Small system

Hi everyone!  I have some newbie questions I hope to get answered through everyones expertise.

I want to install something similar to what was described in the Aug-Sept 2003 issue of Home Power called Renters PV System.  Basically they had a "portable" PV system with three 100W PV panels, two 105AH AGM batteries, a Solar Boost 50 Charge Controller, and a Statpower Prosine 1000.

I was thinking of installing one 125W PV panel, two 105AH AGM batteries, a Xantrex C35 Charge Controller, and a Prosine 2.0
I am hoping that this system will supply 300W for 30 hours a week.  The PV panels will get about 6 hours of sun daily throughout the year.

My questions -

1)  Am I missing anything from this system?

2)  Is one PV panel sufficient?

3)  Is there any benefit to go with larger gauge connecting cables than required, or am I just wasting money?

4)  What is the maximum distance that the PV panel can be from the inverter? 

I think that's it for now.
Thanks for sharing your knowledge!
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Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Small system

    ralos,
    the initial system was a balanced system and what you propose takes it too far from that balance. the charge rate would be under 3.5% so it will take much longer to charge the batteries to reach full capacity no matter that you get alot of sun. 100%/3.43%(per hr)=29.15hrs and add to that charging inefficiencies. do you want to use it down to 50% and then wait a minimum of 2.5 days of full sun to use it again? also reconsider using something as large as the prosine 2.0 on that system as it will have a max draw that far exceeds that which your batteries were designed for. you say you need 300w, which those batteries could provide for about 4hrs, so consider maybe a 600w inverter to allow for future expansion and small term temporary higher loads that the batteries could handle. basically you can't starve the batteries for their charging power and turn around and increase the loads to the batteries. the system you proposed would soon kill those batteries.
  • System2System2 Posts: 6,290 admin
    Re: Small system

    Hi niel,

    Thanks for your response. I wanted the Prosine 2.0 because it was an Inverter/Charger and allowed for expansion. What configuration would you recommend for the Prosine 2.0?

    Also, are there smaller Inverter/Chargers that are reputable?

    Thank you!
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system
    ...The PV panels will get about 6 hours of sun daily throughout the year.

    Ralos,

    Can you tell us a bit more about where you are located?

    More later...
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Small system

    Hi crewzer,

    I am located in the mostly sunny state of Hawaii.

    Ralos
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Ralos,

    I'm using my small system to run a chest freezer. My setup is: 340-Watts of PV, 420ah battery bank at 12-volts, 1100-Watt Inverter, 20a Morningstar charge controller. It's been three (sunny) days so far, and battery voltage is holding up well. I've checked the chest freezer with my Kill-a-Watt meter and found that it uses almost 800 Wh per day. So due to clouds, inefficiencies and losses, I'm probably borderline.

    By the way, I'm located in Southern Illinois, getting about 4 peak hours of sunlight a day.

    John
  • System2System2 Posts: 6,290 admin
    Re: Small system

    Hi John,

    I was wondering how many PV panels you have? I am currently renting that's why I want to keep this setup to a minimum and was hoping I could get away with one panel. I heard that anything above 130W would not be for a 12V system.

    Also, what kind of inverter are you using and is that also a charger?

    Thanks,
    Ralos
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Small system

    ralos,
    maybe if you anticipate larger future loads and will use the grid to back you up through the prosine's charger you should get the batteries you think you will need now rather than later. the small solar you plan for it will not do much for a big battery bank though it may offset some of the charge time. make sure the batteries you get can take the combined charge from the solar and the prosine's builtin charger. batteries are not an item you want to go getting more of in the future as differing ages can cause the new ones to be degraded to the same degree as the old ones, but underusing a larger bank with maintaining the charge will perform better even though bought now unless you plan for the extra loads greater than 5 years (rough figure) from now. in that case you could go with el cheapo type batteries that'll last until then and you'll buy the real good ones you should have later. i'd recommend more than 210ah of batteries in any case. i don't want to go on here as i could confuse the issue with too many other variable possibilities.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system

    Ralos,
    I was thinking of installing one 125W PV panel, two 105AH AGM batteries, a Xantrex C35 Charge Controller, and a Prosine 2.0.   I am hoping that this system will supply 300W for 30 hours a week.  The PV panels will get about 6 hours of sun daily throughout the year.  I am located in the mostly sunny state of Hawaii.

