Just checking

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
I have 2x100, 1x85, and 2x60 pv panels. How many 6v Golf Cart batteries hooked up for 12v operation can i recharge if discharged to 50%. How would you hook up the panels for maximum power to recharge these batteries
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Just checking

    Let's see ...

    2 @ 100W = 200W
    1 @ 85W = 85W
    2 @ 60W = 120W

    Grand total: 405 Watts of mis-matched panels which probably have different Vmp & Imp but I wouldn't worry too much about that on a small, 12V system.

    Potential charge current from those panels: 405 * 80% efficiency (probably less due to mismatching) = 324 Watts @ 14.2 Volts charging = 22.8 Amps which is about 10% of 225 Amps - the 'standard' rating for those golf cart batteries.

    So the answer is: probably you can get away with two 6 Volt 225 Amp/hr batteries in series.
    Please note I didn't include losses from wires and charge controller, environment, etc. But it ought to make the minimum 5% current (11.25 Amps) anyway.

    Harvest-wise, you're looking at roughly 300 Watts * 4 hours of good sun (if you've got it) for approximately 1.2 kW/hrs per day. Not bad for a small system.

    But in AC terms it would probably be more like 800 Watt hours per day.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #3
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    Re: Just checking

    We really need to know more--
    1. What is the Vmp*Imp for each type of panel you have (and how many of each).
    2. What kind of charge controller you have (or are thinking of getting)
    3. Ideally, we really need to look at the loads you want to drive.
    Assuming you can connect everything together (correct panels+charge controller(s)), using the PV Watts Website; using Little Rock Ark, 1kW of solar panels, assuming you are using an AC inverter for your loads (0.52 system efficiency), fixed array (no tracking):
    "Station Identification"
    "City:","Little_Rock"
    "State:","Arkansas"
    "Lat (deg N):", 34.73
    "Long (deg W):", 92.23
    "Elev (m): ", 81
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 34.8"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 7.4 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.78, 59, 4.37
    2, 4.38, 61, 4.51
    3, 5.19, 78, 5.77
    4, 5.67, 79, 5.85
    5, 5.89, 83, 6.14
    6, 5.81, 77, 5.70
    7, 5.93, 80, 5.92
    8, 6.08, 83, 6.14
    9, 5.41, 73, 5.40
    10, 5.16, 75, 5.55
    11, 3.69, 53, 3.92
    12, 3.56, 55, 4.07
    "Year", 5.05, 856, 63.34

    So for a 1,000 watt array, we would expect an average of 53-83 kWhrs per month or 1.8 kWH to 2.8 kWH per day (depending month/seasons) per 1,000 watts of solar panels.
    • 2x100, 1x85, and 2x60 pv panels = 405 Watts of solar panels
    • 1,800 WH per day * 405w/1,000w = 729 WH per day
    • 2,800 WH per day * 405w/1,000w = 1,134 WH per day
    So, load wise, say you have a 100 watt load and run it for XX hours per day:
    • 729 WH / 100 watt load = 2.29 hours per day
    • 1,134 WH / 100 watt load = 11.34 hours per day
    Battery wise, we recommend that you charge your bank at ~5-13% rate of charge (based on battery/bank 20 Hour Amp*Hour rating)--Your 405 watts of solar panels (again, rough rules of thumb):
    • 405 Watts * 1/17.6 volts * 1/0.05 rate of charge = 460 Amp*Hours maximum
    • 405 Watts * 1/17.6 volts * 1/0.13 rate of charge = 177 Amp*Hours minimum
    I think I got everything you asked about--Of course we have the whole controller/solar panel matching / selection questions (details, details).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #4
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    Re: Just checking

    To some extent, the "real" answer is more dependent on your loads. How long does it take you to get to 50% DOD? How fast (or slow) are you willing to be in recharging? How often do you have such loading? Every day is one answer, once a week is another,, and 6 weekends a year is still another.

    That said, other charging options might (should?) change things as well. do you have grid power 24/7 to charge, do you have a gennie,, is it properly sized with an efficient charging system?

    As Marc, suggests, I would be surprised if you see more that ~20 amps out of the PV inot a 12 volt battery bank.

    Just FYI, we live off grid with ~400 watts of PV into 450 ah of trojan T-105 batteries. We see just about 20 amps on an normal day, nearer 30 with winter cold and reflection off the snow. We draw the T-105s down ~60 ah/day on average,, just over 10%. If we don't have sun for three days, I will run the gennie for a few hours, but a full sun day will bring them back from 70% to nearly 100% by the end of the day. (depending on the time of year) January,,no June, maybe but March with long days and lots of snow,, easily.

    Tony
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