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Life of white LEDs powered by lithium button cell

Discussion in 'Lighting' started by Andy, Oct 19, 2008.

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  1. Andy

    Andy Guest

    I have a cheaply made light to be worn on the head. It consists of 5
    white LEDs each of which I guess would each draw about 40mA. (Is that
    right?) So the total current requirement might be around 200 mA.

    The power is 2 lithium button cells in series. These are type 2032 each
    with a nominal capacity of approx 2000 mAh (if a good brand) and probably
    less with the generic cells the lamp was supplied with.

    Simplistically I would say that a 200 mA load can be satisfied for 10
    hours from a supply of 2000 mAh.

    But that sounds like an overestimate and I've no idea what end-
    voltage/end-current is needed to power white LEDs. Nor if the discharge
    of a lithum cell is approx linear.

    So I am asking you guys if anyone is able to provide a better *estimate*
    of long such a 5 LED device would last for?

    (Nope - I don't want to waste a pair of lithium cells if this can be
    determined by theory!)

  2. Jamie

    Jamie Guest

    White LED's operates in the 3.5 volt region.
    I would use an R in series with each LED to the
    series cells.
    In your case.

    R = {E1+E2-Vled}/Iled=(3.0+3.0-3.5)/0.040=62.5

    so this would imply that each LED would need a
    62.5 ohm resistor to properly drop the current.
    You can use what you can find near this size.
    I think a 68 ohm is a common size.

    1/10 (0.1) watt size each should do it.

    + -----/\/\/\/-----|>|----|
    - ------------------------|"
  3. Phil Allison

    Phil Allison Guest


    ** In your dreams - pal.

    About * 200mAh * is the norm.

    ** Lithium 2032 cells can hardly supply 200 mA at all - dickhead.

    That is a massive overload.

    ..... Phil
  4. Are you referring to those coin cells 20 mm in diameter and 3.2 mm
    Such coin cells will have a fraction of 2000 mAH capacity at most, and
    may output 100 mA for minutes before their internal resistance responds to
    the heavy load by increasing.

    I try a bit of web searching on:

    lithium cell 2032 capacity mAH

    and mostly get 180-220 mAH for 2032.

    Many "key fob" style white LED flashlights have two lithium coin cells
    (though smaller than 2032 - such as 2016) in series, connected to a white
    LED with no dropping resistor at all.
    If you want 10 hours with near-full output, plan on "average loading"
    with maybe 20 mA total when the battery voltage is 5 volts, or something
    like that.

    You could use 1 LED with an 82 ohm resistor. Things won't be too much
    different if you use a resistor anywhere from 47 to 150 ohms. Lower
    resistance will favor higher initial brightness and faster fading, higher
    resistance will favor steadier output and output lasting a little longer
    and slightly dimmer on the whole.

    If you have a few LEDs easily in the budget, then use maybe 2 or 3
    of them with individual resistors maybe 2.1-2.5 or 3.2-4 respectively
    (roughly) times the value of one that you would have used with one LED.
    The usual white LEDs are nonlinear, with ratio of light output to current
    being maximized when current is a few mA. Don't fret about resistor value
    - being "a little off" will not change much in this application.

    I would not try to power more than 3 LEDs for 10 hours from a series
    pair of 2032 coin cells. I expect that at Hour 10 the current through
    each of 3 LEDs would be 6-7 mA even if regulated by a linear regulator and
    the 3 LEDs have "current dividing resistors" (maybe 33-47 ohms), less if
    you use only resistors no matter what you do.
    If you go for a switchmode current regulator, then 4 LEDs with
    individual current-dividing resistors around 33-47 ohms can make a
    significant improvement. Total current of all LEDs as high as 25 mA may
    be sustained for 10 hours.
    If a switchmode regulator is in the budget, then maybe also use 5 LEDs
    with current-dividing resistors of 39 or 47 ohms.

    If 5 LEDs are in the budget, consider whether or not to use a smaller
    number of better LEDs such as Nichia ones with D revision letter being the
    last or second-last character of the part number. For 5 mm ones of
    narrowest beam width (15 degrees or so) and "characterized" at 20 mA,
    those are NSPW-500DS.
    However, that does not sound to me like a "cheaply made light".

    - Don Klipstein ()
  5. Jamie

    Jamie Guest

    Yeah true how ever, since this is going into some one's hat, I didn't
    think it was all that important. :)"
  6. You're not going to get a good answer on this.

    First, CR 2032 batteries are around 220mAH.

    Smaller lithium cells are connected directly to the LED. The chemical
    reaction speed of the lithium battery serves as the current regulator.
    It's not how the LEDs or batteries were meant to be used. Efficiency is
    dependent on factors that are held to very broad specifications. The
    LED specs will say that anything below 3.7V at full power passes and the
    battery specs will say that anything above 40% efficiency at 1mA passes.
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