Solar system connection and output

eddyeddy Registered Users Posts: 1

GOOD DAY,


I AM CONSIDERING PURCHASING A GRID TIRED SYSTEM.


THE SPECIFICATIONS ARE AS FOLLOWS;


A 48V BATTERY BANK SYSTEM CONSISTING OF 12 UNIT 220AH BATTERIES TO BE CONNECTED IN SERIES AND PARALLEL.


A 6KW XANTREX INVERTER UNIT 


XANTREX MPPT CHARGE CONTROLLER(S) AND 


20 UNITS 300W SOLAR SYSTEM. (Specification per unit 33.7V; 5.7A)



QUESTION:


WHAT IS THE MIN AMPAGE NEEDED TO CHARGE MY BATTERY BANK IN THE SHORTEST AND SAFEST POSSIBLE TIME??


WHAT IS THE BEST SET UP FOR MY SOLAR PANELS (IN SERIES AND PARALLEL) IN OTHER TO MAXIMISE OUTPUT


WHAT IS THE MAXIMUM EXPECTED OUTPUT GENERATED ASSUMING MY MINIMUM REQUIREMENT IS 3KWH


REGARDS


EDMOND

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 4,280 ✭✭✭✭
    If the batteries will 'live' in float, It will take a very minimal amount of current to over come their discharge. The answer depends on the type of battery.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • BB.BB. Super Moderators Posts: 26,872 admin
    Good day Edmond and welcome to the forum.

    Your battery bank--Is that 12 volt @ 220 AH batteries? If so, that would be 4x 12 volt batteries in 3x parallel strings for a 48 volt 660 AH battery bank.

    The "ideal" charging current for your bank would be around 10% to 13% rate of charge. People do go to ~20-25% rate of charge, but you need to monitor the temperature of your battery bank.

    Many solar charge controllers have remote battery temperature sensors to do this... Basically, as Lead Acid batteries get hot, they require lower charging voltage--Otherwise, you can have thermal runaway in worst case conditions (battery gets hot, battery voltage falls, charge controller pumps more current into battery, battery gets hotter--etc.).

    Typically, for solar 13% rate of charge is around the cost effective maximum. For your system that would be:
    • 59 volts charging * 660 AH * 1/0.77 solar panel+charge controller derating * 0.13 rate of charge = 6,574 Watt "cost effective" maximum array
    Your 6,000 Watt array is "close enough" to maximum for argument sake.

    Assuming you are in/around Lagos Nigeria, and you have a fixed array:
    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Lagos
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 84° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    5.80
     
    5.72
     
    5.34
     
    5.00
     
    4.71
     
    4.02
     
    Jul Aug Sep Oct Nov Dec
    3.91
     
    3.85
     
    3.91
     
    4.59
     
    5.32
     
    5.77
     
    A typical August day would supply (at most):
    • 6,000 Watt array * 0.52 off grid system eff * 3.85 hours of sun = 12,012 WH ~ 12 kWH per average August day
    Your battery bank, if you assume 2 days of storage and 50% maximum discharge would supply:
    • 660 AH * 48 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge (for longer battery life) = 6,732 Watt*Hours ~ 6.7 kWH per day (no sun or over night) per day
    For your AC charger (utility grid and/or backup AC genset), you would want around 10% to 20% rate of charge or ~66 amps to 132 amps. For your branch circuit/genset minimum rating, you would need roughly:
    • 132 amps charging * 59 volts * 1/0.80 typical charging eff * 1/0.80 derating for wiring+breakers+genset = 12,169 Watt/VA minimum
    • 12,169 Watt/VA / 230 VAC = 52.9 Amp minimum rated branch circuit / genset @ 230 VAC
    Your solar array, using "standard" 150 VDC maximum input votlage would be very nice if arranged in 3 series x (21 panels / 3 panels per string) = 7 parallel strings.

    There are "high voltage" solar charge controllers that can take upwards of 400 Volts Vmp (rated array maximum power voltage) or ~13-14 series panels, or 10 panels in series x 2 parallel strings. You have to see what vendors/products are available out there.

    Your 660 AH @ 48 volts will support a 6 kWatt AC inverter and ~6,000 Watt array maximum nicely.

    I am guessing you have unreliable afternoon power? Say 5 hour an evening blackouts on occasion. A 660 AH battery bank would support an average load of:
    • 660 AH * 48 volts * 0.85 inverter eff * 1/20 hour discharge = 1,346 Watt average load (5 hours per night, 2x nights, 50% maximum discharge)
    • 660 AH * 48 volts * 0.85 inverter eff * 1/10 hour discharge = 2,693 Watt average load (5 hours per night, 1x night, 50% maximum discharge)--Actually a bit less (660 AH is probably 20 hour discharge rate)
    Note the 10 hour discharge rate for your battery bank is probably less than 660 AH. You need to check the 10 hour discharge rate for your batteries and plug in to the above equation.

    Is this what you are looking for? Questions? Corrections for my guesses?

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