Battery bank sizing??

Joesouth75
Joesouth75 Registered Users Posts: 33 ✭✭

Hello all,

I'm back again with the same issues. I have 8 t105 225ah batteries in two 24v strings. I have six 320w 24v panels charging them. Midnite solar 200 and an Aims 3000w inverter. I know my inverter is way to large. I only run a few LEDs a 120v well pump intermittently, a tv in the evening and a cpap at nite. I run out of power every nite.

Do I need larger amp hr batteries? I'm estimating 300kwhrs a month usage maybe less since this is my cabin. Any help is greatly appreciated.

Joe

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited March 2019 #2

    The estimated load of 10 Kwh is about 3 times my average, using the exact battery capacity and six 250W modules, the batteries were marginal in that there was little reserve capacity, hense the change to LFP. As you already know the inverter is oversized, but what model is it? some of the 3000W Aims inverters use over 4Kwh per day just being on.

    To get a better answer list the loads, how many hours they operate and the inverter information. Without this is all speculation, but 10Kwh per day seems high.

    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.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    Joe,

    I do not remember if I ran a calculation on what your system should produce (Watt*Hours per day). So, using our basic rules of thumb.

    A battery bank should have two days of storage and 50% maximum discharge--Or be ~4x your daily loads. Working backwards:

    • 450 AH battery bank * 24 volts * 0.85 AC inverter eff (Aims is much worse eff) * 1/2 days storage * 0.50 max discharge = 2,295 WH per day average load suggested.

    Then there is sizing the solar array... Suggest 5% to 13+% rate of charge. 5% can be OK for weekend/summer cabin. 10%+ recommended for full time off grid.

    • 450 AH battery bank * 29 volts charging * 1/0.77 panel+controller derartings * 0.05 rate of charge = 847 Watt array minimum
    • 450 AH battery bank * 29 volts charging * 1/0.77 panel+controller derartings * 0.10 rate of charge = 1,695 Watt array nominal
    • 450 AH battery bank * 29 volts charging * 1/0.77 panel+controller derartings * 0.13 rate of charge = 2,203 Watt array "cost effective" maximum

    And we also need to size the array based on hours of sun per day. I don't remember where you are located, but say Chicago Illinois. Using this link:

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Fixed array, facing south, tilted to 48 degrees from vertical: runs from 2.6 hours of sun in December to 5.36 hours of sun in July. Lets use this month March ~4.13 hours of sun per day.

    The above system suggests a "balanced system design" of 2,295 Watt*Hours per day and your 1,920 Watt array should be producing (long term average) of:

    • 1,920 Watt array * 0.52 off grid AC system eff * 4.13 hours of sun per day = 4,123 WH per day (March average)

    So, your array is larger than the 10% recommended "nominal" array, and this month, your system should produce 2x the "balanced" output of the battery bank.

    Is your system performing this well? Certainly not anywhere need 10 kWH per day or 300 kWH per month (2,295 WH per day if running purely from battery for 2 days of "no sun").

    Now we look at your loads... Since you have not given us the numbers used by your loads using a Kill-a-Watt type meter, some guesses:

    The CPAP can run from 25 Watts (humidifier turned off) to 120-300 Watts with humidifier turned on full. Lets assume 200 Watts for now (pure SWAG).

    The TV--Say LED TV at 50 Watts for 5 hours (40W*5H=200 WH)

    5x 9 Watt LED lights for 5 hours (45W*5H=225 WH)

    CPAP 200 Watts for 8 hours (200W*8H=1,600 WH)

    AIMS 3 kW Sine Wave inverter Tare 45.6 Watts (45.6W*24H=1,094 WH) (note: if you turn off 1/2 the day, or run in standby at ~20 Watts, will save some energy)

    https://www.aimscorp.net/3000-Watt-Pure-Sine-Inverter-Charger-24-Volt.html

    ============Total=================

    Worst case = 200WH+225WH+1,600WH+1,094WH= 3,119 WH per day

    Your numbers could very well be less... But you are pushing your system (using most of your energy at night, means battey bank must be fully recharged during the day--Difficult to do a 1x day solar charge if battery bank is dragged below 75% state of charge--Simply not enough hours of useful sun per day -- i.e., 6 hours of charge time vs closer to 8+ hours needed to recharge Flooded Cell Lead Acid battery bank from 50% State of Charge).

    I did not even take the well pump into account (probably not that much energy usage unless you are irrigating your yard)--At this point, you are (possibly) drawing more energy than the system can provide in the Winter--And right know, you are just a little better than breaking even... (based on my multiple guesses).