    1)  Am I missing anything from this system?

    You’ve got the basic sub-systems. You’ll need circuit breakers between the PV array and the controller, between the controller and the batteries, and between the batteries and the inverter.
    2)  Is one PV panel sufficient?

    The short answer is “no”. Let’s go through the analysis.

    > 300 W x 30 hours/week / 7days/week = 1,286 Wh/day net.
    > Assuming 85% efficiency for the inverter and the related cabling, the batteries will have to supply 1,513 Wh/day.
    > Assuming a nominal 12 V battery bank, that’ll be 126 Ah/day.
    > Problem #1: A good rule of thumb is to size the battery bank at ~6X the average daily energy requirement. Instead of 2 x 105 Ah = 210 Ah, you need to consider a bank rated at ~750 Ah, or three size 8D batteries wired in parallel.
    > Assuming 98% coulombic efficiency for the batteries (Ah out / Ah in), you’ll need a PV array that is rated to supply ~129 Ah/day.
    > Worst monthly case average daily insolation on a south-facing PV array tilted up at latitude for Honolulu is 4.8 hrs/day in December.
    > To generate an average of 129 Ah/day, you’ll need an array rated at 129 Ah/day / 4.8 hrs/day = ~27 A Imp.
    > Problem #2: Assuming a Vmp of 17 V, you’ll need a PV array rated at ~460 W STC, or almost 4X the single PV module envisioned.
    3)  Is there any benefit to go with larger gauge connecting cables than required, or am I just wasting money?

    There’ll likely be no benefit in using cables larger than “required”.
    4)  What is the maximum distance that the PV panel can be from the inverter?

    This depends, as the controller and batteries will be connected between the PV panel(s) and the inverter. In general, you want to keep these low voltage cables as short as possible. See Table 3-2 in the inverter’s manual. One solution might be to keep all of the DC cabling short, and then run long cables from the inverter to the AC loads (low current).
    Also, are there smaller Inverter/Chargers that are reputable?

    I’m not aware of any quality inverter/chargers that are smaller that ~2 KW. However, you might want to consider an individual charger and a separate inverter. For example, the Xantrex TC-20+ and -40+ are excellent chargers. Exeltech makes very good true sine wave inverters. For your 300 W load, a 600 W unit should work well. For expansion purposes, and 1,100 W unit might suffice.

    See: http://store.solar-electric.com/xaprbach.html
    and: http://store.solar-electric.com/exsiwain.html

    Finally, note that inverters draw some amount of power even when their AC loads are switched off. This can range from as low as ~ 2W for the ProSine when it’s in “search” mode, to as high as 25 W when idling. Considering that there are 168 hours in a week, that will influence the calculations above unless you turn off the inverter when not in use with your planned loads.

    HTH,
    Jim/crewzer
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Hi Ralos,

    I have 4 panels, 85-watts each, connected in parallel for a total of 340 watts at 12 volts. Since you need portability, you might consider those (Kyocera KC85T) as well. They measure less than 2 feet by less than 4 feet each.

    My inverter is Exeltech True Sine Wave 1100-watt. It is not a charger. It is quiet, and durable. I love it, and would recommend it to anyone. Since I started with an el-cheapo MSW inverter, I really appreciate the difference.

    As others have said, it seems that you'll need a little more PV capacity and battery capacity than you've proposed from your system to get 300-watts times 30 hours. My math goes something like this:

    340 watts of PV times 4 hours of sun equals 1360 watts of production on a sunny day.

    The load, a chest freezer, requires 800 Watt/hours per day, therfore I have a surplus of power.

    My 420ah battery bank seems to be plenty big to handle the load.

    Hope this helps.
    John

  • mike95490mike95490 Solar Expert Posts: 8,961 ✭✭✭✭✭
    Re: Small system

    You could restrict yourself to having a timer shut down the freezer at night, and and avoid the battery drain, or maybe allow it only one 2 hour cycle

    And more panels so you can recharge
    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 ,

  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Mike90045
    You could restrict yourself to having a timer shut down the freezer at night, and and avoid the battery drain, or maybe allow it only one 2 hour cycle

    Would there be any significant advantage to doing that? The timer is an additional load.

    Additionally, I wouldn't want to risk having my frozen food partially thaw once a day.

