2100 ah bank not enough solar array

dhellerdheller Posts: 6Registered Users
This system was built with the intention of adding more solar panels in the near future. Well things have changed and the solar array is too small to efficiently charge the 2100 ah battery bank. That is 5 strings of 4 420 ah batterys for a total of 2100 ah at 24 volts. The solar array is able to produce 1890 watts flowing through a T80HV Apollo solar charger which I think might be able to take 3 more similar solar panels to help out. As it is the array can just barely generate enough power to do an equalize charge on the sunniest of days. I am concerned that I am going to lose the battterys due to not being able to charge them adequately. There is a back up generator that has to take up alot of the slack. What does everyone think of dividing the bank up into two appropriately wired up banks and charge one and use it for a week or two and then switch to the other half. The idea being that the exsisting array could more efficiently charge a smaller battery bank.
David

Comments

  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: 2100 ah bank not enough solar array
    dheller wrote: »
    That is 5 strings of 4 420 ah batterys for a total of 2100 ah at 24 volts.
    <snip>
    I am concerned that I am going to lose the battterys due to not being able to charge them adequately.

    You have two issues: Not enough array and too many parallel batteries. Your suggestion is reasonable and was just discussed an hour ago:
    http://forum.solar-electric.com/showthread.php?22272-over-sized-battery-banks

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Posts: 27,689Super Moderators admin
    Re: 2100 ah bank not enough solar array

    Assuming a 5-13% rate of charge for the battery bank (on solar):
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.025 rate of charge = 1,977 Watt array minimum to "equalize"
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 3,955 Watt array minimum to "cycle"
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 7,909 Watt array nominal
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 10,822 Watt array "cost effective maximum"

    Your array is roughly:
    • 1,890 Watt actual peak * 1/0.77 panel+controller losses = 2,455 Watt array (STC ratings) (rough guess)

    You only need about ~2.5% (minimum) to equalize a battery bank... And a ~1,977 Watt (STC) array will do that--Your present array looks OK if you are not cycling very much...

    Are you actually cycling the bank today? How much energy are you using per day?

    It might be a better choice to split the bank in 1/2 and just run 2-4 weeks on a bank for now (assuming the bank you leave in storage is fully charged).

    -Bill "my guess" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dhellerdheller Posts: 6Registered Users
    Re: 2100 ah bank not enough solar array
    BB. wrote: »
    Assuming a 5-13% rate of charge for the battery bank (on solar):
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.025 rate of charge = 1,977 Watt array minimum to "equalize"
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 3,955 Watt array minimum to "cycle"
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 7,909 Watt array nominal
    • 2,100 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 10,822 Watt array "cost effective maximum"

    Your array is roughly:
    • 1,890 Watt actual peak * 1/0.77 panel+controller losses = 2,455 Watt array (STC ratings) (rough guess)

    You only need about ~2.5% (minimum) to equalize a battery bank... And a ~1,977 Watt (STC) array will do that--Your present array looks OK if you are not cycling very much...

    Are you actually cycling the bank today? How much energy are you using per day?

    It might be a better choice to split the bank in 1/2 and just run 2-4 weeks on a bank for now (assuming the bank you leave in storage is fully charged).

    -Bill "my guess" B.

    Good Morning

    When you say cycling I assume you mean drawing power, I realize I have a lot of terminology to learn here so I just want to be clear.

    The system is off grid and powers a full time homestead in the cloudiest state in the Union. On the day I was monitoring the system I saw 8 amps ac on the inverter and at the same time -41 amps dc on the solar charger. If I am calculating that correctly for an 8 hour day at 24 volts that is roughly 1536 watts used. The 90 day average solar generation is 1.79 kwhr/day. I don't own this system, it is actually my neighbors whom I have a weekend camp by. So my familiarity with the system is from visiting on the weekends.

    My system is similar in components only I have a 435 ahr battery bank with 420 w array. So because of the similarities of system components, same inverter, (yes a 4000w inverter on 420 watts of solar, my system) and being neighbors I am trying to help him make the most of his present situation.

