12V battery solar charger

Discussion in 'General Electronics Chat' started by porky67, Nov 19, 2010.

  1. porky67

    Thread Starter New Member

    Nov 19, 2010
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    hye guys.. I am new here so I not sure which category should i post my question to... if i'm wrong, please guide me ok? =)

    i would like to talk about my situation first..
    i am involve in a project of MPPT solar charge controller..
    for now, i'm starting from scratch..
    may i know if wattage of my solar panel is important?
    as the battery is 12V, can i get myself 18V, 5W solar panel(because within my budget range)?

    i read somewhere that in order to lower charging time, u need bigger current..
    my solar panel is just producing about 0.28A(and that is ideal case)..
    should i boost up my current? if so, how should i boost the current?(method?)
    or should i parallel few more solar panel before i boost the current?

    Thank you for your time..
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    You are going to want a higher output panel.

    It will take WEEKS to charge a decent battery.
     
  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Voltage can be 'boosted' but current is lowered. By lowering the voltage you can boost the current level. This is done in AC systems by a transformer. In DC systems you must use some form of DC/DC conversion.

    With the solar panels, what you get is pretty much it. You can move voltage and current around with conversion techniques, but the bottom line VOLTAGE x CURRENT product never increases, it will only decrease through conversion losses.

    To get more "power", voltage times current, you MUST increase the number or size of the solar panel(s).
     
  4. porky67

    Thread Starter New Member

    Nov 19, 2010
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    hi.. thank for your reply..
    would you recommend me any method?
    for example, how many solar panel should i add (in series or parallel) for better result in my situation?
    my system have to have a dc-dc converter..
    currently i'm researching sepic converter for my mppt system..
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    Ok, you really have to determine your project goals.

    What is the end load?

    Are you operating lights from this battery? A computer? A home?

    How quickly will the load discharge the battery?

    Once you have this information, you can figure out what size solar system you need to re-charge the battery.

    If you have a sign light that is operated from the battery, and the light will drain the battery in 10 hours, and you use it 5 hours a night, that means you have to recharge the battery in 2 days worth of sunlight in order to have uninterrupted service.

    Without knowing what the draw will be from the battery, you will have no idea how much current you need to top-off or recharge the battery.
     
  6. porky67

    Thread Starter New Member

    Nov 19, 2010
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    hi retched..
    really grateful for your patient..

    for now, there won't be any load for the battery.
    i know that i can just let if charge for a long time..
    but for my system, it is a MPPT solar charge controller..
    which is emphasize on optimizing the solar electric to charge the 12V battery..

    that's why i would like to know what will be reasonable solar panel for 12V battery.. =)
     
  7. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    This one could be charged by your current system, in a long, but reasonable length of time. Best Use would be in a circuit that consumes less than 1 amp of power and only works intermittently. (Example: Gate open/close motor)
    [​IMG] (image:12V7AhSLA)


    This one would need charge current as high as 30 amps and tapering to 5 amps, to be able to fully recharge it in less than 8 hours. Without the high amp output at the start of the charge, charging time from deep discharge could take several days to complete. So an array capable of putting out 18-24 volts and 25 amps would be a minimum starting point for this size battery. $thousands? still, :(
    [​IMG]
     
  8. porky67

    Thread Starter New Member

    Nov 19, 2010
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    kermit2,
    thanks alot for ur feedback..
    actually for now, the battery i might still be able to change to perhaps 9V battery(those looks like in the picture u have posted)...
    but for now, my main concern is how my solar panel should be set up, then only i can proceed with my DC-DC converter and micro controller + charger..
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Are you talking about charging a 12V battery to 9V? or that you have a 9V battery that you would like to use in place of the 12V battery? If the latter is true, then what is the AH rating...it is stated on the battery. Also, you mentioned that "ideally" your solar panel can supply about 280mA, but at what ideal voltage? Under "ideal" conditions you could expect to charge a 2.8AH battery in a reasonable amount of time.

    But now let's talk about a couple of hypothetical, closer to real-world conditions. If you are getting 280mA from the solar cell only 50% of the time, then the battery will take twice as long to charge. Add to this the lost time that your voltage is too low to supply charging current and you again add even more time charging the battery. The best scenario would be to Increase both the Solar panel(s) output voltage and current capability. You can either buy a single panel that would give you 18V - 24V and 560mA or buy more of the same panels you have now. Adding to your existing panels would require you to purchase 3 more identical panels. Therefore, in a real-world situation if you only average 50% of the "ideal" then you still have enough voltage for uninterrupted charging and enough current for a reasonable time to charge completion. This might even let you go back to charging a 12V battery.