    John
  • mike95490mike95490 Solar Expert Posts: 8,961 ✭✭✭✭✭
    Re: Small system
    SolarJohn wrote:
    Mike90045
    You could restrict yourself to having a timer shut down the freezer at night, and and avoid the battery drain, or maybe allow it only one 2 hour cycle

    Would there be any significant advantage to doing that? The timer is an additional load.

    Additionally, I wouldn't want to risk having my frozen food partially thaw once a day.

    Compared to the freezer, a timer that consumes 2 watts is insignificant. A chest style freezer, should keep frozen solid for at least a day, if fully loaded, and the lid kept closed.

    suggestion for timer schedule
    on 10AM sun should be up, and inverter run off solar, then recharge batteries.
    off 6pm
    on 2am run to keep cold overnight (and not kill batteries)
    off 4am

    Set it to run 3 hours at night , or try a remote read thermostat But you'll still need more PV panels to insure recharging the batteries before they sulphate.

    Also, will fresh food be pre-chilled, or will you be loading a batch of warm stuff? A large batch of warm food will really need some power (& time) to chill down.

    What's with a charger (prosine2) ? Are you going to use the grid to recharge the batteries ?
    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 ,

  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Thanks Mike

    If I try the timer plan as you've suggested I'll also connect my Kill-A-Watt meter to see how much of an advantage there is to doing it that way. Understanding that the compressor will have to run longer to re-cool a freezer that has been disconnected from AC power for a few hours, I'm not sure how much advantage I'll experience. It will be an interesting experement.

    Another variable I wanted to mention is that the freezer is part of my emergency plan. When the grid power fails for an extended period of time, I intend to use the freezer to make ice to be used in coolers, keeping the rest of my food refrigerated without using my refrigerator. Because the water in the freezer needs to freeze, I may not want to use the timer plan during a grid-power outage.

    John
  • System2System2 Posts: 6,290 admin
    Re: Small system

    Thank you crewzer for answering all my questions.  Those are some excellent tips that will help me out a lot.  I'll have to rethink this configuration.  So many things to consider, when you think you got it half way figured out you learn something new.


    Also, thank you SolarJohn.  You say your Exeltech True Sine Wave 1100-watt is quiet, are there noisy inverters?  I'm assuming you're talking about the cooling fan noise.  I know I can hear my computer fan going and it is not as quiet as the one I have at work.


    Thanks everyone for this excellent forum, feedback and knowledge!
    Ralos
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Ralos, The cooling fan on my el cheapo MSW inverter was MUCH louder than a computer cooling fan, very annoying.

    And, as you probably already know, motors didn't run at the right speed with the MSW inverter, and it put noise in the audio of a radio, and caused streaks in the tv picture. I can't emphasize enough how happy I am with my Exeltech True Sine Wave inverter.

    The Exeltech does have a cooling fan, but it has never kicked on, even when I connected a heavy load (a microwave oven or toaster). I'm tempted to run a heavy load for a longer time, just to see if it works.

    John
  • mike95490mike95490 Solar Expert Posts: 8,961 ✭✭✭✭✭
    Re: Small system
    SolarJohn wrote:
    Thanks Mike

    If I try the timer plan as you've suggested I'll also connect my Kill-A-Watt meter to see how much of an advantage there is to doing it that way. Understanding that the compressor will have to run longer to re-cool a freezer that has been disconnected from AC power for a few hours, I'm not sure how much advantage I'll experience. It will be an interesting experiment.

    This scheme pushes the electrical consumption into the "off panels" instead of "off battery's" which have recharge inefficiencies. You are using freezer ice, as storage. The Kill-A-Watt meter will have your ultimate answer.
    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 ,

  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system
    My math goes something like this: …340 watts of PV times 4 hours of sun equals 1360 watts of production on a sunny day. …The load, a chest freezer, requires 800 Watt/hours per day, therfore I have a surplus of power.

    SJ,

    I missed this previous post. WADR, a 340 W (STC) PV array will essentially never deliver a net production of 1,360 Wh/day from 4 hours/day of “full” Sun. Factoring is typical PV module operational efficiency, wiring/cabling efficiency, controller efficiency, battery efficiency and inverter efficiency, an off-grid system will typically deliver a net of ~60% to 65% of the “power x hours of Sun” calculation.

    For quickie guess-timating purposes, "2/3 max" is a good rule of thumb, or 340 W (STC) x 4 hours/day x 2/3 = ~907 Wh/day maximum.

