How low can I go?

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rp3703
rp3703 Registered Users Posts: 129 ✭✭
in Off Grid Solar & Battery Systems #1

How low can the voltage drop on my 24V off grid system before I should flip on a generator. Most mornings, the voltage is around 24.5V or so when I wake up and the batteries are at around 80%. As soon as the sun starts hitting the panels, the voltage  will quickly go up to 25V or so but the percentage of charge won’t start increasing till around 10-11am and be at float by around 1-2pm. 


I have a 5500 watt generator that has a 30A outlet and my Victron inverter has connections for me to hook the generator to it and charge the batteries if necessary. I’m just wondering at what point I should do so. 

1860 W (6) Rensola JC310M, Classic 150, 430AH@24V (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
Off Grid Cottage on lake in Northern Ontario 46˚N 
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  • 706jim
    706jim Solar Expert Posts: 515 ✭✭✭✭
    edited July 2019 #2
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    What voltages are you using for bulk and float? You may have read my earlier thread "L16's low voltage" in this section. To properly charge my batteries I now bring them to 29.8 volts bulk and 27.6 float and they seem to be working properly.  I'm wondering if yours are undercharged like mine were. My overnight voltage now goes to 24.8 which was typically what I saw back when I ran 12 GC's.
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 30th year.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
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    You are now balancing the trade off between Generator fuel and battery life.  Batteries - when they sit low for longer times, they start to sulfate.   Or you buy fuel for generator.

    Me,  I watch the weather forecast like a hawk. If there is going to be sun enough to get full by the end of the day, I wait.   If it's a week of rain coming in, i fill the fuel tanks and run the genset 2x a day for an hour or so.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • rp3703
    rp3703 Registered Users Posts: 129 ✭✭
    #4
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    The specs on my Deka/Federal DSGC15’s say Boost Voltage 7.05-7.29 and Float Voltage 6.75. I multiplied those by 4(6V*4=24V) to get 28.2-29.16V Boost and 27V Float. Our Midnite Classic is set to 28.6V Absorb/Boost and 27.5V Float. 

    1860 W (6) Rensola JC310M, Classic 150, 430AH@24V (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
    Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
    Off Grid Cottage on lake in Northern Ontario 46˚N 
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #5
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    mike95490 said:
    You are now balancing the trade off between Generator fuel and battery life.  Batteries - when they sit low for longer times, they start to sulfate.   Or you buy fuel for generator.

    Me,  I watch the weather forecast like a hawk. If there is going to be sun enough to get full by the end of the day, I wait.   If it's a week of rain coming in, i fill the fuel tanks and run the genset 2x a day for an hour or so.
    My experience is, when I relied on lead acid, was if the forecast was for rain, I would use the generator, invariably it would clear and render the generator charging useless. With LFP I now wait until the end of the day to boost as required, have had to do this once in 10 months after 2 days overcast, 4 Kwh in 2.5 hours, no need to worry about fully charging, just enough to grt past the the doldrums.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    edited July 2019 #6
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    I don't use a generator, and also have a 24 volt system. 

    Judging a battery bank's State of Charge (SOC) by the voltage is a very tenuous situation. even minor loads can effect the voltage and minimal charging as well. As Mike stated earlier, it's a trade off between battery life (maximum cycles) and cost of running the generator. I've likely shortened the life of my battery bank after having a very long period of overcast days 3 falls back (13 days with 1 hour of direct sunlight) I was bouncing around the bottom the last few days. Below 50% and as low as 20-25%.


    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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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
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    mcgivor said:    ...............
     if the forecast was for rain, I would use the generator, invariably it would clear and render the generator charging useless.....
    My friends call my generator the "Fog Repeller", 5 minutes after starting, the fog & clouds clear.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #8
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    mike95490 said:
    mcgivor said:    ...............
     if the forecast was for rain, I would use the generator, invariably it would clear and render the generator charging useless.....
    My friends call my generator the "Fog Repeller", 5 minutes after starting, the fog & clouds clear.
    True dat, though I usually just bulk off the genny.  If the sun appears, I just take it as a bonus to do some absorb.  Glass half full and all that ...
    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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  • rp3703
    rp3703 Registered Users Posts: 129 ✭✭
    #9
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    So there is no too low voltage or other indicator I need to look for to determine when it’s time to turn on the generator other than the weather forecast?

    1860 W (6) Rensola JC310M, Classic 150, 430AH@24V (8) Deka DSGC15 FLA's, Victron Phoenix Multi 24/2500 Inverter
    Powering-20.5 CuFt. Fridge, 1 HP Submersible water pump, UV, Washing Machine, Gas Dryer, Gas Stove, Lights and Receptacles
    Off Grid Cottage on lake in Northern Ontario 46˚N 
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #10
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    On a 24V nominal system dipping below 24V for more than a couple of days, without the prospect of good solar forecast, may be time to get a full charge cycle in, this would be where the generator helps, too many days of low input is where sulfation begins it's cancerous process. 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #11
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    Lead Acid battery voltages are dependent on what you are doing (resting, charging, discharging), the rates of charging/discharging, temperature, etc... Using 24 volts as your "genset" point for a "resting" bank and 23 volts for a discharging bank (with a few minute delay to allow for starting a well pump, other heavy surge loads) is not bad.

    Here is a link that shows how Lead Acid battery voltage responds to various conditions:
    note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
    Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf

    vtMaps
    Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf
    vtMaps
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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