Looking for the right cell batteries for a project

Discussion in 'The Projects Forum' started by higuys, Mar 20, 2012.

  1. higuys

    Thread Starter New Member

    Mar 20, 2012
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    Hello to this forum.

    I had a specific project I was working on. I am attempting to light up a 10mm white diffused LED in such a manner that I can achieve a high level of brightness and to have the LED stay at that level of brightness for as long as possible, preferably at least 14 hours.

    I want to use button cell batteries, my question is which size button cell battery (or batteries, plural) would be the best to achieve a consistent level of bright white light for that significant length of time?

    I have tried hooking up the LED to two cr2032 batteries in series and unfortunately the LED goes too dim too quickly that way.

    Thank you for any responses.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Do you know what current your LED requires? That will define what you need; mA times hours = mAh. Most batteries are rated by their mAh capacity. Note that when you place them in series, you gain voltage but not mAh.

    All those solar landscape lights you see get by with a single AA or AAA battery by using a circuit to boost the voltage. An example is the joule-thief circuit but there are many variations. The benefit is that you can run the battery much farther down without the voltage falling below the minimum threshold for the LED. With just a battery, the LED dims quickly due to voltage sag, even though there's still a lot of juice in the battery.
     
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Why are u doing silly things.?

    Do you actually think that a button cell can light a Led for that long. given the power of ur typical LED.

    U cannot run button cells in parallel. it's not safe. It might blow up. :eek:

    Explain ur project clearly.
     
    Last edited: Mar 22, 2012
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  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Take a look at some of the 4.5V and 6V camera batteries with capacities around 500 mAH or more.
     
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  5. higuys

    Thread Starter New Member

    Mar 20, 2012
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    Thank you for the responses, guys.





    Here is a table of the different sizes of button cell batteries and the specifications of each. The LED requires a 20mA current. I am still wondering whether or not I will be able to achieve what I outlined in my original post with one of these batteries? (I attached a file, click on it to enlarge it after opening it).
     
    Last edited: Mar 22, 2012
  6. Audioguru

    New Member

    Dec 20, 2007
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    The CR2477 is the biggest battery cell on your list. It normally has a load of 7.5k ohms so its current with a new battery cell is only 3V/7.5k= 0.4mA. Then it can supply its rated 1050mAh.

    20mA is too high for any of those small battery cells.
    You need 20mA for 14 hours which is 280mAh. Most battery cells have their mAh rating stated when their voltage has dropped pretty low.

    How about using two alkaline AAA cells? The voltage drops to 2.4V as they discharge 20mA in 40 hours. Maybe in 14 hours their voltage might not drop much.
     
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  7. higuys

    Thread Starter New Member

    Mar 20, 2012
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    Thanks. Yes, I guess I will have to go with one or two AAA cells. I will experiment with both.
     
  8. Arkole Blake

    New Member

    Feb 15, 2012
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  9. Audioguru

    New Member

    Dec 20, 2007
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    Li-Po battery cells need a special charger circuit and also need a special circuit to disconnect the load when the cell drops to about 3.2V.

    It is 4.20V when fully charged and about 3.2V when discharged so the voltage drops 24% which is a lot.
     
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  10. higuys

    Thread Starter New Member

    Mar 20, 2012
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    My project changed a bit, I discovered after I last posted on this thread that I needed three of the 20mA LEDs instead of just one. I connected them in parallel to two AAA batteries without using any sort of cicuit. I just used a battery holder from radioshack.

    Now after only three hours the LEDs are visibly dimmer. Is it possible if I used one of these joule-thief circuit or something similar that I could get 14 hours of fairly constant brightness out of my LEDs? I could go with two AAs instead of two AAAs if need be as well.

    Thanks again for all the responses guys.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    14 hours at, say, 30mA for all 3 LEDs is 420 mAh, which is within range for good AAA or AA batteries, so yes.

    Scavenge the circuit out of a solar garden light and replace the battery (usually 600mAh or less rating) with a pair of good batteries in parallel (NOT series). Put your 3 LEDs in parallel, replacing the existing LED. This will run all night for sure, and probably your full 14 hours.

    Experiment to see if you can still get acceptable results with a single AA. I doubt a single AAA can do it, but that's just a hunch.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Aren't you using white LEDs? They need somewhere from 3.3V to 3.7V (they are all different).
    LEDs need a voltage that is higher then a resistor in series with each one to limit the current.

    But brand new alkaline battery cells are 1.6V each (the total for two is only 3.2V) which quickly drops to 2.4V as the battery runs down.

    No wonder you see your LEDs dimming.
    If the LEDs were 1.8V red ones , 2.2V yellow ones or 2.4V old green ones then they are now burnt out.
     
  13. higuys

    Thread Starter New Member

    Mar 20, 2012
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    O.K., I took your advice. One thing I have noticed is that the batteries in parallel do not light up the 3 LEDs with the same brightness from the get- go as does the single CR2032 coin cell battery a single LED> I was wondering why that might be. I thought it might be particular to the circuit I scavenged from the garden solar light from Target (the store). In other words, if I had used a different circuit it might give me better brightness right off the bat?

    I have not yet run the LEDs for the full 14 hours on either the AAA batteries or the single AA. I will post the results once I do (why not?) :).
     
  14. higuys

    Thread Starter New Member

    Mar 20, 2012
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    Yep, that is very true!
     
  15. wayneh

    Expert

    Sep 9, 2010
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    This may depend on the details of the solar light you're using - it might not be able to provide enough current for 3 LEDs. Is it bright with one, with two? Dimmer still with 4? There's not much you can do beyond trying different ones. I'll speculate that removing one parallel battery will have little effect on initial brightness, just how long it stays bright.
     
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