    Your number 1 and 2 energy consumers are probably your CPAP machine and AC inverter.

    For the CPAP machine, you really need to get a Kill-a-Watt type meter and see what it is really using:

    https://www.amazon.com/s?k=kill-a-watt+meter&ref=nb_sb_noss

    And for the AC inverter--If you only run it 12 hours a day, it will save you ~500 WH per day.

    If you get another inverter to run your "small stuff" and only power up the 3kW unit when you need to run the water pump. Samlex 300 Watt PSW 24 volt inverter with 0.4 Amp Tare load (400 mAmp):

    https://www.solar-electric.com/sa300wa24vos.html

    https://www.solar-electric.com/lib/wind-sun/PST-300-12-24-specs.pdf

    • 0.4 amps * 24 volts * 24 hours per day = 230 Watt*Hours per day

    If you want 24 hour per day power, this unit will supply it at 1/4 the energy usage as the Aims 3kW.

    However, you will need to measure the peak and average load of your CPAP machine to see if this 300 Watt inverter s a "good fit" or not.

    The 600 Watt version of the Samlex uses 0.450 Amps Tare... Not much more than the 300 Watt version (very nice).

    https://www.solar-electric.com/samlex-pure-sine-wave-inverter-pst-600-24.html

    The 600 Watt inverter includes a simple remote control panel (15 feet) if you want to turn the unit off during the day:

    https://www.solar-electric.com/lib/wind-sun/RC-15A-specs.pdf

    Your 450 AH @ 24 volt battery bank will support a 2.25 kWatt rated AC inverter--But I would suggest you use as small as inverter as practical for your needs... Or two inverter (big Aims for water pump, small inverter for "rest of home").

    Lots of guesses here (about your loads and how you use your system)... But, at this point, it appears that you are drawing more energy than your battery bank can "reliably" supply for full time use. A larger battery bank and larger array could "fix" the problems... However, reducing your loads will probably be a "cheaper" long term fix.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joesouth75
    Joesouth75 Registered Users Posts: 33 ✭✭

    Bill,

    The Aims is a pure sine wave inverter. Other than that you are pretty much on the money with the numbers.

    I like my set-up as is. Would larger amp hour batteries do the trick and give me more reserve? I could change out my inverter to a 2000w pure sine wave. Anyone in need of a 3000w?

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭

    There are two primary factors to consider when increasing capacity, the condition of the present bank and the ability to replace withdrawals.

    Firstl, mixing old with new batteries can result in imbalanced charging, as the older ones need more time to approach fully charged state this tend to over charge the newer ones, or the inverse if the change profile is tailored to the newer string the older ones suffer undercharging. If the existing batteries are not within 6 months of light useage, I personally wouldn't recommend adding new ones into the bank.

    The second is the capacity of the array to replenish what's taken out, it's very easy to fall behind, this leads to sulfation resulting in capacity loss, which actually occurs over time, often going unnoticed until it becomes noticeable, which may be too late. Generator assistance obviously helps but the nature of lead acid batteries requires them to demand a long absorption period at ever diminishing current, so not very efficient. This was the reason I switched to LFP, they accept full current until the programed voltage is reached, especially useful for generator charging, not to mention partial state of charge is not an issue.

    So the answer is more complexed than just saying yes or no, theoretically yes, additional capacity may help, albeit at the expense of the system as a whole, in some circumstances.

    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.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited March 2019 #6

    Joe, have you actually measured the usage of the CPAP (and other devices) with a Kill-a-Watt type meter?

    Are you near Chicago (hours of sun)?

    What are your plans for winter when your sun falls below 3 hours per day and you have storm clouds for more than a few days? (Genset, not there during winter, etc.?)

    Do you run the AC inverter 24x7?

    I did not bother with the Water Pump energy assuming it only runs for 6 minutes per day or so (1,200 Watt motor * 1/10th hour per day = 120 WH per day). If the pump runs very long (30 minutes per day for irrigation, etc.), that could be a wrong assumption.

    Very roughly, if you add one more parallel string of batteries (or new/larger AH batteries) with 50% more capacity, they would be "happier".

    And, if you add 59% more batteries, you should add ~50% more solar panels too (for full time off grid, you really want 10%+ rate of charge.

    As Ralph suggests, if your battery bank has been damaged by over discharging/under charging, etc., then you might be on the hook for an entire new battery bank.

