Simple solar power questions, pls help

System
System Posts: 2,511 admin
in General Solar Power Topics #1
Hi,

My name is Melvin, and I am very much a beginner in solar panels. I'm planning to build a 2 axis solar tracker using a small solar panel (<5W), a lead acid battery, a microcontroller and 2 motors (its not for everyday usage, more for educational purposes). I have read through a lot of the basic principles of solar panels, but I still have a couple of questions for you guys,

1. What effect does a panel's nominal voltage have on the system ? Is a higher nominal voltage selected just so that it can power higher voltage devices ? For example, a 6V panel is chosen over 3V panel because I have a 5V device ?

2. What will happen if a panel's nominal voltage differs from the lead acid battery voltage that it is charging ? For example, a 6V panel charging a 12V battery, or vice versa.

3. For battery ratings, I understand that 4.5 AH means that if 4.5A is drawn from the battery, the battery's capacity will be enough to last for one hour. Does this mean that at the end of the hour the battery will have 0 charge left (100% DOD) ? Or am I misunderstanding something ?

Thanks for your time, I really appreciate it.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #2
    Re: Simple solar power questions, pls help

    Melvin,
    What effect does a panel's nominal voltage have on the system ? Is a higher nominal voltage selected just so that it can power higher voltage devices ? For example, a 6V panel is chosen over 3V panel because I have a 5W device ?
    It’s really the other way around. For a basic system, the system’s nominal voltage will determine the PV module’s nominal voltage.
    What will happen if a panel's nominal voltage differs from the lead acid battery voltage that it is charging ? For example, a 6V panel charging a 12V battery, or vice versa.
    A flooded-cell deep-cycle lead-acid 12 V battery requires an absorption stage charge voltage of ~14.6 V, depending on battery temperature and chemistry. A 6 V PV module will not charge a 12 V battery. In fact, the typical maximum power voltage (Vmp) spec for a “12 V” PV module used to charge a “12 V” battery is ~17.5 V.
    For battery ratings, I understand that 4.5 AH means that if 4.5A is drawn from the battery, the battery's capacity will be enough to last for one hour. Does this mean that at the end of the hour the battery will have 0 charge left (100% DOD) ? Or am I misunderstanding something ?
    Not exactly. 4.5 Ah actually means 0.225 A for 20 hours, at which point a “12 V” battery under that load and at 77 degrees F will register 10.5 V. The battery will be 100% discharged. If you were to draw 4.5 A from the battery, it might last for only 20 minutes or so. This is known as the Peukert Effect. For maximum economic life, deep cycle battery capacity is discharged by only ~15% to 20 %, and rarely more than 50%.

    HTH,
    Jim / crewzer
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: Simple solar power questions, pls help

    First, you have to understand the difference between volts, amps and watts. Watts are the measure of power, which is the combination of volts and amps,

    VoltsXamps=Watts
    Watts/Volts=Amps

    For example, a 60 watt bulb at 120vac burns 60 watts of power uses .5 amps@120 volts.

    A 60 watt bulb at 12 volts uses 5 amps.

    It uses the same amount of power in each case. As someone told me early on, Watts is Watts!.

    As for nominal volatges of Pv panels/batteries. The nominal output voltage of a solar panel is quoted at some average temp. For example a 12vdc panel would put out its STC (Standard Test Conditons) rated voltage of ~17 volts at 40 degrees C. The panel MUST put out more voltage than the battery in order to be able to charge that battery. In simple terms you could never charge a 12 volt battery from a 6 volt cell. On the other hand, without proper charge controls, charging any battery with too high a voltage will damage the battery beyond repair. For example, if you charged a 6 volt battery with a 12vt panel without regulation, you would soon cook it.

    I suggest that you follow this link: http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    Batteries are rated in amp/hours. The previous link will give you more info.

    Icarus
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Simple solar power questions, pls help

    In my first question, there was a typo, it should be 'a 5V device' instead of 5W. For some reason I cant save my edit on that post.

    So basically, if I choose use a 6Vnominal voltage to charge a 6V battery or a 12Vnom to charge a 12V battery, there will be no problems right ? Also, if I use a 12Vnom panel to charge a 6V battery (with regulation) compared to charging a 12V battery, what would be the difference ?

    For question 3, I get that my understanding is not correct due to the Peukert Effect law. So how do I actually determine how much current my solar panel and battery setup can supply to my system (motors) ? Taking for example maybe a 6V panel and 6V 4.5AH battery. The motors will only require short bursts of current after a certain interval (probably 60 mins). I am really having a hard time with this part...

