Battery Bank Decision ?

Arkansasoffgrid
Arkansasoffgrid Solar Expert Posts: 117 ✭✭
in Off Grid Solar & Battery Systems #1
After multiple load calculations, I'm stuck between 2 sizes of battery bank. Basically being very AH conservative and getting 1 string, or paralleling 2 strings and having more days of "reserve power". The question is, is it better to have 8 batteries "floated" every day and a 25-40% discharge nightly, or 16 batteries "floated" every 2-3 days and discharged 12-20% nightly? These are the batteries http://www.powerstridebattery.com/us-l16hc-xc-6-volt-420-ah-solar-storage-battery
Planning on opportunity loads if we only use 1 battery string, since there will likely be excess PV wattage daily after charging.
8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Battery Bank Decision ?
    Arkansasoffgrid wrote: »
    The question is, is it better to have 8 batteries "floated" every day and a 25-40% discharge nightly, or 16 batteries "floated" every 2-3 days and discharged 12-20% nightly?

    If you spend twice as much on the batteries, they won't last twice as long. my opinion: 1 string is better design, more cost effective, easier to monitor and maintain.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Arkansasoffgrid
    Arkansasoffgrid Solar Expert Posts: 117 ✭✭
    #3
    Re: Battery Bank Decision ?

    No doubt about the one string simplicity, but with my $$ budget 420AH seems to be the 1 string limit.
    8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Battery Bank Decision ?

    A 25 to 40 percent overnight usage is quite a wide range.

    In general 25% daily DOD is a very economical use of battery: good balance between capital cost, power availability, recharging structure, and lifespan.

    If you double the battery capacity you need to double the PV as well in order to be able to recharge it when needed, which means if you only use 20% daily you will have an even larger "surplus" of power available from the PV that likely will not be utilized.

    So ask yourself the question: if I use one string and run out of power in a day (or night) what will the consequences be?

    If you can control loads so that the deep discharge is never reached, great. If you can start the gen as-needed and bulk up whenever the sun fails you, great. If your critical equipment is going to go off in the middle of the night with dreadful consequences, increase the battery bank.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #5
    Re: Battery Bank Decision ?
    Cariboocoot wrote: »
    So ask yourself the question: if I use one string and run out of power in a day (or night) what will the consequences be?

    If you can control loads so that the deep discharge is never reached, great. If you can start the gen as-needed and bulk up whenever the sun fails you, great. If your critical equipment is going to go off in the middle of the night with dreadful consequences, increase the battery bank.

    And don't forget the increase in battery requires an increase in associated solar array. Or run the generator on those days when you really run it down.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
    Re: Battery Bank Decision ?

    or plan c----look at other batteries that may fit better your requirements.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #7
    Re: Battery Bank Decision ?
    Arkansasoffgrid wrote: »
    I'm stuck between 2 sizes of battery bank. <snip> getting 1 string, or paralleling 2 strings
    question asked.

    and answered:
    Arkansasoffgrid wrote: »
    with my $$ budget 420AH seems to be the 1 string limit.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Arkansasoffgrid
    Arkansasoffgrid Solar Expert Posts: 117 ✭✭
    #8
    Re: Battery Bank Decision ?

    What I was trying to say was that if I stay with 1 string, that particular battery seems the best value at 420ah for $299 per battery. My question is, with a array of 18-255w panels would it be best to stay with 1 string at 420ah or is that array sufficient to charge 2 strings at 840ah total ???
    8-420AH US Battery L-16s, Midnite Classic 150cc, Whiz Bang Jr, Magnum MS4448PAE inverter/charger,  4590w Canadian Solar panels. Honda EU2000i generator.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: Battery Bank Decision ?

    18*255 watt panels = 4,590 Watt array

    Using the 5% to 13% typical rate of charge for solar:
    • 4,590 Watt array * 0.77 panel+controller derating * 1/59 volts charging * 1/0.05 rate of charge = 1198 AH @ 48 volt bank maximum
    • 4,590 Watt array * 0.77 panel+controller derating * 1/59 volts charging * 1/0.10 rate of charge = 599 AH @ 48 volt bank nominal
    • 4,590 Watt array * 0.77 panel+controller derating * 1/59 volts charging * 1/0.13 rate of charge = 461 AH @ 48 volt bank minimum
    The above battery bank range is based on the recommended rates of charge for a battery bank. 5% is the minimum recommended--Weekend use and requires lots of load management to ensure battery bank is properly recharged.

