Solar cell recharging the 1.5v rechargable batteries.

Discussion in 'The Projects Forum' started by bundick, Jan 13, 2011.

  1. bundick

    Thread Starter Active Member

    Dec 19, 2007
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    My outside Temperature sensor/transmitter and my outside rain gauge fall/transmitter both use AA batteries at too high of a rate.

    I'm wondering if there is a small Solar Cell charger that will handle 4-:AA Rechargeable batteries?

    My idea is to hide the Temp sensor from the Direct sunlight, up under the eve or out on a pole, then put the Solar cell in direct view of the sun.
    This charger would have to intelligent enough to keep it from over charging.

    Any ideas?:)
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    If you click on NiMH application manual on this page, it seems an indefinite charge of C/40 should be OK which would be 50mA for a 2000mAH battery so for anything less than that all you need is a diode to stop it discharging at night.
    http://data.energizer.com/SearchResult.aspx
     
  3. wayneh

    Expert

    Sep 9, 2010
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    The first place to start is to estimate or measure your current draw. That's what will determine the needed size of the solar panel. You need to add enough juice during the day to power the thing through the night, and I think you'll want to "oversize" the panel to keep the batteries topped up almost all the time, as opposed to running dead once in a while in cloudy weather.

    I assume your batteries are in series, for ~6v? That makes it a little trickier because, if you were just using 1 or two cells, you could use the electronics from one of those cheap solar landscape lights. There may be one using 4 cells out there, but I haven't seen one.

    Anyway, as noted, the simplest circuit is just a blocking diode and a panel that's not big enough to toast your batteries. Beyond that it gets complicated quickly, although there are lots of circuits already out there you could use to protect the batteries from overcharge.

    Question to others here, one I've been wondering about for a while: If you have a one-cell charger such as one pulled from a solar light, and you wire 4 of them together in series, could you use that as a 4-cell charger? Do these things even have overcharge protection?
     
  4. Gundalf

    New Member

    Jun 22, 2010
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    The tricky quite obviously is to stop it overchargeing - as someone said provided that you dont exceed the magic C/40 rate for your cells or whatever the rate is then it'll be fine.

    The solution that I used was to fit cells designed for continuous charge and then simply oversize the panel so it can easily cope with the load and regulate it with a 7812 or similar 1.5A voltage regulator. If you connect the output through a resistor sized to drop 1.25V at the required current and then connect the sense/adjust pin to the far size of that you create an effective and cheap current reg that easily copes with solar charging up to typically 37V input. The only disadvantage is that you have a fair voltage drop across the reg and the output so it can be a bit wasteful of energy.
     
  5. bundick

    Thread Starter Active Member

    Dec 19, 2007
    97
    1
    I'm getting excited Gundalf. I think my head is not working well lately though. I'll have to draw out what you guys are saying and I may be back.

    They sell little solar chargers for the Rechargeable batteries. I looked at on on Ebay that put out more than 3 volts. That should handle a Twin AA device.
    I don't know if they are intelligent though.

    Also, if the Rate the batteries are being used , is more than the Charger can provide, will some internal resistance cause a short(Burn) in the Battery or the Charger?
     
  6. Gundalf

    New Member

    Jun 22, 2010
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    If the current is regulated then no it wont cause any problems. What's the current draw of the sensors etc that are powered by these batteries?
     
  7. bundick

    Thread Starter Active Member

    Dec 19, 2007
    97
    1
    I have no idea what the Current draw is. It might be listed on the device or maybe I could look it up on the internet.(its in storage now, and it's too cold to go look for it)
    But it cant be a whole lot. Two AA's lasted a month or so.
    This is what I bought from Ebay:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270683147441&ssPageName=STRK:MEWNX:IT

    I think my days of 'creating' are long gone by. I'm better off purchasing "Appliances" .
     
  8. wayneh

    Expert

    Sep 9, 2010
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    That bodes extremely well for a solar solution. Your current draw can't be more than a few mA. So a 'small' solar panel that has little risk of overcharging your batteries will still be able to keep up with the power demand.

    Now the bad news: I don't think that strip you bought on e-bay is quite up to the task. Maybe, but maybe not. If it really performs as advertised and you have it in a sunny spot, maybe. It needs to supply MORE voltage than the battery in order for current to move, and the voltage will drop as current flows.

    The piece that you may not have factored in is that a blocking diode, absolutely required to prevent reverse current at night, will drop about 0.7v across itself. A Schottky diode is often used for this because its drop is a bit less at ~0.5. What this all means is that your battery can only charge when the panel is fully lit by direct sun. Anything less than ~3.5v at the panel will give no charging at all of two AAs in series.
     
  9. bundick

    Thread Starter Active Member

    Dec 19, 2007
    97
    1
    I need a Diode to keep the batteries from discharging into the Solar Array?
    When I get this thing and see how it works, I may have to lean on you guys again.
    Hopefully it's going to come with all that.
    Thank you for all the care! You guys are great.
     
  10. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    Yes, without a diode, at night the solar cell voltage drops below the battery voltage and the battery discharges itself through the cell.
     
  11. bundick

    Thread Starter Active Member

    Dec 19, 2007
    97
    1
    OK, thanks.
     
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