Life of white LEDs powered by lithium button cell

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!)

Andy

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Reply to
Andy
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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.
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Reply to
Jamie

White LED's operates in the 3.5 volt region.

That throws away 40% of the battery energy.

Another approach is to use a switching regulator setup to regulate current rather than voltage. That also keeps the lamp brightness constant at the end of battery life. (at the cost of having a very sharp cliff on light output vs time which might surprise you)

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Reply to
Hal Murray

Are you referring to those coin cells 20 mm in diameter and 3.2 mm thick?

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 ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

Is there some reason that you can't just measure the actual current being used?

Reply to
Anthony Fremont

Yeah true how ever, since this is going into some one's hat, I didn't think it was all that important. :)

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Reply to
Jamie

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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Reply to
Kevin McMurtrie

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