Advice for Panel and Interface for Simple System

mckinney3mckinney3 Registered Users Posts: 6 ✭✭
edited January 31 in Solar Beginners Corner #1
I want to gain some experience with solar at a very simple and elementary level. I also learn better if I have a project.

I have an Auto-Zone hybrid Battery(27 DC-2)  -  It is a group 27 , 12 volt battery and has 85-90 amp hour capacity. I am using it to power a 20 watt load for about 8 hours per day. I understand that when in use, I am drawing 1.6 amps. I understand that I really should not draw down the battery past 50% of capacity before recharging.  So I figure that I need to "top up" 45 Amp*Hours maximum.  I understand that the 45 Amp*Hours would be depleted in  28 hours by calculation so lets make it a 24 hour time frame to keep it simple.  So if I run my load for 8 hours then I need to add back 12.8 AH during the next day.  This would also give me a 24 hour use window if no recharging is applied.  I currently recharge with a simple 120 volt battery charger at 8 amp setting and that usually arrive at Full rating in about 6 hours if I have drawn the battery down 50%. 

I would like to "pull the plug" on the recharging step and insert a panel and interface.

I am seeking input on the panel and interface that I should buy.  I would like a specific brand/model recommendation as well as the general specifications that I should look for so I can also shop around and understand the "why" behind the recommendation. I would also like a critique of my assumptions that I have stated.

It looks like I need the capability of adding back 12.8 Amp*Hours during an 8 hour sunny day in south carolina.  But need the upward capacity to add back 38.4 Amp*Hours over an 8 hour day to allow for 2 days of poor sun conditions.

thank you

{edited by Bill "BB."--To Amp*Hours vs "amps" where needed}

Comments

  • BB.BB. Super Moderators Posts: 27,061 admin
    McKineey, I have edited your post a bit to make it easier to follow (amps is a "rate" and "amp*hours" is an amount... Sort of like miles per hour vs miles driven).

    From a purely mathematical point of view, more or less assuming standard losses (~81% derating for solar panels, ~95% charge controller eff, and 80% flooded cell lead acid battery charger efficiency), and--I like to convert to Watts (volts*amps) because, for many systems we are talking about different voltage elements (12 volt battery, 17.5-30+ volt solar panels, 120/240 VAC power). Watts is a "complete unit" -- Amp*Hours works fine in a "fixed" voltage system (where everything is assumed to be "12 volts" as in a car).

    So, you have a 1.6 amp load @ 12 volts and want to run it 8 hours per day:
    • 1.6 amps * 12 volts * 8 hours per day = 154 Watt*Hours per day
    • 1.6 amps * 8 hours = 12.8 Amp*Hours per day

    Either works, but I will go ahead and do the rest of the math in Watts and Watt*Hours... I can do it in Amps and Amp*Hours if you would like to see that instead.

    So, to supply a 154 WH load in a day, for Charlston S.C., and assume a fixed panel facing south:

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

    Charleston
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    3.83
     
    4.37
     
    5.31
     
    6.06
     
    6.09
     
    5.59
     
    Jul Aug Sep Oct Nov Dec
    5.67
     
    5.39
     
    5.02
     
    5.04
     
    4.38
     
    3.85
     
    And we are about to hit February with 4.37 hours of "average sun" per day:
    • 154 Watt*Hour per day * 1/0.61 average DC solar system efficiency * 1/4.37 hours of sun = 58 Watt array (minimum "break even" for February)
    Or, if you want 3x more energy (2 days of dark days, and 1 day of sun+load to fully recharge), then:
    • 154 Watt*Hour per day * 3 days of power * 1/0.61 average DC solar system efficiency * 1/4.37 hours of sun = 173 Watt array (minimum "break even" for February)

    Now, we nominally want 5% to 13% rate of charge for your battery bank. 5% is good for weekend/sunny weather, 10%+ is good for full time off grid usage:

    • 90 AH battery * 14.5 volts charging * 1/0.77 panel+controller losses * 0.05 rate of charge = 85 Watt minimum array
    • 90 AH battery * 14.5 volts charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 169 Watt nominal array
    • 90 AH battery * 14.5 volts charging * 1/0.77 panel+controller losses * 0.13 rate of charge = 220 Watt "cost effective" maximum array
    The above gives you some "brackets" for sizing the array (occasional usage, full time usage, winter/summer/etc.).

