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97TJ
97TJ Solar Expert Posts: 68 ✭✭
in Off Grid Solar & Battery Systems #1
I'm going to upgrade my small off grid system that is 10 years old.  Currently a 12 volt system with 600 watts of panels but I purchased 200 watts more so I will have 800.  PWM controller and Crown lead acid batteries.  The Crown batteries have finally given up the ghost so I want to go lithium with a mppt controller.  So, with 800 watts of panels, what size battery bank of lithium would be appropriate.  I had (4) 6volt 230 amp in series parallel.  Could I purchase a couple of lithium batteries then add another in about 6 months?  I'm thinking maybe a 60 mppt controller with (3) 100 AH batteries in parallel. 
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    What is your final goal?

    If just 800 watts of array (panels), you could stay at 12 volts, but if you want to grow more you might consider going to a higher system voltage and go to an all-in-one inverter MPPTcharger.

    Another thing to consider is do you live near this system or is this for an off grid system that will be exposed to below freezing temperatures?
    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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  • 97TJ
    97TJ Solar Expert Posts: 68 ✭✭
    #3
    800 should be enough for what I have.  Small fridge, lights and a TV.  I have been running the lead acid set up for 10 years and really just going to lithium.  Can I add a additional 100 AH battery in 6 months without issues.  Seems like when building a lead acid system you were told to buy your batteries at the same time because the older batteries would be degraded. Will a 800 watt array be sufficient for three 100 AH batteries in parallel?
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  • 706jim
    706jim Solar Expert Posts: 519 ✭✭✭✭
    #4
    97TJ said:
     Will a 800 watt array be sufficient for three 100 AH batteries in parallel?
    Yes I think so.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • Wheelman55
    Wheelman55 Registered Users Posts: 244 ✭✭✭
    #5
    You can add another lithium in 6 months no problem. 
    Off-Grid in Terlingua, TX
    5,000 watt array - 14 CS 370 watt modules. HZLA horizontal tracker. Schneider: XW6048NA+, Mini PDP, MPPT 80-600, SCP. 390ah LiFeP04 battery bank - 3 Discover AES 42-48-6650 48 volt 130ah LiFePO4 batteries
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #6
    Without looking at your location, hours of sun, Watt*Hours/Amp*Hours of loads specifically... I like to use the 5% / 10% rule of thumb as a sanity check... For a "weekend system" (mostly summer usage, etc.)... 5%+ minimum rate of charge. For a full time off grid system, a 10%+ minimum rate of charge:
    • 300 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 282 Watt minimum array (weekend)
    • 300 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 565 Watt minimum array (full time)
    This is based on charging needs for FLA batteries... But seems to work well with any off grid system.

    You Li Ion batteries will work "better" than the FLA/AGM batteries--And since you had a long life with the original lead batteries--The Li Ion (with proper care/watching minimum/maximum bank voltages) should be very happy too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 97TJ
    97TJ Solar Expert Posts: 68 ✭✭
    #7
    And will 60 amp 12 volt mppt charge controller be adequate.  I've seen some 60 amp 12 volt charge controllers rated for a 750 watt array.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    edited December 2022 #8
    Assuming a "good quality" MPPT charge controller, you can "over panel" them and they will work safely and reliably (as always, need good cooling/airflow to keep electronics happy).

    An 800 Watt array on an MPPT controller typically (rare cool/clear day near solar noon) would output:
    • 800 Watt array * 0.77 panel+controller deratings * 1/14.5 volts average charging = 42.5 Amps charging a 12 volt battery bank.
    PWM controllers should not be "over paneled" (always refer to manual)... So they will be designed to manage a "worst case" design (i.e., the panels should never supply more that rated current)... And "the powers that be" use an 80% rating for solar power:
    • 60 Amps * 18.0 Vmp typical array * 0.80 NEC solar derating = 864 Watt array "max" PWM charge controller
    For a 60 MPPT controller, an "optimum" "over paneling" would be (clipping would be "rare" on "perfect" solar noon days):
    • 60 amps * 14.5 volts * 1/0.77 panel+controller derating = 1,130 Watt array "optimum" over paneling for MPPT charge controller
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 97TJ
    97TJ Solar Expert Posts: 68 ✭✭
    #9
    Thanks!
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