Alkaline Battery Capacity Question

Discussion in 'The Projects Forum' started by Morpho, Dec 22, 2013.

  1. Morpho

    Thread Starter New Member

    Dec 13, 2013
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    I have 2 AAA Energizer Max Alkaline batteries, in series, powering a 2.5V 25mA 10mm LED with a 22ohm resistor in front of the LED. The spec sheet on Energizer's web site led me to believe that I should expect about 1300mAh capacity with a constant 25mA load giving me about 52 hours of constant use. My question is, "Why is the LED still bright as ever after 72 hours?" If anything, I'd have expected the spec sheet to over estimate what I would really see in my application. I'm sure I'm missing something obvious, but could somebody please shed some light on this for me? Thanks very much.
     
  2. #12

    Expert

    Nov 30, 2010
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    The spec sheet doesn't tell the guaranteed fail point, it tells the minimum guaranteed performance. Besides that, your eyes aren't all that good at telling when an LED is running at half power. Measure the voltage across the 22 ohm resistor to find that answer. Then, stand up, take a deep breath, and be happy that you got better performance than you expected.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Another factor is that the battery voltage, and hence the LED current, drops over time; so the average current may be well below 25mA.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Another factor is that the LED stays bright (as perceived by the eye) even though the current has dropped off a lot. I have used a standard ultra-bright 20mA indicator LED as an ON-indicator in some battery-powered equipment where I set the current through the LED to only 250uA, and you can still see it in direct sunlight...
     
  5. Morpho

    Thread Starter New Member

    Dec 13, 2013
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    Thanks for the info and suggestions!
     
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    http://data.energizer.com/PDFs/E92.pdf

    Voltage of battery drops off in the first 24 hours. As voltage drops, current also drops and battery lasts longer. See voltage vs time at various current draws o. PDF link above.
     
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  7. Morpho

    Thread Starter New Member

    Dec 13, 2013
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    So looking at the voltage vs time graph with 250mA discharge at 21 degrees, can you extrapolate my 25mA load to get a reasonable expectation of how long the battery will last?
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Now that i look deeper at the datasheet and what you are really doing, No, I can not extrapolate with any math - just gut feel.. There are some competing issues...

    As your project runs, battery voltage drops and, therefore, current across your 22 ohm resistor and LED drop. The graphs are all inconstant current control (you are not constant current, and no constant voltage)

    I don't know what voltage your LED will still generate light. Or acceptable levels of light.

    Also, from the bar graph on page 1, you see that a batterthese dry cell batteries last longer at lower amperage discharge rates.

    You can make a bunch of assumptions and try to calculate it, or, you can assume you are about 50% to 75% of the way through a lab trial and just record the time when your battery goes dark (even if you heck every 12 hours at this point, you will be within 15% accurate. If it stays on another 3 days, you will be within 7%). In any case, the time method will be better than any extrapolation from Datasheets that are not even displaying what you are actually doing. Once you have that number, it should be fairly repeatable if you make a bunch of them.

    Finally, my gut feel, based on experience and a fairly good history of estimation, your batteries will run dark after a total of 5 to 7 days.
     
  9. Morpho

    Thread Starter New Member

    Dec 13, 2013
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    That helps! Thanks again for the opinions and insight.
     
  10. trader007

    Active Member

    Feb 27, 2010
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    All of it is very good advice. Spot on answers.

    I'll just add, every battery made has a little different performance, which can make a difference when working with such low current... AND add to that the fact that each LED made has even more variance. You can do all the math you want, but youll be lucky to get it within 20% when you have such a capacitance/load difference.
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I was surprised when I bought some cheapo "9V" NI-MH batteries from Harbor Freight and tested them. They exceeded their ma-hr rating by about 50%.

    Battery makers make several different "grades" of batteries they sell for a range of prices. I think that they may be the same battery just labeled differently.
     
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