    The other thing to remember is that, very roughly, a flooded cell lead acid battery will take full charging current (13% to even 20-25% rate of charge) from "0%" to ~80% state of charge. Assuming 50% starting point to 80% full, that is "30%" charge and at 10% rate of charge, that portion will take about 3 hours. The time to go from 80% to ~100% state of charge is ~2-6 hours of steadily reducing charging current (absorb stage--when the battery is charging at ~14.75 volts). 2 Hours if the battery is not deeply discharged to ~6 hours if the battery is discharged below 50% SoC. So you are looking at ~3 hours + 6 hours ~ 9 hours total for a deeply discharged battery. Depending on season/weather/etc., you may only have 6-8 hours of enough "direct sun" for charging (mid morning to mid afternoon). Even if you have "enough panel" to fully recharge the bank, you may not have enough time under sun to fully recharge, and have to continue the balance of the charging the next day.

    I will stop here--You asked your questions is a bit different way than I (we) normally answer... I tried to give you the answers to the questions you asked.

    Please feel free to comment and ask more questions--It is possible that I have confused you more than helped you.

    Generally, we start with the loads and times (AH or WH per day), size the battery bank, then size the array to keep the battery bank "happy" and the loads supplied with enough electricity. You sort of started in the middle (have battery, have loads) and are fitting them to what you think the system can supply (sort of).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mckinney3mckinney3 Registered Users Posts: 6 ✭✭
    Bill, Thank you very much for your generous reply.

    I understood every word thanks to your very clear explanations.  I liked the way that you worked in some "teaching" and that is just what I was looking for!

    My project (I learn from projects) is a small 100 sq ft tiny house/shed with a 50 sq ft "porch" with shed roof.  The shed is in our orchard and has no power.  I plan to start simple with powering a few creature comforts like lighting,etc.  So I like the ability to expand.

    From your excellent lesson, I believe that I will start out with a 100 watt panel.  The roof which faces south will hold 4-5 24' panels.
    The information for the package I am looking at will handle up to 4 of these panels.

    I realize that placing brand names of times in this forum may be a no-no so I am going to include three  screen shots of the specs for the panel and controller to see what you think.  I can private message you or email if specifics are needed.

    I am seeking a discussion on if this system will meet my needs.





  • EstragonEstragon Registered Users Posts: 1,819 ✭✭✭✭
    4 panels may be pushing it a bit with a 20a controller, but you could always add a second controller if/when the time comes.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators Posts: 27,061 admin
    Note that solar electric panels do not tolerate shade at all... Shade even one cell, and you can kill 1/2 (or more) of the panel output in otherwise full sun (i.e., ideally, trim trees, no cables/ropes over panels, etc. so that they are shade free from ~9am to 3pm, at least).

    Using brand names is fine (and model numbers+links for operational/configuration questions). Our forum is funded and maintained by Northern Arizona Wind & Sun out of Flagstaff Arizona. We try to avoid blatant advertising and links to the cheapest panels of the week sale...

    Otherwise, pretty much every one here is volunteering their time and energy (including me).

    System details do matter (length and AWG/diameter of wiring, where will use put fuses/circuit breakers/on-off switches, use conduit if you have little climbing/chewy critters, etc.). Keep the charge controller close to the battery bus (short heavy cables, ideally less than ~0.05 to 0.10 maximum voltage drop for charge controller cable to battery bus under maximum charging current; and under ~0.5 volt drop from battery bus to your DC loads). If you need to send power further, or want to use small AC phone/laptop/etc. chargers, AC inverters are not a bad way to go as your system grows.

    Lead Acid batteries can output pretty good current in a short circuit situation (100's of amps)--Fire safety is a major concern of ours (you are there, we are not, so we try not to assume too much when helping). Cover the top of the battery / put in box so that kids and tools do not mix with your wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mckinney3mckinney3 Registered Users Posts: 6 ✭✭
    Thank you Bill and Estragon!

    I plan to keep everything neat and tidy and safe.  I want the installation be "show worthy!"   Thank you for your concern.
    Here is a link to the "Kit."

    https://www.amazon.com/dp/B010FLW6DC/ref=asc_df_B010FLW6DC5355279/?tag=hyprod-20&creative=395033&creativeASIN=B010FLW6DC&linkCode=df0&hvadid=198064505516&hvpos=1o2&hvnetw=g&hvrand=649209663028083260&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9010612&hvtargid=pla-390130293817

    Do you have any recommendations for type of lighting? 1.) I plan to have an overhead light and a reading light. 2.) Should I use regular house switch and house wiring and will that reduce voltage loss? 3.) Outside on the porch, I want to run a string of decorative lights for fun as well as the lighting for "sitting on the porch" at night. Recommendation.  4.) Later I want to add a small pump to drip irrigate hanging planters from the rain barrel reservoir.  I'll use the usb outlet on the controller for those pesky  devices.   Remember this is a demonstration of use of solar and i want to "show off" the installation with correct practices.