    Is there any reason why your would not want to pay $180 to $240 for a new 300-600 Watt AC inverter (that you run 24x7, or with remote on/off to save power when you are not there/not using devices) and see if your present system would do better, especially as you are heading into summer? Vs buying 1-3 strings of batteries and 50% more solar (and possibly a second charge controller for the larger array)?

    Right off the bat, that (a new 300-600 Watt efficient AC inverter) would save close to 800 WH per day--Assuming you run the 3kW only when you run the water pumps a hand full of minutes per day (AIMS inverter "off", not in standby mode which still uses 18 Watts--A ridiculous amount of power for standby too).

    And get a Kill-a-Watt type meter for less than $30. So you have a better idea about your energy usage (some public libraries will have Kill-a-Watt meter to "check out" and take home).

    Adding batteries only to your system and nothing else (and even if you add 50% more solar panels too)--That would be way down on my list of suggestions to do right now. Measuring your loads first and doing energy conservation second (like a new inverter) is how I would proceed.

    I may be good (and my wife may disagree), but me estimating your energy usage 2,000 miles away with no real data--I would not bet a $1,000+ on me, and you should not either.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joesouth75
    Joesouth75 Registered Users Posts: 33 ✭✭

    I will look into the kill a watt device.and go from there. Thank you

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited March 2019 #8

    Joe,

    The batteries are the one major item that will decay/fail with improper usage (under charging they tend to sulfate--A permanent loss in capacity).

    Do you have a voltmeter to make sure the battery bank gets fully charged, at least, once a week (>90% state of charge)?

    Do you have a hydrometer to measure (and log) the specific gravity of your cells (you are looking for min/max values for state of charge, and the "spread" between the low and high cells--High spread, you probably need to Equalize Charge "EQ" your battery bank (controlled over charging to bring up the "low cells". The typical range to begin EQ is between 0.015 to 0.030 spread between high/low cells. Do not EQ the cells too much or too often (typically once a month is "often enough"). EQ is hard on lead acid batteries (as well as beneficial--the old double edge sword).

    You need to see how your batteries are now (state of charge, how well they are getting charged, etc.)--And make sure that they are fully recharged sooner rather than later. That is a big investment in batteries.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuilt
    wellbuilt Solar Expert Posts: 763 ✭✭✭✭

    hi joe , my system and usage is very close to yours ..

    when setting up my system my out back inverter 36/48 8 gc215 battery’s and 1800 watts of solar would use about 1% of power every hour it was on .

    My loads are a small frig 1000 watts radio 230 watts 50” tv 200 watts led lights 20/30 led 6watt units 180 x 10 1800watts water pump 100 watts cpac. 20 watts with out heat 300 watts wth heat for 6/7 hours with all types of tools when the sun is out .

    The heat on the c pac used a lot of power.

    I think your inverter is runing your system down My outback uses 50 watt being on

    now I use 14% with 16 battery’s .

    The cpac keep the system running all night I may use a small inverter with a 12vbattery and charge of ac power at noon .

    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • DaangeroussDan
    DaangeroussDan Registered Users Posts: 23 ✭✭
    edited April 2019 #10

    I would check for a water leak. If your pump is coming on much more now, it would explain everything. And get one of these https://www.youtube.com/watch?v=GO_d9Ek5nXg

    Chinese inverter (if 1500 watts is enough) since they only pull 5-8 watts sitting idle. 180 bucks for 95+% efficiency. And seperate your water pump from the house inverter so you can turn it off when you sleep, just in-case of leaks. Here is the link to this product I have been using for over a year, it is a good investment and performs above its specs. https://www.ebay.com/itm/EDECOA-Power-Inverter-1500W-3000-Watt-Pure-Sine-Wave-12V-dc-110V-120V-ac-LCD-RV/232635461270?epid=14031086292&hash=item362a276e96:g:71gAAOSwm8Na~A4h

    yes, you would have to pull 12 volts off one side of the 24 volt system, and probably rotate your battery order a couple times a year to balance battery wear. But al least you will not lose everything because of a leak in the water system one night.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    Dan, we have seem the Ebay link for that inverter a bunch of times since yesterday.

    Please stop posting it as the solution for all problems.

    And splitting 12 volts of a 24 volt AGM battery bank is both going to kill the back in short order and runs the risk of a battery back fire or explosion.

    Unless the person is using 2x independent 12 volt chargers and or a large active 12/24 volt battery converter/balancer. A single 24 volt battery charging system on a 12/24 volt split battery bank... don't do it.

    Bill "moderator" B.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭

    I smell a time out on it's way.

    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.

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    I try reasonable and rational first.

    Ban and delete second.

    Bill "hint hint" B.

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