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Inexpensive solar panels.

Discussion in 'Renewable Energy' started by Hero999, Dec 15, 2008.

  1. Hero999

    Hero999 Banned

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    I have connected up a couple of 8W 24V fluorescent luminaries to a pair of 14Ah lead acid batteries in series to light my shed. I would have gone for mains but this would be inconvenient as it's all the way up the end of the garden and I'm not digging it or the patio up to lay cable. Besides I got both the lights and the batteries for free.

    I would like to buy a small inexpensive solar panel to keep the batteries topped up as recharging them is inconvenient.

    The lights draw about 750mA and are only used for an hour a week at the most.

    I live at a fairly northerly latitude about 52° so the days are only 8:45 hours long in winter. Despite this the weather is relativity mild with an absolute minimum and maximum temperature of -12°C to 35°C respectively. Ideally it should be weather proof but I could put it behind glass if it isn't water proof enough to stand the heavy rain we get.

    So far the best deal I've found is at Maplin's but there's no mention of the temperature range. This is also assuming that I can connect them in series - i.e. the 0V rail isn't bonded to their cases.

    The open circuit voltage is 17.5V and the current ouput is 125mA maximum, although I know full well it would be a fraction of this in winter. Will I need to connect a zener in parallel with the batteries to stop them from overcharging, bearing in mind the lights will probably never be used in summer?

    Solar Powered 12V Battery Trickle Charger > Maplin
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would suggest the chances of over-charging batteries in the UK are pretty slight :D

    14AH batteries should be able to take 125mA continually, and it's never going to reach that in the UK - perhaps 80-100mA peak innthe summer, for a short period a day?.
     
  3. HarveyH42

    HarveyH42 Banned

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    Those aren't really intended for charging batteries. They're for keeping a charged battery from self-discharging, while sitting unused for a while. I tried 3 in parallel, and could get much of a charge to a 7 Ah battery. There are 5 watt panel sold for a similar purpose, I bought 3 off Woot.com a while back ($14.99 each). Haven't had many clear days to play with them. Futurlec has charge controllers, the 5 amp is around $25, will take care of your battery, whether you use it or not...
     
  4. dave

    Dave New Member

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  5. Hero999

    Hero999 Banned

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    Thanks for the comments.

    It looks like I won't have to limit the voltage.

    DIYman,
    A zener isn't inefficient in this case because it only starts to draw power when the breakdown voltage is exceeded.

    The circuit on the site you linked is better but a low dropout regulator might be required as it will take a higher light level before the batteries start to charge. It would also need modifying to work with a 24V system which is what I have.

    Is this solar panal good value, can anyone suggest anything which represents better value for money.
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I would suggest it's far too small for the job, the specified output is only at midday on the equator, in the UK in summer you might average 50mA for ten hours (if you're lucky?).

    No need to regulate it, as it's never even going to reach maximum trickle charge currents.
     
  7. Hero999

    Hero999 Banned

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  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I'm sure cheap Chinese manufactruiers ship their panels to California to be tested :p

    Buy one and do the tests on it - as far as I'm aware the rated values are never going to be approached in the UK, even in the middle of summer.
     
  9. codyrozalyn

    codyrozalyn New Member

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    I can assure you will never never maintain that "maximum" output current the panel manufacturers post. Assuming you have a proper mount with sun-tracking circuit(s) and motor(s) you may get 75% of what they list during the summer in full sun, <10% in winter. I would suggest designing an efficient method of sun input before you fork over the cash for a panel. I cannot stress this point enough, the lossy nature of glass covered panels makes them at times unreliable especially when proper angles are not maintained. On 100W panels I can lose 6~7 A out of my 8.5 per panel per 15 degrees degradation.

    You can waste your time and money with charge controllers but the size battery you will be implementing wont subsidize the cost of the controller. Although a properly rated Schotchkey diode will be needed for reverse flow during period of no sun.

    Check this site out for a circuit diagram of a sun tracker. LED3X Solar Tracker
     
  10. electronic spark

    electronic spark New Member

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    seems like it will not be enough in winter when you want to use it, if there is wind you could use a small wind charger that you can make yourself?
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You Brits are being ripped off.
    I bought 1.8W (12V at 150mA) solar panel for 1/4 of the price you pay.
    At noon in spring its output in direct sunlight is 120mA and behind clean glass it is 80mA.

    In the middle of summer its output at noon in direct sunlight is 170mA and in winter it is almost nothing. On cloudy days and at night its output is nothing.
     
  12. Hero999

    Hero999 Banned

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    The problem is it needs to be weather proof, I still haven't found a cost effective weather proof panel.

    We get plenty of wind so a mini wind turbine would work. Do you know where I can buy a cheap one?
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Most of my solar garden lights have their solar panel fried by the heat from sunshine. Guess where they are made?
     
  14. mneary

    mneary New Member

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    Toronto? :D
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    When I was little, a cheap Japanese radio was made in USA, Japan. Their city was called USA.

    The Chinese probably have a city called Toronto.
     
  16. electronic spark

    electronic spark New Member

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    doesn't seem like anybody sells micro wind turbine <10w. You may have to make one yourself. For ideas see Wind Generator from old scanner [updated]
    Instead of stepper motor which will probably give you 5v, you can try a 12vdc (ac/dc) portable fan such as 12" fan from walmart for $10. Search around any permanent magnet motor or toy motor can be used or maybe some other member has an idea, may have to use two of those to get 24v. I have one such fan and may do that myself, just will have to change the blade to a 2 or 3 blade instead of what comes with it.

    That only produced a few volts at 300rpm. I tried a scanner, printer head and paper stepper motors with similar results.

    Then I got an old 5 1/4" floppy drive stepper motor for head movement, it is a 12vdc, see pics. This one produced 7v, 11v, 11v and 14VDC (red and white wires on outside) at 300 rpm! That's promising. It's about 2.25" round and about 1/2" thick.
     

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  17. duffy

    duffy New Member

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    Did you check to see that it was the panel and not the nicads, Audioguru?
     
  18. Hero999

    Hero999 Banned

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    A stepper motor looks like a good idea.

    It might work if I add a voltage multiplier to boost the voltage to get 28VDC.
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My solar garden lights are cooking in the sunshine all day long every day.
    Some have the solar panel and Ni-Cad battery cell failing soon from being cooked to death. Others last for years.

    They get so hot from the sunshine that they burn you if you touch them.
    I wonder if the "solar panel" uses heat, not light.
     
  20. duffy

    duffy New Member

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    Maybe you could get an extra watt out of it with a heat module on the back of the solar cell connected to a heat-conducting rod driven deep in the ground for a heat-sink.
     
  21. Hero999

    Hero999 Banned

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    That's a thought, I wonder if I could but a cheap batch of 20 solar lights and connect the panels in series.
     

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