    Thanks for your help and I am greatly enjoying the forum.

    David
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: 2100 ah bank not enough solar array
    dheller wrote: »
    On the day I was monitoring the system I saw 8 amps ac on the inverter and at the same time -41 amps dc on the solar charger. If I am calculating that correctly for an 8 hour day at 24 volts that is roughly 1536 watts used.

    Something isn't adding up... Your inverter is putting out 8 amps at 120 volts = 960 watts.

    On the DC side you are drawing 41 amps from the batteries at approx 24 volts = 984 watts. That 984 watts, plus whatever is coming in from the solar array is powering the inverter. Of course the inverter is not 100% efficient, so it takes more than 960 watts on the input to produce 960 watts on the output.

    I'm not sure what '1536 watts used' means or how you calculated it.

    Also, you mentioned an 8 hour day... if you were drawing 960 watts for 8 hours that would be 7.68 kilowatthours.
    dheller wrote: »
    When you say cycling I assume you mean drawing power

    Cycling means discharging and recharging your battery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Posts: 27,689Super Moderators admin
    Re: 2100 ah bank not enough solar array

    Everything vtMaps says...

    Be very careful here--You are speaking of two systems--And confusing me between the two systems. And not using paragraphs to separate thoughts/systems.

    I will "fix" your paragraphs to make it easier to read/understand.

    And a 4 kW inverter on 420 watt array--That is an "unbalanced" system. Your large inverter is drawing around 20-60 watts just to "turn on" with no other AC loads. Say 25 watts of "tare" losses:

    24 watts * 24 hours always on inverter * 1/0.52 end to end losses * 1/4 hours of sun = 277 watt array minimum just to run inverter 24 hours per day

    And there is the 5% to 13% rate of charge for a 435 AH @ 24 volt battery bank:
    • 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 819 Watt array minimum
    • 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,638 watt array nominal
    • 435 AH * 29 volt charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,130 watt array "cost effective maximum"

    Plus, battery banks have maximum "practical" capabilities... Another rule of thumb would be a maximum inverter sizing of ~0.5 kW per 100 AH @ 24 volt battery capacity... Or:
    • 435 AH * 0.5 kW * 1/100 AH (at 24 volt) = 2.175 kW = 2,175 Watt practical (reliable) maximum AC inverter output for this battery bank

    With solar power--The old real-estate adage of Location, Location, Location applies. Guessing your location is somewhere around Concord New Hampshire, using PV Watts for a fixed array tilted (43 degrees from horizontal) to latitude:
    Month    Solar Radiation (kWh/m 2/day)
    1      3.80     
    2      4.56     
    3      4.89     
    4      5.19     
    5      5.40     
    6      5.42     
    7      5.59     
    8      5.47     
    9      4.93     
    10      4.01     
    11      3.13     
    12      2.87     
    Year      4.61
    

    Tossing the bottom 3 months as (you will have to use the generator)--Then the typical amount of "average sun" (+/- ~10% over 20 year average) would give you a "break even" solar/generator use month of October at 4.01 hours.

    A 420 watt array (ignoring significant inverter tare losses) would be around:
    • 420 Watt * 0.52 end to end system losses * 4.01 hours of sun = 876 Watt*Hours = 0.9 kWH per day from solar in average October

    I can believe you are using a lot of generator run-time... And a lot of fuel. There is a whole set of questions/calculations on generator sizing to loads/battery bank too... Just like driving a vehicle and figuring out your best fuel economy, there is something similar to balance your electrical loads to generator sizing and operating algorithm.

    More or less, a standard genset gets best fuel economy if you run it at 50% or more of rated load, and gets very poor fuel economy at 25% or less load (i.e., use 2x or more fuel than you really need to). Sometimes people end up with two generators--A large one to "bulk the battery bank" and run shop tools... And a second, smaller genset to finish up charging and run a few loads (like a Honda 1.6-3 kWatt or similar Yamaha inverter-genset).