    HTH,
    Jim / crewzer
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Crewzer wrote: 
          Quote
    My math goes something like this: …340 watts of PV times 4 hours of sun equals 1360 watts of production on a sunny day. …The load, a chest freezer, requires 800 Watt/hours per day, therfore I have a surplus of power.

    SJ,

    I missed this previous post. WADR, a 340 W (STC) PV array will essentially never deliver a net production of 1,360 Wh/day from 4 hours/day of “full” Sun. Factoring is typical PV module operational efficiency, wiring/cabling efficiency, controller efficiency, battery efficiency and inverter efficiency, an off-grid system will typically deliver a net of ~60% to 65% of the “power x hours of Sun” calculation.

    For quickie guess-timating purposes, "2/3 max" is a good rule of thumb, or 340 W (STC) x 4 hours/day x 2/3 = ~907 Wh/day maximum.   

    You might be right about the ~907 watts.  But the freezer is still running, and the batteries seem to be charging back up each day.  It's been 17 days now.  I've had a few partly cloudy days, but mostly sun.  My batteries, charge controller, panels must be pretty efficinet huh....

    Frankly, I've never understood why I'm supposed to consider only the peak hours of sunlight per day.  I see current flow from my panels early in the morning until late in the evening.  Sure, maximum current flows from about 11am until 2pm, but the off-peak hours produce some charging as well.  What gives? 

    Another thing to consider is that I don't have high-performance batteries, just ordinary marine deep-cycle ones from Sams club. 

    John
  • BB.BB. Super Moderators, Administrators Posts: 30,936 admin
    Re: Small system

    I believe the irradiance measurements are total kWatthrs/sqmeter (or whatever your local units are).

    Roughly, full sun is about 1kW/sqmeter, so when somebody has 6.2kWhrs of TOTAL Irradiance per day in July (average), people just short hand it for 6.2 hours of Full Sun Equivalents.

    In reality, your system produces about 3 hours at full sun, but (I am guessing) really produces about 5+ full sun equivalents per day (these weeks of good weather).

    Remember too, that the numbers we through throw--duh around here are generally average irradiance per day/month/year... Take a look at this link:

    http://rredc.nrel.gov/solar/pubs/redbook/ (link into data sets)
    http://rredc.nrel.gov/solar/pubs/redbook/PDFs/HI.PDF (PDF File for Hawaii)

    Pick the city nearest you... But you can see that, for example, Hilo, some months (in the 30 some-odd year sample) produced only 3kWhrs per day, and other years produced almost 6 kWhrs per day. Weather, jetstream clouds, pollution, etc. all have quite dramatic effects on "average" production. And, for Silicon Solar Panels, even temperature of the panels have a good 10-20% effect on production (as well as dirt on panels, etc.).

    In the end, most people here try to error a bit on the conservative side when estimating system performance.

    If the folks here "sold you a system" that would barely work on an average day in an average year--would you be happy when half of the years in the future the system failed to give you enough energy?

    Well, it depends, if you are on-grid and can easily use utility power to backup/charge your system--probably not. However, if you were dependent on OFF-Grid Tie and you had a 50/50 chance on any week/year of having your system fail to deliver power, have your food go bad, and your batteries only last 1/2 of their expected life--you probably would not be very happy.

    Grid Tie is a great way to go because of these issues... No battery expenses, virtually unlimited (and cheap) utility power as backup, and if you have net metering--your power can be "used and stored" over the period of one year. Only down side with grid tie, if you lines are knocked out in a storm, your panels/inverter do you absolutely no good to supply backup power.

    If you don't have Grid (utility) power--then one has to be very aware of the vagaries in the weather and the ability of solar panels/batteries to supply the necessary power. Many (if not most) folks have a fuel driven backup generator because it is just not practical (cost/size/maintenance) to expect a solar (or wind) system to supply 100% of the power 100% of the time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system
    You might be right about the ~907 watts.  But the freezer is still running, and the batteries seem to be charging back up each day.  It's been 17 days now.  I've had a few partly cloudy days, but mostly sun.  My batteries, charge controller, panels must be pretty efficinet huh....

    Another thing to consider is that I don't have high-performance batteries, just ordinary marine deep-cycle ones from Sams club.

    John,

    Considering your ~800 W freezer load and your long run of good weather, I’d expect for your system to be operating satisfactorily from the estimated average net production capacity of ~907 Wh/day.

