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battery capacity calculations metal nickle halide

Discussion in 'Electronic Basics' started by ilaboo, Nov 21, 2004.

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  1. ilaboo

    ilaboo Guest

    really confused about this rating

    i am using a 3,500 mah c cell to powe a 20 ma led

    how may days ( about--i understand the fall offf of volatage) can i run
    the led--i amusing a vanson smart charger to charge the c cells (2) in
    series.

    i would think the calculation
    would go like this



    3500/20 == how many hours the led should light


    ((3500)/20) /24 how many days the led sould be lite

    i get about 5 days running the led right now after fully charging the
    cells--about 4 hours

    what is wrong with my calculation

    tia
    peter
     
  2. Bill

    Bill Guest

    Most cells are rated based on a 10 hour rate which means if your cell
    is rated for 3500mAH, you should be able to draw 350mA for 10 hours.
    In my experience, when you draww less than the 10 hour rate you
    usually get more than rated capacity. Since you say you are only
    drawing 20ma, your calculations appear to be correct and come out to a
    little under 7.5 days and I would expect a little more than this based
    on what I said above concerning more capacity at loads under the 10
    hour rate. If you are not getting that then you must either have more
    current draw than you think you do or the battery is not delivering
    rated capacity (which would not surprise me).
     
  3. The OP didn't describe the circuit (single LED? series resistor?).So
    another reason for the apparent discrepancy could be that the voltage
    across the battery is falling below the level at which the LED
    illuminates.
     
  4. ilaboo

    ilaboo Guest


    circuit is simply a led across the + and _ pole of the battery
    i think what is happening is the voltage is dropping such that the led
    cannot light

    thanks
    for all the info
     
  5. John Fields

    John Fields Guest

     
  6. ilaboo wrote:
    [...]
    http://www.emanator.demon.co.uk/bigclive/joule.htm
    That is a small device which should keep you going some longer...
    Good luck with it:)
     
  7. Indeed!
     
  8. Rich Grise

    Rich Grise Guest

    Not only that, but it's surprising that the LED has not already been
    destroyed - or is this a 1.2V cell and red LED?

    One thing about LEDs, or any diode, for that matter, is that it has a very
    low dynamic resistance - very little change in voltage can cause a much
    greater change in current.

    Please tell us everything you know about your battery, your circuit, your
    LED - we aren't mind readers, after all! :)

    Thanks,
    Rich
     
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