39 volts from panels.. is that normal?

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mike_littlerock
mike_littlerock Registered Users Posts: 9 ✭✭
in Solar Beginners Corner #1
I have four 100Watt panels. panels A and B connected positive on A to neg on B.. same with panels C and D. then combine the two positive to one. and same with negatives. I have a renogy controller (for now). i have four SLA 12v batteries in a 24 v config ( and yes. know that long term i will need to invest in some better batteries.. newbie mistake). 
I measure 24.8 volts on battery bank, and looks like 39 volts mid-day coming into the charge controller.. My renogy MPPT charge controller has a flashing green battery indicator which looks like it means "battery full". 

my bit question. is 39 volts from the panels wired in 24 volt configuration normal? whats the range i should expect?
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #2
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    Likely you're fine, My guess is you disconnected the panels to measure the (VOC)open circuit voltage?

    I think Renogy panels run VMP (voltage charging) at 18.9 volt, so VOC would be higher by 15-20% and lessened by the heat from the sun. so 39volts would be in the ball park. I don't know if that is enough higher for the MPPT charge controller. I have no experience with Renogy charge controllers. Does the charge controller give you any voltage reading?

    If you measured 24.8 volts I would think they haven't fully charged OR you had the array (solar panels) disconnected and had a load on the batteries.

    You should be able to measure the battery voltage with the batteries charging, In the morning before the panels have much of anything to work with, the batteries will likely be between 24-25.6 volts. With no loads the voltage will rise to about 28.6-29.4 volts, then the charge controller will back off until the voltage reaches float, 26.6-27.6.

    Of course the best way to check your state of charge is by measuring the specific Gravity of the electrolyte in the batteries cells. Voltage can be confusing way to figure out what the current state of charge is...

    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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  • oil pan 4
    oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    #3
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    That sounds perfect.

    If I put 2 of my "12 volt panels" in series I would get right about 35 to 42 volts (I use a few different kinds on as many systems), depending on which panels and if they are pointed straight at the sun or laying flat.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

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