  • EstragonEstragon Registered Users Posts: 1,819 ✭✭✭✭
    As the distances are short, it may be worth considering LED lights designed to run on 12vdc. Marine/RV versions can work well for this. Some LED lights run on 12vdc natively anyway, but are normally powered by 120vac "wall warts". Some can be run directly off 12vdc, but may not be able to take the range of voltage (~11-16v) seen in dc charge-discharging. Marine/RV types are generally designed to handle the range.

    The advantage to using 12vdc is avoiding losses inverting from 12vdc to 120vac and back to 12v. If distances are long though, this advantage is offset by wire losses and/or the need for heavier wire.

    If using USB/load terminals on the controller, note that these tend to be for low power devices. The USB may be a couple amps at 5v (10w) or so.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators Posts: 27,061 admin
    For the charge controller, the "load" outputs are not always the best choice for connections to loads. Connecting your loads directly to the battery bank has less restrictions (especially for heavier/non-lighting loads like motors and AC inverters).

    If you have a flooded cell battery, get a hydrometer (glass or less likely to break integrated plastic dial, not floating balls).

    http://www.midnitesolar.com/sizingTool/index.php

    Make sure to rinse a couple times with distilled water before putting away (get sticky if you don't).

    LED lighting is the way to go. There are many lights that are >6,000 Kelvin color temperature. They tend to look blue white and wash out colors--but they are very efficient. 4,000-5,000k lamps are not bad--Better color rendition and still pretty efficient. For indoor reading/general lighting, <4,000k is warmer.

    You can get 12 VDC MR-6 LED bulbs (aka 12 volt track lighting) can work very nicely from 12 VDC battery bus.

    https://www.bulbs.com/LED_MR_Bulbs/results.aspx

    I don't know the vendor or their products--Just an example for further research.

    For outside lighting--Motion detector LED are the way to go... Running a security light for 12 hours over night--Just eats lots of power (I like motion lights--If you see the light on, you pay attention to whats happening.

    https://www.amazon.com/s/ref=nb_sb_ss_i_3_18?url=search-alias=aps&amp;field-keywords=12+volt+led+motion+sensor+light

    Also, a cheap and nice DC Current Clamp DMM is great for debugging and understanding your system. Sears (if they are still in business tomorrow) sells a "good enough" meter for $60 (AC/DC current clamp + digital multi meter). You can get them for ~$100 from Amazon:

    http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P

    Getting a small DC AH/WH meter is nice to measure your loads (lots of brands and models/knockoffs available these days):

    http://www.rc-electronics-usa.com/

    There are lots of other places to drop your money (battery monitors, etc.). But the above will get you started.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ILFEILFE Solar Expert Posts: 364 ✭✭
    mckinney3 said:
    I am seeking a discussion on if this system will meet my needs.


    Personally, I would suggest purchasing a quality controller, rather than using one of those "included" controllers. Northern Arizona Wind & Sun will carry quite a number of these controllers for you to choose from.

    I know some people may disagree with me here. But, I see that many "package deals" tend to provide cheaply made Chinese junk controllers. I have seen similar controllers destroy a quality set of batteries. So, before simply buying any old controller that is an "included" part of the deal, definitely have a look at NAWS's solar controllers for sale. Do you really want to pay good money for batteries, just to learn your controller boiled them a couple / few months later? 

    Options I would look at, in no particular order, are: Midnite, Outback, Morningstar, or Schneider Electric (formerly Xantrex, I believe?)
    Paul
  • BB.BB. Super Moderators Posts: 27,061 admin
    I need to add--regarding LED lights/"bulbs", buy one first before you purchase 10. There is so much variability, color, how they reactive to 12 VDC power, that it is difficult to predict which you will love or hate.

    Also... the human eye works "logarithmic" scale... More or less, a 2x difference in "Lumens" between two lights--You can barely detect a difference. A 10x difference is a "day/night" difference (the lower wattage lamp "disappears" when compared to a 10x brighter lamp).

    A secondary number to look at is the CRI--Color Rendering Index... Over >90 will render colors pretty accurately (fabrics, wire colors, etc.). Less than 80, colors look washed out, or one or two primary colors may stand out. Lower Kelvin temperature LEDs tend to have higher CRI numbers (not always, look at documentation, buy one and see if you like).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mckinney3mckinney3 Registered Users Posts: 6 ✭✭
    @ILFE  No argument here!  Now I am coming closer to finding "real" equipment.  =)

    OK, I can get started with the NAW listed

    Solarland SLP100-12U Multicrystalline 100 Watt 12 Volt Solar Panel


    Which of the controllers do you suggest to start with that can can handle the addition of 2-3 more panel? 

