24vdc Battery Query

rooduJardin
rooduJardin Registered Users Posts: 7 ✭✭
in Solar Beginners Corner #1
There's a heap of info about 12v batteries around and I'm satisfied with my 12v system calculations and setup, including battery charging from solar panels. 

Different story for 24v, little to be found. I'm using 2 x 12v 100ah cabled in series for a 24v system. I noticed my inverter behaving oddly, a 24vdc to 220vac 4000w continuous power. I put it down to not knowing what was the maximum voltage I could send to my batteries. I was just guessing hoping to hit the right number for best efficiency.

Got sick of this quickly as it was taking too much time time, so rang the manufacturer. Within a minute I had the answer, they'd obviously done a quick calculation or looked up a table. They gave me 29v.

But where does this number come from? How do I calculate it. I may want to add another group to bring up my ah. There's no info in the battery manual so how does one come by this number? 
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
     Solar PV power is time limited, and so to be able to reliably push 120% of last nights consumption back into the batteries, you have to tune your PV array/Charge controller to deliver enough voltage, to push enough amps back into the batteries before the sun goes down.

    Sometimes, this means having to jack up the voltage to 14 or 15V (per 12v increment) to complete the charge in 1 day, or you risk chronic deficit charging and slowly killing your batteries.

    14 to 14.5 V is common starting point, but your battery manual will have the exact voltage they recommend starting from. Sealed batteries are very touchy, as a tiny bit of overcharge can pop the overpressure seal and you loose precious electrolyte that cannot be replaced.
    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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  • Tecnodave
    Tecnodave Registered Users Posts: 437 ✭✭✭✭
    #3
    Proper charge voltage for flooded lead acid batteries is 28.8 absorb , 27.6 for float.  Bigger issue......100 a.h. battery minimum per 1000 watt output.......200a.h. per 1000 watts for a full system, for a 4000 watt inverter that's 400-800 a.h. of battery to run that inverter. .....time to think out if you need that big of inverter or need lots more battery.......batteries don't work well in parallel..........4 X .......L-16's.....6 volt 428 a.h. ...Is more appropriate at a minimum

    I have 8...X.....L-16's....850 a.h. battery for my MagnaSine MS-4024AE 4000 watt inverter.....
    2 Classic 150, 2 Kid, 5 arrays 7.5 kw total  2ea.  2S6P Sharp NE-170/NE-165, 1ea. 12P Sanyo HIT 200,  2ea. 4/6P Sanyo HIT 200, MagnaSine MS4024AE, Exeltech XP-1100,  2 Banks L-16 battery, Rolls-Surette S-530 and Interstate Traction, Shunts with whizbangJr and Bogart Tri-Metric, iCharger i208B  dc-dc buck/boost converter with BMS for small form lithium 8S 16650 or LiFePO4,
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #4
    Tecnodave said:
    Proper charge voltage for flooded lead acid batteries is 28.8 absorb , 27.6 for float.  Bigger issue......100 a.h. battery minimum per 1000 watt output.......200a.h. per 1000 watts for a full system, for a 4000 watt inverter that's 400-800 a.h. of battery to run that inverter. .....time to think out if you need that big of inverter or need lots more battery.......batteries don't work well in parallel..........4 X .......L-16's.....6 volt 428 a.h. ...Is more appropriate at a minimum

    I have 8...X.....L-16's....850 a.h. battery for my MagnaSine MS-4024AE 4000 watt inverter.....
    Charging voltage at 25°C, for every °C above subtract 0.06V,  for every °C below add 0.06V (values for 24V nominal) or better yet use a controller with a remote temperature sensor to automatically modulate the charging voltage.
    Plus 1 on inverter being oversized for the battery capacity. 
    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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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,100 ✭✭✭✭✭
    #5
    Your battery bank is undersized for that size inverter.
    For 24 volt systems you simply double the 12 volt voltage parameters, 48 volt, quadruple 

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,609 admin
    #6
    Remember that a battery is composed of a bunch of 2 volt (Lead Acid) cells in series... So, the numbers for a 12 volt bank * 2 for charging a 24 volt battery bank... And many specs you will see them talking about a single cell charge/discharge/rest voltages in industrial batteries... Just xN batteries in series for actual bank voltages.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rooduJardin
    rooduJardin Registered Users Posts: 7 ✭✭
    #7
    Thanks for all responses ~ good information. 
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  • spacebass
    spacebass Registered Users Posts: 87 ✭✭
    #8
    Thanks to Bill who made me search it out and this thread I have just ordered a 1500 watt sine wave inverter to replace my 3000 watt sine wave inverter for 2 x 210ah 12 v in series. Nearly right.
    Will I notice any difference?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,609 admin
    #9
    If you use your inverter 24x7, your battery bank should be a little less discharged in the morning. And in winter/poor sun conditions, your genset usage should be a bit less.

    You probably will not notice your batteries lasting 5-6 years vs 4-5 years (better charging conditions)--But it should be "easier" to keep your batteries a bit happier (less deep discharges, less genset needed).

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