Storage calculators?

rickeolis
rickeolis Solar Expert Posts: 110 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
Hi everyone,
I have recently doubled the numbers of panels in my system, and have put a nice charge controller in the mix as well, but now am finding that I am not storing enough to get me through the night.
I should recheck my math using a system calculator to verify that I'm running the right number of batteries and panels for the load sizes I use.
Does anyone recommend an easy spreadsheet or calculator for this?
Thanks-

-Rick-
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Storage calculators?

    You need to better characterize your system:
    • Watt*Hours of kWhrs of energy used per day
    • Watt*Hours / kWhrs of energy generated
    • State of Charge of your battery bank before sun up and at sun down
    I would also suggest a Battery Monitor (I did not see one in your sig line).

    Is this an off-grid or grid tied system?

    In the end, power in, power stored, and power out is what you need to know.

    (6*180 watt + 6*190 watt) * 5 hours of sun * 0.52 efficiency = 5,772 watt*hours per day

    Say you need 18 hours of power over night:

    5,772 watt*hours / 18 hours = 321 watts of average power over 18 hours...

    If you use no power during the day, you can run ~320 watt load for 18 hours...

    The minimum battery bank size:

    5,772 watt*hours / 24 volt = 240 Amp*Hours per day of available energy...

    Assuming 3 days of no-sun backup and no more than 50% maximum discharge:

    240 AH per day * 3 days * 1/0.50 = 1,440 Amp*Hours of battery bank @ 24 volts

    If you have more than 240 AH of battery bank--it sounds like your batteries may:
    • not be getting fully charged
    • damaged by over discharging/under charging
    • other damage (boiled dry, electrolyte contamination, etc.)
    • have wiring problems (corroded connections, unbalanced parallel connections, etc.)
    A Battery Monitor will be your best friend here. At least do a few Specific Gravity measurements morning/evening (make sure all cells are good--then just measure a sample cell daily) until you understand the behavior of your battery bank in this system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Storage calculators?

    rick,
    you upped your pv capacity and have a good controller so that leaves the batteries and the loads on the batteries barring no faults to the controller or pvs. did you increase the draw on your batteries? if so, then you may need more capacity if all of your batteries check out to be good. you already know of mixing old and new batteries so i won't go in that direction.
    as to a calculator, i believe bad apple had one and he had a link to it on one of his posts. i didn't go into it very far, but it looked like it was quite elaborate and covered much ground.
    in general on the surface it sounds as if you're loading down your system more than it was originally capable of as far as the battery capacity is concerned. if that's all too much for you to figure out then load us down with all system specs and usages as best you can and maybe we'll come up with something.

    edit to add:
    in searching for the calculator it was a thread he created on sept 10, 2007.
    http://forum.solar-electric.com/showthread.php?t=1124
    post 11 gives a zip file link.
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  • rickeolis
    rickeolis Solar Expert Posts: 110 ✭✭✭✭
    #4
    Re: Storage calculators?

    Thanks Bill and Niel,
    I have 24 Trojan T-105 batteries (4 banks of 4 each) and they are all less than 2 years old, but some have seen deep discharges at times. I read them all and looked at the water last weekend and all show more than 6 volts and had enough water.
    I'd have to say that I do use too much power consistently I think. Most days while I look at the pull from the inverter, I see typically around 4-5 amps drawing from the house. This is around 600 watts, all the time, that's a lot.
    Bill, your suggestions: (And my answers)
    1) Not be getting fully charged (I think that is fine, most end of day readings show over 28 volts)
    2) Damaged by over discharging/under charging (This could be possible, a monitor should help me see that)
    3) Other damage- boiled dry, electrolyte contamination, etc. (Visual checks appear OK)
    4) Have wiring problems- corroded connections, unbalanced parallel connections, etc. (I solder all my connections, and it is bone dry where I live. I have an electrical background, so the wiring and connections I feel are OK)
    5) A Battery Monitor will be your best friend here. At least do a few Specific Gravity measurements morning/evening make sure all cells are good--then just measure a sample cell daily until you understand the behavior of your battery bank in this system. (This might be the suggestion that will help me solve the problem. I need to buy a Prometric or something, as well as begin taking specific gravity readings of my banks)
    I am replacing a large TV this weekend with a new LCD. My toddler loves his video games, so that should help some. We have 2 adults and 4 kids here, so there is only so much I can do! Also, my fridge is about 15 years old and is due as well. Otherwise, I have gone to a tankless gas water heater, all CF lights, and we try harder to keep things turned off when un-needed.