    Assuming enough Sun and a large enough system, actual production will be limited to recharging the batteries, load requirements, and system losses (i.e.., floating the batteries). “Extra” energy will be left behind, as there's no place to store it. Specifically, the system produces no energy when the controller’s PWM duty-cycle circuitry opens the connections to the PV array while maintaining target voltages in the Absorb, Float and EQ stages. The “buzzing” sound your Morningstar controller makes during these stages is a characteristic of this on-off operation.

    In terms of system efficiency, the flooded-cell batteries are the largest single hit: They’re typically ~80% efficient (Wh out / Wh in). However, if your loads run for ~6 hrs/day directly from the PV array and for ~18 hrs/day from the batteries, then their operational efficiency is ~85%.

    The array’s operational efficiency is fairly easy to estimate. If the max battery voltage is 14.4 V using a PWM controller in bulk charging stage, the array’s max voltage might be ~14.6 V measured at the array terminals. Multiplying this by the KC-85’s Imp of 5.02 A means that each “85 W” module may supply ~73.3 W at mid-day near at the end of the bulk charge cycle, or ~293.2 W from the “340 W” array”. That’s ~86% of STC spec.

    The XP-1100-12 inverter may well be the next big hit: ~87% at 1/3 rated power, according to Exeltech. And, if left on when the freezer is off, the 1100’s “no-load” draw spec is 20 W (10 W with the X2 option). If the freezer is on for ~ 8 hrs/day, the inverter will consume an additional 160- to 320 Wh/day while in standby mode when the freezer is off.

    (In fact, a way to perhaps save yourself a bit of energy would be to wire a timer switch to the XP-1100's external on/off switch connections and have the timer turn off the inverter from, say, midnight to 6 AM. The freezer will still use as much energy per day, but the inverter standby losses would be cut by 60- to 120 Wh/day.)

    The collective efficiency hit from these three components/subsystems is 85% x 86% x 87% = 63.6%. Add in small losses in the wiring and the controller, and the overall system efficiency will be in the ~60% range.

    This isn’t a hit on your system. All off-grid systems suffer similar losses, and part of the common challenge it to find more efficient but cost-effective components, such as some monocrystalline PV modules, MPPT controllers (sometimes), VRLA batteries, and high-efficiency inverters.

    YMMV.

    Regards,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: Small system

    Hi Bill,

    Thanks for the great tips.

    Ralos
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Thanks for the tips Jim.

    I'm going to continue the test into this winter when less sunlight is available. I'm also thinking about adding two more batteries (105ah marine deep cycle), and I suspect that I might gain some efficiency as a result of peukerts law. I know I'm not supposed to add batteries to an existing array, but mine are gently used and only about a year old. I'll also be changing the angle of my PV array twice a year to compensate for the sun's position in the sky.

    It's a good feeling to squeeze more energy from the system than it is expected to produce.

    John
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system

    John,

    Peukert's Law addresses battery capacity w/r/t load, so I'll be surprised if you see any effect on efficiency when you increase battery capacity. In fact, recharging efficiency might actually drop a bit with a larger battery bank and under the same energy demand, as you'll be using less capacity from each battery and top-of-charge recharging is the most inefficient.

    A steeper tilt angle (~ latitude + 15 degrees) for your array should increase "winter" (late September to late March) Ah production capacity. It may not help with actual production unless you've got sufficient load on the system, and I'd expect your freezer's winter energy requirement to drop a bit. On the other hand, the change may make the difference between not enough energy and just enough.

    So many variables -- I'll be interested in the results of your experiments!

    Regards,
    Jim / crewzer
  • BB.BB. Super Moderators, Administrators Posts: 30,936 admin
    Re: Small system

    John,

    Adding batteries, past a certain point, is not really going to help and can, in fact, start causing you issues too.

    If your discharge rate is over 20 hours for the battery bank, adding batteries will probably not add much more efficiency because of the Peukert factor. The factor is an exponent and as the rates get less, the amount you pick up will be less... Below the 20hour rate on your battery bank, just a SWAG will would probably be you would pickup single digits in efficiency not matter how many more batteries you add.