    Lastly, which should be first--You really need to measure and log temperature corrected specific gravity for each cell in your battery banks. And make sure you are operating it in a way that will ensure long life for the battery bank. Usually something like recharging >90% State of charge a couple times a week, and avoid discharging below 50%, and don't let unused bank set below 75% SOC. Check water usage (watering every month maybe over charging; adding water every 6 months or less, probably undercharging, etc.).

    Lots of reading:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    Anyway, at this point, more questions than answers right now.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dhellerdheller Posts: 6Registered Users
    Re: 2100 ah bank not enough solar array

    Yes I agree with the statement more questions than answers at this time. I appreciate the time, thought and knowledge you have put into the difficulties both of these systems have. The operator certainly being part of the difficulty. The smaller system of my own is only used on the weekends so I very rarely have to use a generator. I certainly need to become more efficient in going forward with this system.
    I am more concerned with the 2100 ah system of my neighbors and will be changing that to two smaller battery banks that can be cycled independently. However before I do anything I have some recommended reading to do. Thanks for sharing your knowledge and giving me some much needed guidance.
    David
  • SolInvictusSolInvictus Posts: 138Solar Expert
    Re: 2100 ah bank not enough solar array

    Before you split the battery array, what type of inverter does the neighbor have and are high powered inductive loads (motors) being operated? Are the batteries kept in a warm or cold place? What is their temperature if they are cold? If the inverter does not regulate its output AC voltage, big motors are being started and the batteries are cold, the voltage on a smaller battery array may be insufficient to start the loads.

    A 4,000 W inverter with a 24 V, 2,100 Ah battery array would discharge the batteries in less than 12 hours at full power. The battery array is not sized at the 20 hour discharge rate at the continuous rated power of the inverter. There may be a reason why the system is sized this way.

    The neighbors could also be using more power than their PV array outputs. Maybe that is a problem in wintertime.

    The PV array could be malfunctioning and outputting less power than normal. 41 A at 24 V = 984 W is about half of the 1890 rated watts for the PV array. Was that due to the sun angle, shading, cloud cover or something else?

    A T80HV Apollo solar charger (80 A output, 200 V input, MPPT Charge Controller) has a maximum DC current output of 80 A at 24 VDC, and the current output of a 1,890 W PV array would vary between 67 A (at 24 VDC, 85% efficiency) and 56 A (at 28.8 VDC) going into the battery array. Adding another series string of PV panels would probably waste some power. Expansion of the PV array probably means another charge controller or a 48 V inverter and battery array. I see someone is arguing 28.8 V * 80 A / .96 = 2,400 W maximum PV array which would waste some power.
  • dhellerdheller Posts: 6Registered Users
    Re: 2100 ah bank not enough solar array

    The inverter with the 2100 ahr bank is a Xantrex SW Plus 4024. A vacuum cleaner and on a rare occasion a band saw and meat grinder are the motor loads. The generator is started up whenever the bandsaw is used. Batteries are always in a warm place. The day I was there it was mostly overcast. So the panels were not producing their max. What started me wondering if everything was alright with this system is the inverter will indicate the batteries are in float most everyday with help from the genny but the solar charger has only records 18 hrs of float time since the bank was installed 8/20/13. Six of those hours were the result of an equalize charge on 1/6/14. The solar charger never indicates above 86% SOC. There is a Midnight Solar battery monitor on the bank that indicates 100% all the time since it was installed at the same time the equalization took place. The owner is very good about not letting the SOC go lower than 75% but that means running the generator once a day for 3 hours.
  • SolInvictusSolInvictus Posts: 138Solar Expert
    Re: 2100 ah bank not enough solar array

    Because the Xantrex SW Plus 4024 regulates output voltage and the motor loads are not so severe, the voltage sag on a split battery array under load probably will not be a problem.
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