    Configuring the panels:
    http://partsonsale.com/learnwiring.htm
     
    Last edited: Nov 20, 2010
  10. Audioguru

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    Dec 20, 2007
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    I think his "9V" lead-acid battery is actually only 6V but it is looked at upside-down.
    A solar panel produces its rated current for only about 1 hour in summer on a sunny day. At noon. Its current is less at other times and obviously on a cloudy day and at night.
     
  11. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Not sure you deduced this...?

    Thus my "real world estimates" were a bit optimistic! Hmm...
     
  12. porky67

    Thread Starter New Member

    Nov 19, 2010
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    [​IMG] (image:12V7AhSLA)

    @Ionic, thank you for ur detailed response. what i mean is if 12V battery is hard to implement because of needed more solar panel, my project allow me to CHANGE my battery from 12V to 9V(project requirement are flexible because the main reason for my project it to create a maximum power point tracking solar charge controller)

    because i am making a MPPT solar charge controller system, i need to maintain my solar panel output at around 13.8V for uninterrupt charging of my 12V or 9V battery.

    let say my solar panel rating is 18V 5W.. so the current I will be getting is 0.28A right? however, if i limit my voltage at around 14.4V, can i enlarge my current while maintain power at 5W? if can, please tell me how to do so?

    if cant, is there any other way to enlarge my current while limit my voltage at 14.4V and power at 5W?

    thanks guy.. really need alot of feedback..
     
  13. Audioguru

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    Dec 20, 2007
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    There is no such thing as a 9V lead-acid battery because each cell is 2V, not 1.5V.
    6V and 12V lead-acid batteries are common. I have never heard of an 8V nor 10V one.

    If your 9V battery is an alkaline type then it cannot be recharged.
    If your "9V" battery is actually a 7.2V or 8.4V Ni-Cad or Ni-MH type then a complicated charger circuit is needed.

    Solar panels are made to charge 12V batteries, not 18V batteries. I have never heard of an 18V battery nor a solar panel rated at 18V but most 12V panels produce 18V with no load current.

    If you want more current from a solar panel then add more panels in parallel or buy a larger solar panel.
     
  14. porky67

    Thread Starter New Member

    Nov 19, 2010
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  15. Audioguru

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    The ad for the 18V solar panel says 5W but also says only 2W.
    Its voltage is 21.7V with no load and its shorted current is only 0.3A so I think it is a little 12V panel that produces maybe 200mA. Then its power is only 2.4W.

    I bought a 12V/1.75W solar panel for $10.00 at a local store. Its voltage is 18V to 20V with no load and its shorted current is 250mA. It produces 12V at 150mA only at noon and in summer. Its area is almost the same as the one in your ad.
     
  16. porky67

    Thread Starter New Member

    Nov 19, 2010
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    Audioguru..
    For now, with 12V battery, how much solar panel should i put in parallel for decent charging time of the battery?
    let say i have the same solar panel as yours..
     
  17. Audioguru

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    Dec 20, 2007
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    The little 12V/7Ah battery can be charged in 20 hours at 350mA. Then two of my solar panels in parallel will charge it in 23.3 hours but the sun goes to sleep each night so the charging will take about 4 sunny days.

    There is a big advertising sign in a field beside a major street. The owner did not want to pay for electricity to be wired to it so he put a huge solar panel. It didn't work following a cloudy day and didn't work long enough in winter. So he added a pretty big windmill generator and now it is fine.
     
  18. Audioguru

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    Dec 20, 2007
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    The little 12V/7Ah battery charges with 350mA for 20 hours. You can simply calculate how many solar panels you need.

    Hint: the sun is the brightest at noon on each sunny day in summer. So if the battery needs to be charged for 20 hours then it takes about 4 sunny days in summer or a couple of sunny weeks in winter. You might need a charger controller circuit so the battery does not over-charge at noon on sunny days in summer.
     
  19. porky67

    Thread Starter New Member

    Nov 19, 2010
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    "The little 12V/7Ah battery can be charged in 20 hours at 350mA."
    how do you get 350mA? you already parallel them is it?

    FYI, i'm living in a country which get alot of sunshine.. summer everyday..
    then things is, i'm currently cracking my brain to figure out what DC-DC converter to be used.

    Solar panel -> convert to AC -> transformer(to boost the voltage and current) -> regulate them for higher V and I -> convert back to DCl -> DC-DC converter for optimizing charging of battery..

    is the above got the rational to get it work? about using the transformer, i get the infomation from MPPT description which they mention they use transformer to boost up V and I, regulate them and convert back to DC and using DC-DC converter for MPPT optimizing
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    The battery is printed "7Ah at 20 hour rate".
    So it charges and discharges at 7A/20= 0.35A which is 350mA.

    EDIT: If you use a converter to boost the voltage then the current is reduced. You cannot get more power out of a solar panel.
     
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