    Icarus, I have actually read that page before, but I'm still a little confused. I understand volts, amps and watts, i think, it was just a typo in my post as stated above.


    Melvin
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Simple solar power questions, pls help

    Fyi,

    To save your edited posts (when the Save function just hangs), click on Go Advanced button and hit save--Has always worked for me--even during the original edit/save hangs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #6
    Re: Simple solar power questions, pls help

    Melvin,

    The answer is complicated. You have to determine not only how big your average daily load is, but you have to answer how many days of reserve you wish for.

    The former is fairly easy to determine. Add up the total load/time taking into account the starting loads of the motors as well as the running loads will give you a watt/hour or amp hour calc. That will give you the load calc.

    There are guys here who are way smarter than me who can give may be able to give you a detailed calc. of the supply side. In our world, we build our systems as big as we can afford! My intuition is if you have the loading calcs,
    if you triple or quadruple that calc for the supply you will be in the ball park. Add in a figure for no sun days etc and you should get close.

    My one observation is that your choice of a 4.5 amp/hour battery is a bit small except for tiny loads. If you use an average draw down of less than 50%, (20% would be better!) you would only have 2.25 amp/hours of load.

    The rule of thumb is that you should charge a battery with a minimum of 3-5% of battery capacity, and a max of 20% . 20% would be about .2amps or about 3 watt panel.

    Hope this helps,

    Icarus.

    Jim, or Bill will probably chime in before I post this and once again make me look silly! lol
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #7
    Re: Simple solar power questions, pls help

    I hate to get into the details by trying figure out some overly complex calculation that would be covered by a good rule of thumb (and I have a 50% chance of getting wrong the first pass through :roll: ).

    It would be easier if we had some real numbers--but what I would do?

    Look at a capacity vs ampere draw for a representative battery and look at capacity vs your load. For the sake of argument, lets say that you have a very short, but high current, application. And that looking at the battery curve that your "10 amp 1/20 second load" reduces the battery 20 hour discharge rate to 33%... That would indicate that your solar charging capacity would (very roughly) need to be 1/33% or 3x larger than the "measured" amp*hours of your load.

    If your want the battery to last a long time, take your "normalized" or corrected current requirement (3x measured daily load) and mutiliply it by 2x (for 50% maximum discharge level), and multiply it by 3 days (if you want three days of no-sun storage).

    Now that you have picked the actual battery capacity--you can go back and look at its performance curve and double check that that your "normalized" ampacity requirements are still met (a larger battery, or different type will have different derating values).

    Next, you need to figure out how much sun you have in your location... Look here for a good list of US locations for sun (by collector angle, tracking or not, and by month/year). They also have about 30 years of data so you can see variations (due to weather, pollution, etc.) between typical and min/max sun per month.

    Sizing the solar panels, basically, the "number of hours of Sun" times the panel rating in STC Watts * ~85% (for PTC rating) (actually, I would be more conservative and use closer to ~75% of STC rating to allow for dirty panels occasional poor weather, hot days, and such).

    And, you need to take any other inefficiencies into account. Losses due to battery charging (~80% power efficiency for wet cell lead acid, approximately 90% for AGM type sealed batteries).

    So, the end rule of thumb equation would be (for 1 day of solar panel to power 1 day of load):

    Solar panel size = Amp*time*batt volt (Watt*Hour) * 1/hours-of-sun * 1/battery factor * 1/75-85%-solar-panel factor * 1/80-90%-battery-charge * 1/solar charge controller efficiency

    If, you are building a small system (less than 400 watts of solar panels), then you can take the Watts out of the equation and work with just amp*hours (PWM controller is ~100% efficient for amps, Wet Cell batteries are approximately 90% efficient for current and AGM are ~100% efficient, do your motors draw more, same or less current as voltage drops from 14.5 volts charging to below 12 volts during the night and under load, etc.).

    I am going to stop here because I am not sure I am really helping here.