    -Rick-
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Storage calculators?

    rick,
    from what i see here the batteries are being depleted. you have 6 pvs supplying 6.95a@24v for a total of 41.7a. you have 24 t105s for 6 rows of 4 for 1350ah @24v. that is a 3% charge rate only during the full sun rating of the pvs. that alone means they are undercharging unless you allow the pvs to bring up the battery bank to full charge before using and that is a 20hr rate and if at 50% dod that would mean 10hrs of full sun. that was a big if as you are draing them far below 50% and 10hrs sun is around 2 days.
    the 2nd wammy to this is the constant 600w load you have on it for 14400w drained every day(600w x 24hrs). that far exceeds what the pvs are capable of delivering per day as the pvs at full stc ratings of 180w x 6 of them is 1080w (most likely less than this) and if you get 5hrs of full sun this equals 5400wh. i would say you have pretty much been killing the batteries by about 3 fold.:cry::cry::cry:
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  • PhilS
    PhilS Solar Expert Posts: 370 ✭✭✭
    #6
    Re: Storage calculators?

    Seeing a constant 600w load I'm guessing your 15 year fridge may be your biggest problem. Refrigerators have come a long way and you can simply verify your load with a Kill-A-Watt.

    The fridge I bought a year ago uses about 100w when it's running, nothing between cycles. As long as our weather is good our battery bank has no problem keeping up, and it's half the size of yours (a dozen T-105s).

    Double check the power usage of the TV you plan on buying too. I have a 7 year old projection TV and it uses about 200W. I'd planned on buying LCD when this one goes FUBAR but recently I've seen it mentioned that the larger LCDs and most of the plasma units actually use more power than tube/projection. I haven't verified this but you should be since you are buying now.

    We also have "entertainment" on a power strip and it gets turned off when we retire for the night. The phantom loads might be hurting your overnight supply.

    Phil
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  • rickeolis
    rickeolis Solar Expert Posts: 110 ✭✭✭✭
    #7
    Re: Storage calculators?

    Thanks guys,
    Yeah, I need to put my KillaWatt on the fridge again to see what it's doing. The 33" CRT television HAS to be a problem too, so I'll KillaWatt it too to see.
    Thirdly, I need to identify the rest of the phantom loads using the KillaWatt too. The power pack for our Bose is quite large and we forget to turn the switch off on the power strip that it's plugged into at times. Maybe I could consider a timer for that...
    So, based on what you guys just said, it looks like I'm using more power than what I need to top off the batteries with. THAT is something I can change. I just need to identify the reasons first.
    I really appreciate the input guys!

    -Rick-
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Re: Storage calculators?

    i agree with phil that the tv you go to may use just as much power or more. i'm guessing you'll see somewhere around 100w give or take with a killawatt on your present tv. if you have the room and the cash, consider adding some more pvs too. you may have shortened the life of your current batteries so this is a near future expense and we just can't say when specifically. you do need to get those batteries fully charged now though and with an eq.
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  • rickeolis
    rickeolis Solar Expert Posts: 110 ✭✭✭✭
    #9
    Re: Storage calculators?

    You really do learn something new every day- I did read the old (w/KillaWatt) CRT 33" TV and the new 42" LCD TV, and you were right, the new one uses a bit more power than the old one. (113 watts vs. 97)
    I had heard that the step up transformers that produce the really high voltages needed for the CRT guns (80,000+ volts) used a lot of power, where the LCD's didn't need that.
    Anyway, I am currently looking at a set of 4 LH-16 batteries to add to my bank. Close to a grand, but I need them.
    I do equalize about every 90 days, for about 4 hours. The new FlexMax 80 allows me to program in EQ times, so I set it for 3 hours of EQ every 30 days, hoping that's pretty good...
    This morning before leaving for work, while everyone was asleep still, I looked at the inverter load, and it was only one amp, so the fridge must have been off. It shows to use about 170 watts when running. I did unplug a power pack that my computer uses for the external speakers, and will simply use the cheaper built in monitor speakers instead for now.
    Yeah, you can never have enough PV panels, but I was hoping I had enough power to be off-grid at this point. I am getting close...

    -Rick-
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #10
    Re: Storage calculators?

    rick,
    i'm sorry i overlooked the 6 canadian solar 190s as that would put you in theory in the right charge range with pv if parallel to the 6 evergreens, at least until you add more to the battery bank capacity. i said in theory because a constant load like 600w will effectively reduce the charge current seen by the batteries and again making the percentage seen by the batteries into the low range. that still leaves you as using too much of the power without it being replaced. if there's anything wrong with the capacity levels of the old batteries, it'll drag down the capacity of the new batteries and you'll still need more pv from a power usage standpoint because of your not replacing all that is used. overall, this is looking like a deficeit charge and can sulphate your batteries.
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  • rrbv
    rrbv Registered Users Posts: 16 ✭✭
    #11
    Re: Storage calculators?