    10%-13% rate of charge is recommended for daily off grid life. Can use some power for day time loads, less load management needed.

    Using PV Watts for Springfield MS (not sure where system will be use), fixed array, tilted to latitude (37 degrees from horizontal):


    Month
    Solar Radiation
    (kWh/m2/day)


    1
    3.93


    2
    4.20


    3
    5.18


    4
    5.54


    5
    5.71


    6
    5.80


    7
    5.92


    8
    5.99


    9
    5.44


    10
    4.95


    11
    3.83


    12
    3.29


    Year
    4.99



    Toss the bottom three months as needing a generator during bad weather, gives us 4.2 hours minimum sun for February:
    • 4,590 Watt array * 0.52 typical off grid AC system eff * 4.2 hours of sun per day (Feb) = 10,025 WH = 10 kWH per day

    Sizing the battery bank to 2 days of storage and 50% maximum discharge (for long battery life, cost effective/balanced system design):
    • 10,025 WH per day * 1/0.85 inverter eff * 1/48 volt bank * 2 days * 1/0.50 max discharge = 982 AH @ 48 volt battery bank
    Or, based on "optimal" battery bank sizing of ~10% rate of charge, the "over night" nominal load would be:
    • 599 AH * 48 volts * 0.85 inverter eff * 1/2 days * 0.05 max discharge = 6,110 WH "overnight" optimal load
    So, based on your array sizing, we have the nominal (February generator break even month), and sized the battery somewhere between 599 AH to 983 AH @ 48 volts as being a "balanced" system design--Depending on your loads and daytime vs overnight loads.
    You can also size the battery bank based on maximum continuous loads (pumping/AC/etc.) and/or maximum surge loads (motor starting surge for well pump, etc.)...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
    Re: Battery Bank Decision ?
    BB. wrote: »
    • = 1198 AH @ 48 volt bank maximum
    • = 599 AH @ 48 volt bank nominal
    • = 461 AH @ 48 volt bank maximum

    is that a typo? I think you are trying to say that the minimum (cost effective) battery capacity is 461 AH.

    Of course, a smaller battery bank (420 ah) can be used, but it would be a good idea then to set the controller to limit the max current into the bank. Over paneling battery banks has become more popular as panels become cheaper, and it may allow for a complete charge on some days of marginal insolation.

    I do agree, though, that over paneling to exceed a 13% charge rate is usually not cost effective unless there are some opportunity loads to use the extra power on sunny days.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Battery Bank Decision ?

    Or:

    'Ideal' battery bank size for that much array would be 736 Amp hours @ 48 Volts.

    It is a very large array for only 420 Amp hours @ 48 Volts so you would be 'wasting' some of it on the best days.

    For 849 Amp hours @ 48 Volts the peak charge rate would be around 8.7%. A tad low, but viable.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: Battery Bank Decision ?

    Thank you vtMaps--Fixed. Use cut and paste to make equations.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #13
    Re: Battery Bank Decision ?
    BB. wrote: »
    Depending on your loads and daytime vs overnight loads.

    I agree - spend more time on moving loads to the daytime and the typical guidelines might not be very applicable. On the other hand, most things don't make that so easy.

    I am available for custom hardware/firmware development

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #14
    Re: Battery Bank Decision ?

    One reason I don't focus too much on moving loads to daytime is that many people would still like to have electricity on demand even when there a a couple days of clouds (2 days of "no sun"--Heavy clouds can reduce solar array output to 5% of normal).

    If you have a business, kids studying, etc... You usually need the power whatever the weather is like. If you do not use power much during the day (work outside the home) and/or have variable/optional loads (like cutting back on irrigation and clothes washing during poor sunlight)--Then the Day/Night/Optional load "juggling" can be a viable consideration.

    Energy usage is highly personal. I try to make the system meet the needs of the owner, rather than fit the owner to the capabilities of the system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
    Re: Battery Bank Decision ?