    Please look at my original context
  • mckinney3mckinney3 Registered Users Posts: 6 ✭✭
    I wrote the folks at NAWS.  I stated that I wanted to build a demonstration system for the shed that included all the bells and whistles like Combiner and Circuit Breaker boxes and Charge Controllers, etc with my constraints,  I look forward to what they say.

    dan

  • ILFEILFE Solar Expert Posts: 364 ✭✭
    edited February 1 #13
    mckinney3 said:
    @ILFE  No argument here!  Now I am coming closer to finding "real" equipment.  =)

    OK, I can get started with the NAW listed

    Solarland SLP100-12U Multicrystalline 100 Watt 12 Volt Solar Panel


    Which of the controllers do you suggest to start with that can can handle the addition of 2-3 more panel? 

    Please look at my original context
    Sorry, man. I live in Cambodia and keep weird hours.

    At most, you would max out at 400 watts, if I understand you right? That is, I am under the impression that additional panels would be rated at 100 watts, as well. So, my question for you is, which type of controller, PWM or MPPT? 

    Here are two formulas used to determine controller size:
    PWM Output Current = Input Current
    MPPT Output Current = Panel Wattage * 0.77 Efficiency / Nominal Battery Voltage

    EDIT: Oh, personally, I would go with an MPPT controller, if I had the option. An MPPT controller will convert unused voltage to additional amperes needed for charging.

    Larry Crutcher wrote a very informative article on MPPT technology, years ago. I see the original site he posted it on is now defunct. However, below is a link to a copy of that article: http://www.ebay.com/gds/Why-MPPT-PV-Solar-Charge-Controllers-Are-Important-/10000000176770209/g.html
    Paul
  • mckinney3mckinney3 Registered Users Posts: 6 ✭✭
    @ ILFE Paul  Have I got this correct?
    I would decide between these two similar Charge Controllers:
    https://www.solar-electric.com/blue-sky-energy-sb3000i-mppt-solar-charge-controller.html
    https://www.solar-electric.com/blskyscpwmch.html

    dan


  • ILFEILFE Solar Expert Posts: 364 ✭✭
    mckinney3 said:
    @ ILFE Paul  Have I got this correct?
    I would decide between these two similar Charge Controllers:
    https://www.solar-electric.com/blue-sky-energy-sb3000i-mppt-solar-charge-controller.html
    https://www.solar-electric.com/blskyscpwmch.html
    Either would work for you, I'm sure. MPPT, as stated, would be more efficient of course. But, to be honest, I use both MPPT and PWM controllers on my various arrays. However, I'm not trying to squeeze every last ampere into a battery bank. Days of autonomy are few and far between, because I live in Southeast Asia. More specifically, I am at 13° North Latitude. Full days of bright sunshine are no problem.
    Paul
  • BB.BB. Super Moderators Posts: 27,061 admin
    MPPT tend to work better if your array is 10's to 100's of feet from your charge controller+battery bank (send "high voltage" from array and down convert for short trip to battery bank. Typically >800 watt systems are more cost effective (large >200 watt panels with Vmp>30 volts use MPPT). GT type solar panels (>200 Watts) tend to be cheaper $$$/Watt vs 150 Watts or less.

    PWM tend to be cheaper (small array, short distance). Typically less than ~400 watts. Usually panels less than 150 Watts with Vmp~17.5 volts (for 12 volt bank), Vmp~35 volts for 24 volt bank, etc.

     -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 1,819 ✭✭✭✭
    IMHO, the main reason to spring for an mppt controller is if the array is far away from the controller. If so, it makes more sense to put panels in series. Running at higher series voltages keeps current down and allows for use of smaller wire, so it comes down to local wire costs vs higher controller cost. It sounds like your panels will be nearby, so a pwm controller is likely to work out cheaper. In your climate, there likely won't be much difference in performance.

    Mppt controller also tend to include more features and adjustibility, but not really necessary in many applications.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • littleharbor2littleharbor2 Solar Expert Posts: 747 ✭✭✭✭
    One thing about Blue sky controllers I don't care for is their low Voc. limit. This particular one, the SB3000 has a paltry 50 volt open circuit maximum. For a 24 volt system it really is useless as a single 24 volt panel by itself can hit 50 volts.

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

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