    Also, as you add more batteries, you have more losses from self discharge, require more/longer current to float, etc. Also, if you have marginal solar power, it will take longer to bring back the batteries to full charge--and the longer the batteries stay under full charge, the longer lead sulfate has to harden (and hardened lead sulfate will generally not convert back during charging). From what little I have read, lead sulfate actually begins to harden in as little as 24 hours. (by the way, supposedly, AGM batteries, according to the manufacturers are not--as?--susceptible to sulfation as flooded cell batteries and may be discharged to 80% of capacity without issues).

    It seems from reading here and doing a few back of the envelope calculations, that about 3 days of battery power (assuming discharge to 50% capacity to ensure long life for flooded cell batteries) seems to be a nice optimum between low current draw, carry through without sun, and still allow float and reasonably quick recharges (remembering that you have to support both the original load plus the load of recharging the batteries).

    If your load is close to 100% of your daily generation, then the batteries will never be fully recharged by using the panels alone during poor weather/winter time (probably OK if you have Grid Power, or use a generator often for backup).

    -Bill

    PS: What Jim said.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Update (340-watts of PV - 420ah battery bank - 800 watt/hour per day load):

    Weather for the Memorial Day weekend was mostly rain and clouds. I was concerned that the battery voltage might dip to a point where it would be necessary to turn off the inverter and freezer load. Fortuately, Monday afternoon was mostly sunny, and battery voltage climbed to more than 13.09 volts. (I'm not continuously monitoring, just checking with a DVM now and then). If tomorrow is sunny, I suspect that my battery bank will fully recharge by the end of the day.

    The lowest voltage I measured was 12.11 volts at about 8:00am this morning. After that, I watched the voltage climb in the morning, even though the sky was cloudy. By noon, the battery voltage climbed to 12.81 volts, and by 2:00pm (with partly sunny skys), battery voltage was 12.96. The highest reading was the one I took at about 5pm (13.09 volts).

    I welcome other interpretations, but it seems to me that I have ample charging capacity and I could probably add to my battery bank without taking a performance hit.

    John
  • BB.BB. Super Moderators, Administrators Posts: 30,936 admin
    Re: Small system

    Do you have a battery monitor installed?

    Measuring the battery voltage during charge (or discharge too) is not very accurate in determining state of charge. Disconnecting the load to rest the batteries, measuring temperature, voltage, and/or electrolyte specific gravity (against the mfg. charts) will give you a better idea of battery levels.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system

    SJ,

    As Bill has indicated, measuring battery voltage (system voltage, actually) while the battery is being discharged or charged does not usually yield an accurate indication of battery state-of-charge, or SOC.

    The fundamental problem is a phenomenon known as “surface charge”. For example, while recharging, a thin layer of relatively high SG acid forms immediately adjacent to the positive plates. Eventually, the high SG acid will dissipate throughout the electrolyte (like tea steeping), but, while under charge, and for a while afterwards with no load, a battery voltage measurement will yield a false high due to the high SG of the electrolyte in contact with the plates.

    Conversely, low SG electrolyte in direct contact with the plates during discharge yields a false low reading. However, once the load is removed, the battery voltage will recover a bit.  8-)

    If using battery voltage to determine SOC, the battery has to removed form all loads and/or chargers and allowed to “rest” for several hours. A voltage measurement then taken will yield a useful indication of SOC. Specifically, once an absorption stage at a charge voltage of 14.4 V is complete and the charger and loads removed, the resting voltage of a fully charged flooded-cell deep-cycle lead-acid “12 V” battery at 77 F will gradually drop to and stabilize at ~12.7 V.

    Alternately, you can use in-service voltage measurements and your Morningstar PS-20 controller’s operational states to derive an idea of battery SOC and overall system operation. For example, your system voltage should reach 14.4 V while the controller is in the absorption (PWM) stage (flooded-cell battery setting, @ ~77 F, measured at the battery terminals). You should be able to hear your controller “buzzing" while operating in this mode.

    Once the absorption stage is complete, the system voltage should switch to float mode and drop to ~13.7 V. That would be a credible indication that your batteries are fully recharged. On occasion, I believe your controller will automatically execute an equalization/maintenance charge after the absorption stage is complete. Check the controller’s specs and instructions.

    On edit:

    Achieving a maximum system voltage of 13.09 V by the end of the day may be good news, but it could also be bad news. If you got to 13.09 V on Monday, higher still on Tuesday, and then get in a complete absorption charge today (assuming it’s sunny), then that would be good news. You really want to get in at least two or three full recharges per week.