    This is why we do initial designs based on all the energy being stored in and pulled from the batteries; you never know when they're going to use it. So the 20 hour rate for battery capacity is used (4 hours of the day for charging) with 24 hours power usage from that capacity; zero energy use direct from panels. Thus the over-all efficiency of 52%. Most people will use at least some power during the day and have an improvement over that percentage.

    If you can manage the loads, fine. My system runs much higher than 52% efficiency but requires active load management. That can be a big problem for some people.

    If you can't, then you need the fudge factors working to your advantage. No one ever complains about having too much power capacity, but plenty have complained about running out before the next charge cycle begins.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #16
    Re: Battery Bank Decision ?

    And do not forget seasonal loads... Fans, some A/C, refrigerator/freezers/water pumping all use more power in hot/sunny weather. Right when there is (for many places) lots of summer sun.

    However, you need more hours of lights, possibly clothes washing because of mud/wet, moving around heated air (or even a heat pump system)--Not much sun, you are pretty much forced to run a generator (as efficiently as you can) to make up for short days/poor weather.

    How many people can predict their seasonal needs--Not many. To size a solar power system and spend your money optimally--You want to really know you power needs something like +/- 25% of their true value. If you are off by a factor of 2x or 1/2--Your system is over sized and potentially expensive to maintain (too much battery bank, may more panels than you need)--Or too small, and would require major changes (upping battery bank voltage, what to do with old battery bank/"wrong DC voltage" AC inverter and AC backup battery charger, perhaps too small of genset, etc.).

    There are no easy answers regarding "knowing" your power usage--And to a degree--Going off grid is going to change you and your power habits. It just depends on how much you (and your spouse/family) are willing to bend vs $$$ out of wallet costs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #17
    Re: Battery Bank Decision ?

    I don't know about A/C.
    It's June 30th, 9C, and the wood fire is burning.
    What is this "Summer sun" of which you speak? :p
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #18
    Re: Battery Bank Decision ?
    Cariboocoot wrote: »
    I don't know about A/C.
    It's June 30th, 9C, and the wood fire is burning.
    What is this "Summer sun" of which you speak? :p

    Oh man it is currently 36C here @ 11 AM, head to 41C later today. A/C is pounding hard already. Consume 350 kWh off the net-meter reserve last month and bought another 587 kWh of off peak (for $13.23).
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #19
    Re: Battery Bank Decision ?
    solar_dave wrote: »
    Oh man it is currently 36C here @ 11 AM, head to 41C later today. A/C is pounding hard already. Consume 350 kWh off the net-meter reserve last month and bought another 587 kWh of off peak (for $13.23).

    The Canadian "advantage": if we want ice in our drinks we just go chip a piece off the nearest glacier. Even in August. :p
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  • SkiDoo55
    SkiDoo55 Solar Expert Posts: 414 ✭✭✭
    #20
    Re: Battery Bank Decision ?

    -Cariboocoot. You can bring all of the Ice/Water you want to get rid of! I can melt it for you. Had about 42.5C (109F) yesterday and about 39C today. First day this year I haven't produced any surplus in production. Made about 58kWh and used the same. Looks like I will break even today for all of the A/C that has been running. Wife doesn't like the heat! Was up in Yellowknife one fall working for Buffalo Airways, doing some engine buildup for Joe, so I have experienced some Canadian snow and Ice. Lived in Upstate NY for 18 years too!

    keep up all of the Great information you put out. I enjoy the information and sharing of knowledge and experience.
    Will see if my new XW4024 gives me any problems. Get batteries this week. Let controller charge them up and flip the switches and see how it works.
    Also, know the kind of heat the Solar Dave gets have spent many summers working Tucson pulling aircraft out of storage there.
    GT3.8 w/4600W Trina 230W, TX5000 w/5000W ET-250W, XW4024 w/1500W ET-250W, 4 L16, 5500W Gen. (never had to use) Yet!!
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,891 ✭✭✭✭✭✭
    #21
    Re: Battery Bank Decision ?

    If I had unlimited ice and Canadian Whiskey I would not get up in time to see the sun rise. What are you cooling with Skidoo to use 58.8 kwh ? We were 99F yesterday and turned the mini split on at 6 am and ran it until midnight, about 24 KWH. My record is about 30 KWH and I still needed more load to max out. The only problem you will have with the xw will be related to programming the sell function. I happily do not have to do that. It can be done!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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