    Please ask more questions and/or supply some real numbers and what type/brand of batteries, etc., that you are looking to use.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #8
    Re: Simple solar power questions, pls help

    Melvyn,
    So basically, if I choose use a 6Vnominal voltage to charge a 6V battery or a 12Vnom to charge a 12V battery, there will be no problems right ?
    Charging a nominal 6 V battery from a nominal 6 V PV module and a charger will generally work. However, there are many “ifs”, “ands”, or “buts” to consider, such as actual specs for the module and battery, PV module temperature, size of battery with respect to PV module, etc.
    Also, if I use a 12Vnom panel to charge a 6V battery (with regulation) compared to charging a 12V battery, what would be the difference?
    Assuming that you’d use a regular ol’ PWM charger, ~60% of the 12 V PV module’s rated power will be lost due to environmental factors and the drop in module operating voltage (from “12 V” to “6 V”). It’s also possible that the 6 V battery’s relatively low voltage could be below the charger’s minimum operating voltage, and then nothing would happen.

    Some 12 V PV modules consist of two 6 V sub-modules wired in series to make 12 V. Such modules can sometimes be rewired to make a 6 V module with twice the current.
    So basically, if I choose use a 6Vnominal voltage to charge a 6V battery or a 12Vnom to charge a 12V battery, there will be no problems right ?
    Charging a nominal 6 V battery from a nominal 6 V PV module and a charger will generally work. However, there are many “ifs”, “ands”, or “buts” to consider, such as actual specs for the module and battery, PV module temperature, size of battery with respect to PV module, etc.
    Also, if I use a 12Vnom panel to charge a 6V battery (with regulation) compared to charging a 12V battery, what would be the difference?
    Assuming that you’d use a regular ol’ PWM charger, ~60% of the 12 V PV module’s rated power will be lost due to environmental factors and the drop in module operating voltage (from “12 V” to “6 V”). It’s also possible that the 6 V battery’s relatively low voltage could be below the charger’s minimum operating voltage, and then nothing would happen.

    Some 12 V PV modules consist of two 6 V sub-modules wired in series to make 12 V. Such modules can sometimes be rewired to make a 6 V module with twice the current.
    For question 3, I get that my understanding is not correct due to the Peukert Effect law. So how do I actually determine how much current my solar panel and battery setup can supply to my system (motors) ? Taking for example maybe a 6V panel and 6V 4.5AH battery. The motors will only require short bursts of current after a certain interval (probably 60 mins). I am really having a hard time with this part...
    How much current? Or, how much power (V x A), or even how much energy (V x A x time)? We need to have a better idea of how much current the motor draws, and for how long. It’s one thing to draw 1 A for two minutes every sixty minutes, and another to draw 5 A for 10 minutes every sixty minutes.

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #9
    Re: Simple solar power questions, pls help

    Thanks for the replies, sorry I did not give enough info on my system, I'll try to this time.

    Basically my tracker consists of these main parts,
    light sensor circuit (either LDRs or small solar cells)
    solar panel
    solar charger circuit
    battery
    PIC 16F877A
    2 motors (haven't decided whether to use DC gearmotor or servo motor)

    For the solar charger I will not be buying a PWM charger but making my own simple charger using this circuit by Oscar den Uijl : http://www.xs4all.nl/~odu/solar.html (the second one) which is meant for a 12V battery.

    The battery that I'll be using will probably be a 12V 7.2AH one (brand is GP, model GPP1272)). I cant find the datasheet for this exact model but here is a link to a similar one but different brand, http://www.csb-battery.co.uk/catalog/GP/gp_1272.pdf

    For the motors, if I use DC gearmotors then I will have to add 2 h-bridge circuits. I would like to use DC motors because they are cheaper but I haven't decided yet because I'm still unsure about position control and accuracy for DC motors (still researching right now). If anyone has any idea pls let me know...

    This is basically how I would like my system to work. The PIC will be always on, and in the morning when the amount of light is above a certain value, the sensors will detect it, tell the PIC, then begin the tracking. The PIC will check the position every 30 mins or so (not sure yet because now I'm not quite sure how fast the sun moves), and then power the 2 motors (one runs at a time) to adjust the position accordingly. The motors will probably be powered for about 2 mins ?
    Then at night the sensors will tell the PIC and the operation stops.

    Current consumption for when motors not running/standby :
    sensor circuit - not sure, approx 10mA
    charger circuit - 0.5mA
    PIC - 0.6mA
    h-bridges - not sure, approx 20mA
    total - 31.1mA

    When motors running (for short periods eg. 2 mins) :
    total - 31.1mA + approx 1A = 1031.1 mA


    The panel will not be supplying any electrical appliances, etc, just these components. How would the panel size calculations be like for these numbers ?
    I will try out the formulas by Bill and Icarus and post back if I can get results.

