smart solar controller/charge

Discussion in 'General Electronics Chat' started by bug13, Feb 13, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi Guys

    I am looking for a smart solar controller/charger here, does anyone come acorss a really smart one?

    due to the weather conditon I am at, winter time is going to be a challenge to matain the charge. (on average we have 3.3 hours sunlight per day in winter, and we are at hight latitude, 41 degree south)

    So I am looking for a controller that can PWM/reduce the light to say 50% when the battery is running low, or somehting like that.

    And also, it can do the normal stuff, like over charge/discharge protection etc...

    Thanks guys
     
  2. SPQR

    Member

    Nov 4, 2011
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Solar systems are a delicate balance between what energy the load needs, the capability of the battery bank, and what the panels can produce. Exactly what needs to be done depends strongly on how much energy is needed each day.

    You need two controllers unless you contract out a custom design. To get the most out of what power your panels make for you a MPPT (Maximum Power Point Tracking) charge controller. These are switching power controllers that do constant measurements to get all the powerout of your panels and convert it for the battery to use. They are "high 90's" efficient in the conversion... any other charging method would run perhaps 75% efficient... and the get worse in cold weather as the panel makes a higher voltage.

    I have seen MPPT charge controllers cost some 4x times a comporable PWM charger (same manufacture, similar rating, even the same box) but the circuitry is more complex, and I imagine some extra profit is in there too.

    In the US we need to comply with a national standard called "the code" where every part used in an electrical system must be registered ("listed") with a testing agency, and that listing costs money so listed chargers carry that in their price. If you do not need any such certifications then the inexpensive devices I've seen on EBay may work well for you.

    I'm (very slowly) working on a solar battery bank monitor to record charging and load currents so I know what goes in and what comes out. Something like that may work as part of the load controller... but I can't share what's not complete.

    What is the load on the system?
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi SPQR,

    Thanks for your replay, it doesn't say much on the website, but I have send them an email and ask for the datasheet.
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks for you replay ErnieM,

    The load on my system is two 12V 0.75W LED, self regulated, no external circuit needed, just connect to 12V battery.

    The goal is to get 5 days backup lighting on the system. MPPT controller is ideal, I am hoping that I can find a MPPT controller, which it can do 50% PWM the load when the battery have around 80% capacity left, and if it come with a programmable timer, that will be even better.

    So far, all I can find is those disconnect the load when the battery's voltage drop to 11.8 or even lower, which I think it's not very good for the battery.


    anyone have some suggestion for me
     
  6. Brownout

    Well-Known Member

    Jan 10, 2012
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    That is good for the battery. Just not what you want.
     
  7. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks for correcting me, that what I really meant! :)
     
  8. tindel

    Active Member

    Sep 16, 2012
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    Good news! You only need a 15Ahr battery... which isn't a ton. 8 NiMH batteries would give you that capacity. Also, you don't really NEED a peak power tracker for a job like this, but your array size will be bigger when you do this.

    Assuming a 6 hour winter day (assuming a low latitude, which NZ certainly is) you need 2.5A to fully charge 8 NiMH batteries in that time frame. Not a huge array, but not a tiny one either, unless you go with a newer technology cell. A 30-40W solar array (at 12V) would probably be good. A slightly smaller array would work with a MPPT, but it's probably not a whole lot smaller.

    You'll also need control circuitry which will waste some power... there are many IC's available to assist in battery charge control and boost converters to produce 12V from 8 NiMH batteries.

    Also, 10 NiMH batteries would give you an average of 12V. Might as well use that to your advantage, and not have a boost converter... you still have to control the charge current though... NiMH might not be a good solution since 2.5A is ~1C charge rate - pretty high.
     
    Last edited: Feb 14, 2013
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Two .75W LEDs is only 1.5W, that's a very reasonable load. For a 12V system it's only 3 AH a day (24 hours)

    How long does the load run? Over 5 days are we talking 5*24 hours?

    Is the 5 day backup for cloudy weather? Should the battery recharge 100% on the 6th day?
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi tindel,

    Thanks for your reply, I am not familiar with NiMH battery (I have only investigate AGM, GEL and LifePO4), would you kindly show me your calculation, thanks
     
    Last edited: Feb 15, 2013
  11. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    On a typical winter night over here, it need to run for at least 14 hours a night, so we are talking about 14hr x 5 days = 70 hrs.

    The design requirement is on a full charge, it can run 5 cloudy days, 14 hours a night, on the 6th day, it depends on the weather condition, it might be a good day, it might be another bad day, the system is supposed to be fully off-grid, and only be powered by the solar panel.
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Good luck with that in NZ winter!

    I think you are going to need big solar panels and big batteries, and very small lights.

