
Given a choice of 12 volt panels with different VMP specs of 17.5v and 19.5v I assume the one rated at 19.5 would be a better choice but do not know why.  Is it that the higher VMP panel will stay above the minimum charge rate (~13.5v) for a 12 volt battery with less available sunshine?

Would the CC type matter, PWM or MPPT?

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  • Raj174
    Raj174 Solar Expert Posts: 795 ✭✭✭✭
    #2
    The higher voltage panel is likely better because high temperature panels cause lower voltage output. Only the PWM controller will work with a single panel or panels in parallel.
    4480W PV, MNE175DR-TR, MN Classic 150, Outback Radian GS4048A, Mate3, 51.2V 360AH nominal LiFePO4, Kohler Pro 5.2E genset.
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  • garynappi
    garynappi Registered Users Posts: 120 ✭✭
    #3
    Thanks Raj174. That makes sense, but an MPPT CC cannot be used with a single or multiple panels in parallel is news and confusing.  
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #4
    It's not quite as simple as it will/won't work with mppt.  An mppt cc needs some overhead pv voltage to buck down to charging voltage.  I've seen suggested minimums of 20-30%.

    The actual overhead will depend on the temperature of the panel (lower when warmer) and battery (should be higher when cold).  Panel Vmp is rated at STC, 25°C, but will generally operate at higher temp and lower Vmp in full sun.  There will also be some wire loss voltage drop.

    In other words, an mppt may work bulk charging a room temp bank on a breezy cool day, but may not be able to hold absorb voltage on a hot day for a cool bank.  Doing an EQ may not be possible.

    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
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  • mountainman
    mountainman Registered Users Posts: 439 ✭✭
    #5
    Imho Even with pwm The 19.5 is definitely the best choice if you have 100f + Summers. 

    2kw array 6 345 q cells  make sky blue 60 cc
     6 230ah GC @36 volts 
    18 amp accusense charger. 3650 champion 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #6
    ...and  of course, if your array is close to your battery bank and you use PWM the lower voltage panel will provide enough  voltage for proper charging and more current for the same wattage. Hence the better value.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • garynappi
    garynappi Registered Users Posts: 120 ✭✭
    edited January 2020 #7
    Estragon said:
    It's not quite as simple as it will/won't work with mppt.  An mppt cc needs some overhead pv voltage to buck down to charging voltage.  I've seen suggested minimums of 20-30%.

    The actual overhead will depend on the temperature of the panel (lower when warmer) and battery (should be higher when cold).  Panel Vmp is rated at STC, 25°C, but will generally operate at higher temp and lower Vmp in full sun.  There will also be some wire loss voltage drop.

    In other words, an mppt may work bulk charging a room temp bank on a breezy cool day, but may not be able to hold absorb voltage on a hot day for a cool bank.  Doing an EQ may not be possible.

    The overhead issue is what I never read of before, thanks for clearing it up! Gads, this site is GR8!
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