    Next, you need to figure out how much sun you have in your location... Look here for a good list of US locations for sun (by collector angle, tracking or not, and by month/year). They also have about 30 years of data so you can see variations (due to weather, pollution, etc.) between typical and min/max sun per month.

    Sizing the solar panels, basically, the "number of hours of Sun" times the panel rating in STC Watts * ~85% (for PTC rating) (actually, I would be more conservative and use closer to ~75% of STC rating to allow for dirty panels occasional poor weather, hot days, and such).

    Thing is, I am not from the US, so how can I manually estimate this value ?

    Also, as this tracker is just for learning and presenting purposes, I will probably not keep it running for long periods. So, I do not need too many no-sun storage days (probably 1 day is enough) in order to save cost (I am on a tight budget).


    Melvin
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  • System2
    System2 Posts: 6,290 admin
    #10
    Re: Simple solar power questions, pls help
    Look at a capacity vs ampere draw for a representative battery and look at capacity vs your load. For the sake of argument, lets say that you have a very short, but high current, application. And that looking at the battery curve that your "10 amp 1/20 second load" reduces the battery 20 hour discharge rate to 33%... That would indicate that your solar charging capacity would (very roughly) need to be 1/33% or 3x larger than the "measured" amp*hours of your load.

    Do you mean a capacity VS ampere graph ? I cant find that graph (or similar) in the battery datasheet ?
    Solar panel size = Amp*time*batt volt (Watt*Hour) * 1/hours-of-sun * 1/battery factor * 1/75-85%-solar-panel factor * 1/80-90%-battery-charge * 1/solar charge controller efficiency

    daily load requirement = 0.86746 AH (not "normalized/corrected)*2 (50% discharge) * 2 (2 day storage) = 3.46984 AH

    size = 3.46984* 1/6 * 1/0.9 * 1/0.75 * 1/0.9 = 0.9519 A

    Is the calculation correct ? Seems like quite a high value.


    Anyway, from looking at shops for the prices, It seems like I can most probably only afford a 5W panel, such as this
    http://my.farnell.com/jsp/search/productdetail.jsp?sku=1462491
    its a 17V with 0.3A max current. Is this adequate ?
    If you have any idea pls leave a comment. Thanks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #11
    Re: Simple solar power questions, pls help
    rngd wrote: »
    daily load requirement = 0.86746 AH (not "normalized/corrected)*2 (50% discharge) * 2 (2 day storage) = 3.46984 AH

    size = 3.46984* 1/6 * 1/0.9 * 1/0.75 * 1/0.9 = 0.9519 A

    Is the calculation correct ? Seems like quite a high value.

    There is one "mistake"... The "Daily Load Requirement" you calculated is really the storage capacity of the battery.... Assuming that your 0.86746 AH is your Amps*Hours raw PIC/Motor/etc. usage...

    The solar panel "size"--you should use 0.87 AH instead of the 3.46984 AH--You basically sized the system such that it would recharge 4 days of usage in one day... Your actual solar panel current rating calculation would be:

    panel size = 0.87AH * 1/6 * 1/0.9 * 1/0.75 * 1/0.9 = 0.24 Amp rated solar panel (assuming a PWM or analog controller). That would roughly equal your 5 watt panel rating...

    If this is an important application--I would probably double the solar panel size to 10 watts (2x5watt in parallel) to ensure you have "enough solar panel" for cloudy days and such... Not many areas get 6 hours of full sun per day...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: Simple solar power questions, pls help

    Melvin,

    Also, here is a generic data sheet for a small 6 volt battery (PDF File, 2 pages) that shows some of the graphs that will be helpful for you to size and understand your battery's capabilities and proper operation.

    If you look at the second page, this battery will supply (roughly) to 5 volts DC:

    30 amp*9min/60minperhour = 4.5 amp*hours
    0.5 amp * 20 hours = 10 amp*hours

    This is what we are typing about that high currents will "drain" batteries such that it reduces the total amount of power (or Amp*Hours in this case) vs a steady slow drain.

    Of course, every application is different and battery temperatures matter quite a bit too (20-25C is about ideal temperature for Lead Acid)...

    Hope this helps.
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #13
    Re: Simple solar power questions, pls help

    Bill,

    Thanks for the correction. I see the graph you were referring to was the voltage vs discharge time graph.

    I think I will go with the 5W panel and 12V battery for now and see how it goes. Thanks for all the help guys.

    Melvin
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