    You can check online there are maps etc showing average monthly insolation for your region, made for just that purpose; calculating how many solar panels you need. Some good sites will also show "worst case" insolation, you are not going to get 3.3 hours a day every day.
     
  13. tindel

    Active Member

    Sep 16, 2012
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    You're an engineering student, it looks like - give the calculations a whack! If your numbers match mine, then you understand the math - if not, post up your calculations, and I'll show you where you (or I) am wrong. Really, all I used was ohms law and the battery capacity, which is pretty obvious if you look at the units! I did calulate 24 hour operation for 5 days.

    I looked at those batteries that you have been looking at, and it looks to me like they'd work fine. A 15-20Ahr lead-acid car or golf cart battery would probably be perfect, huge, but perfect. I've heard that you can just hook up a solar array directly to lead-acid batteries, and that is good enough (they won't overcharge), but I'm not an expert on this battery chemistry. Maybe ErnieM can comment - it sounds like he knows his stuff.

    If you can just hook your SA directly up to your battery (don't forget to diode isolate) - then you could just hook up the lights directly to the 12V battery - then you're done. This could be a real easy setup, really expensive ($200 USD) and huge for two little light bulbs, but easy.

    It will probably take a decade or more to recoup the cost of just running power to your lights, is my guess.
     
    Last edited: Feb 15, 2013
  14. Brainbox

    Member

    Nov 15, 2010
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    Just try to catch as much solar energy as you can.
    It´s probably cheaper to buy some extra solar panels in stead of making a complex circuit to get a few more percents out of one with a expensive MPPT charger.
     
  15. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Yes my calculation shows me that I need a big panel, or a very intelligent controller, which dim the light when the battery is running low, dim the light low enough for security purpose only.
     
  16. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Great, so .125A * 70 hours = 8.75 AH for the net loading.

    You generally don't want to discharge a battery more then 25 or 50%, so the battery capacity should range from 8.75/50% to 8.75AH/25% or from 17.5AH to 35AH

    Over the 3.3 hours sunlight per day you need to supply current plus the amount lost to charge efficiency, ballpark that at 75%, so the current is 8.75AH/3.3H/75% = 3.5A

    For a 12V system this is a 12V * 3.5A = 42 watt panel.
     
  17. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I haven't investigate NiMH battery, but for what I know about SLA (GEL or AGM, which most of the solar system is using), it generally have 80 - 90% charging and discharging efficiency for full capacity, but it's charging and discharging efficiency drop to less than 50% from 80 - 100 % state of charge.

    The capacity of SLA is usually rated at 25 degree, every 10 below 25 degree, it's capacity drop about 10%, our winter here can reach 0 degree.

    On top of my head, SLA is usually recommended to operate at 20% of depth of charge, while deep cycle is about 50% I guessing, for longer battery life.

    All these together, it will end up with a huge panel just to survive winter, but in summer, it will likely over the battery as the sunny hour will be much longer than winter. also the large current would exceed the charging current limit.

    it get too complicated, that's why I am hoping I can get an off the shelf smart controller.
     
    Last edited: Feb 15, 2013
  18. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Thanks ErnieM,

    Your calculation has confirmed mine, I am more confident now. (I have been worrying that my calculation is wrong)

    Now what make thing complicated is, the solar panel has to be lied flat. (I have no control over the orientation and angle of the panel)

    The Altitude angle of the Sun in winter from 12.00 - 13.00 is 25 degree, my rough estimate efficiency of a flat panel is about 35%.

    that's why I want to look for a very smart controller: survive winter with minimum lighting, avoid big panel overcharge battery (also due to budget, big panel is expensive)
     
    Last edited: Feb 15, 2013
  19. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi THE_RB,

    Thanks for your reply, I found some data of my region here: http://solarone.co.nz/main/Service/Insolation_North.html#1, we get 1.62 kWh/m^2/day.

    This concept is new to me, I have no idea what to do with it, can you briefly show me how to use these data, if not too much troubles, thanks.
     
  20. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Is that summer or winter or a year round average?

    Generally you factor in about 15% efficiency for the panel cells, and calc area of the panel size:cell size. On some panels it is close to 1:1 on some it is much less.

    So for a 80W solar panel which is about 0.4 sq metre would be about 1620 watt/hr * 0.4 metre * 15% = 97 watt/hour per "average" day.

    Or look at it in another way; on an "average day" the 80W panel might make 80w for a bit over an hour.

    At your low latitude and knowing NZ's reputation for rain and clouds it might be a few times worse than that in the middle of winter.

    Anyway, IF you can get 97 w/hr per day, and charge batteries at about 70% efficiency you can get about 68 w/hr of energy in the night. You said light on for 14hrs a night, so that gives you a light wattage of 68/14 or 4.8W lighting from that 80W panel.

    And remember in winter it will be significantly worse than the average.
     
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