    If, on the other hand, your system is but occasionally achieving a maximum system voltage of 13.09 V (or anything less than a sustained 14.4 V several times a week), then that’s not at all a good thing. and there are likely problems. These could include too large a load, too small an array, and/or declining battery health. Additionally, it likely means that your batteries are being subjected to what’s known as “deficit recharging”, and this style of operation will gradually reduce both their useable capacity (Ah) as well as their useful life (cycles).

    I hope that your system is doing well and that this information is useful to you. You can verify of lot of these operational guidelines by running your freezer from the grid for a few sunny days, and checking battery voltage and controller operation during that period. A battery monitor (i.e., a Xantrex Link-10) can also help, but, to be accurate, it typically has to learn the battery bank's coulombic efficiency (Ah out / Ah in) first, and that requires that the system be able to regularly complete the absorption cycle.

    Regards,
    Jim / crewzer
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    Jim said: 
    Achieving a maximum system voltage of 13.09 V by the end of the day may be good news, but it could also be bad news. If you got to 13.09 V on Monday, higher still on Tuesday, and then get in a complete absorption charge today (assuming it’s sunny), then that would be good news. You really want to get in at least two or three full recharges per week.   

    I believe that is what will happen today.

    Based on past experience, I suspect that my battery voltage will reach 14.4 volts today.  Except for the period of three full days of cloudy/rainy conditions, my batteries have been fully recharging each day.  Not having a battery monitor, I rely on observations and voltage readings.  For example:  If the array output measured at the input to the charge controller rises to 19 volts or so, I interpret that as meaning that the the batteries are no longer using all of the energy that the PV array can provide.  Charge current is limited by the charge controller because the batteries are fully charged.  When array voltage is about the same as the battery voltage, say 12.7 volts for example, then the charge controller is passing heavier recharge current to the battery bank to recharge it. 
       As Bill has indicated, measuring battery voltage (system voltage, actually) while the battery is being discharged or charged does not usually yield an accurate indication of battery state-of-charge, or SOC.

    The fundamental problem is a phenomenon known as “surface charge”. 

    I believe that the load is "burning off" the surface charge.  Still, I agree with your statement, it is difficult to get an accurate SOC measurement since charge/discharge conditions affect the meter readings.  Not having a battery monitor, I'll settle for ballpark SOC indications.  I have to take into consideration time of day, and whether or not the freezer's compressor is running at the time of the voltage reading. 

    Thanks for the advice/info.

    John

  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Small system

    I said:
    Frankly, I've never understood why I'm supposed to consider only the peak hours of sunlight per day. I see current flow from my panels early in the morning until late in the evening. Sure, maximum current flows from about 11am until 2pm, but the off-peak hours produce some charging as well. What gives?

    Over this cloudy Memorial Day weekend I noticed that significant charge current flows even when the sky is cloudy. How cool is that?

    John
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Small system

    John,
    If the array output measured at the input to the charge controller rises to 19 volts or so, I interpret that as meaning that the batteries are no longer using all of the energy that the PV array can provide.
    Correct. The higher voltage reading is the result of your DVM integrating the battery voltage and the array’s open circuit voltage, both with respect to time, while the controller is operating in its PWM mode. For example, if the PWM duty cycle is 30%, the Voc is 21 V, and the battery voltage is 14.4 V, the you'd see 21 V x 70% + 14.4 V x 30% = ~19 V. Everything else being equal, the indicated array voltage should increase as the charge current decreases.
    Charge current is limited by the charge controller because the batteries are fully charged.
    That’s certainly true when the batteries are fully charged, However, the controller also limits current during the absorb stage, when the batteries are typically not fully charged. This is how the controller maintains the target "constant voltage".

    As you have determined, the controller is not delivering all available PV power when it’s operating in PWM mode (absorb or float). This is a good time to power an "opportunity load" from the “extra” power. For example, once my controller has been in absorb mode for ~1/2 hour, there’s enough "extra" power available from my PV array to power a 200 W attic fan without affecting the controller's ability to deliver enough charge current to maintain target battery voltage.
    I believe that the load is "burning off" the surface charge.

    If there’s not much net charge current into the battery, then there won’t be much surface charge error.
    Over this cloudy Memorial Day weekend I noticed that significant charge current flows even when the sky is cloudy.  How cool is that?

    8-)

    Regards,
    Jim / crewzer
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