    I agree that a good net current based battery monitor is essential to an off grid system. Why does no one to the best of my knowledge, make a shuntless net current meter? You can buy all kinds of inexpensive clamp on current meters. Sounds like a great opportunity for someone to build and sell one. I have an offgrid house, with a state of charge meter hooked to a shunt in series with the inverter and never any problems, but my electric side by side only has a cheap voltmeter type of monitor. The true state of charge meter measures the current in, subtracts the current out and with the battery capacity information, displays a true state of charge, not some voltage measurement subject to huge errors. Would be easier to install a good meter if it were clamp on type!
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #12
    Re: Storage calculators?
    rrbv wrote: »
    I agree that a good net current based battery monitor is essential to an off grid system. Why does no one to the best of my knowledge, make a shuntless net current meter? You can buy all kinds of inexpensive clamp on current meters. Sounds like a great opportunity for someone to build and sell one. I have an offgrid house, with a state of charge meter hooked to a shunt in series with the inverter and never any problems, but my electric side by side only has a cheap voltmeter type of monitor. The true state of charge meter measures the current in, subtracts the current out and with the battery capacity information, displays a true state of charge, not some voltage measurement subject to huge errors. Would be easier to install a good meter if it were clamp on type!



    Most of the DC clamp ons are notorious for having to reset the zero on each reading. They drift pretty good between readings, hence the shunt is the best solution.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #13
    Re: Storage calculators?

    More or less, it is more difficult and expensive to measure DC current with a current clamp of some sort. The typical DC Current Clamp meter uses a Hall Effect Transistor that needs to be calibrated every few minutes (+/- depending on application/meter/accuracy required) because of drift.

    AC is very easy to measure... You put a current transformer around the wire--And use a 10:1 (or whatever ratio) to step down (or up) the current. The CT is very accurate and and the output of the CT connects to a shunt and volt meter to read the scaled current--Pretty much what is done with a DC Current Shunt--Without the easy isolation/scaling of a current transformer.

    At this point, the current shunt + Kelvin Contacts is the best you are going to get (as far as I know).

    One thing that makes the DC Current shunt a bit safer is to make its electrical connections on the "grounded side" of the battery bank. That way, and shorted wires and such are virtually at zero volts--So no extra fusing required for sense leads (very common to use resistors or similar to limit excessive current flow anyway).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • drraptor
    drraptor Solar Expert Posts: 218 ✭✭
    #14
    Re: Storage calculators?

    any commercial products based on current shunt + kelvin contacts ? or even better any open-source project ?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #15
    Re: Storage calculators?

    I am not quite sure I understand your question... Pretty much the majority of Battery Monitors (and Victron) use a current shunt (with Kelvin contacts).

    The Midnite WhizBang Jr. does too--It just has an Analog to Digital converter board mounted on the shunt--Makes it easier to add general purpose I/O digitally to the Charge Controllers (vs having a dedicated A to D in the charge controller).

    Here are several battery monitors that measure voltage only (no shunt):

    MidNite Solar MNBCM Battery Capacity Meter
    www.smartgauge.co.uk

    Poster NSASpook has his own DIY Battery Monitor that (I believe) he opened source. Also is using Hall Effect transducers.:

    DIY battery monitor    /charger

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • drraptor
    drraptor Solar Expert Posts: 218 ✭✭
    #16
    Re: Storage calculators?
    BB. wrote: »
    I am not quite sure I understand your question... Pretty much the majority of Battery Monitors (and Victron) use a current shunt (with Kelvin contacts).

    The Midnite WhizBang Jr. does too--It just has an Analog to Digital converter board mounted on the shunt--Makes it easier to add general purpose I/O digitally to the Charge Controllers (vs having a dedicated A to D in the charge controller).

    Here are several battery monitors that measure voltage only (no shunt):

    MidNite Solar MNBCM Battery Capacity Meter
    www.smartgauge.co.uk

    Poster NSASpook has his own DIY Battery Monitor that (I believe) he opened source. Also is using Hall Effect transducers.:

    DIY battery monitor    /charger

    -Bill
    Of these three methods battery monitors with shunt based current measurement + voltage is considered the most accurate ? e.g Victron and midnite WhizBang Jr. Can we use Whizbang Jr. without the midnite's CC and record the result on a SD card or a PC ?

    While I believe the Voltage only method is the least accurate.

    Hall effect sensor based approach require frequent recalibration.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #17
    Re: Storage calculators?

    The WhizBang Jr. is "simply" an analog to digital converter with (I think) a one wire bi-directional digital bus (it certainly can be a two wire bus).

    You would need to contact Midnite to see if they have released the interface specifications/driver for the board for others to use on a separate PC.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ILFE
    ILFE Solar Expert Posts: 364 ✭✭
    #18
    Re: Storage calculators?
    BB. wrote: »
    I would also suggest a Battery Monitor (I did not see one in your sig line).


    Bill, you may be using an old URL bookmark? Here is the correct one.
    Paul
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