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« on: June 05, 2008, 05:57:52 PM »

Does anyone know how long it should usually take until IR LEDs kill a battery? I just opened a new case of 9V batteries, and I checked with a voltmeter to make sure a battery was 9v, and it was. I used it for a continuous total of maybe four hours over the course of two days, and now it's sitting at 3V of power left. That's pretty quick!

The little coin-sized batteries in a set of glasses must not last very long if they're powering IR LEDs. Someone told me IR LEDs should actually drain a battery LESS than regular LEDs, but my experience shows otherwise.

Does anyone else have a report on their battery life with IR LEDs?
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benpaddlejones@live.com.au View Profile WWW Email
« Reply #1 on: June 06, 2008, 03:02:08 AM »

More info needed:

Sourve Voltage: 9v
LED Forward Voltage: Huh
Resistor: Huh
Battery brand & type: Huh (NIMH/NICAD/etc)

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« Reply #2 on: June 06, 2008, 06:06:41 AM »

Well, assuming it's an alkaline battery, that's is a bit fast.  Alkalines are generally about 550mAh, so if you really did kill it in 4 hours that would mean you were pushing over 100mA through your LED.  Not impossible (there's LEDs rated for 100mA that could probably take 120+ and still be happy), but not good either.  As benpaddlejones  said, we need to know what resistor and LED(s) you were using to say for sure if something weird is going on.
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« Reply #3 on: June 06, 2008, 05:21:05 PM »

hi,

The little coin-sized batteries in a set of glasses must not last very long if they're powering IR LEDs. Someone told me IR LEDs should actually drain a battery LESS than regular LEDs, but my experience shows otherwise.

certainly not true.
standard leds usually want 10-20mA current for a good visibility.
low current leds reduce this requirement to 1-2mA. their visibility is still good, but they (obviously) arent as strong as standards regarding their emission (check the datasheets, they should state the emission intensity in mW, sometimes at different current levels).

for ir leds to work as intended, this emission usually needs to be quite high (the higher the better), therefor most ir leds are built for MUCH higher currents... typical leds are specified for 20-50mA, most of them even work up to 100-150mA limits. some special high performance ir leds for very long range uses even work with 500mA+.
generally, ir leds driven with less current than you would drive standard leds with (<20mA) most likely wont give you good results.

if you want to make the most out of your battery lifetime and still get as good results as possible you should
1. use the wavelength the sensor is most sensitive to (apparently 940nm for the wii camera, 950nm - which is quite a standard - should work, too and give much better results over other ir leds in the 850's range).
2. use leds with the best mW/mA ratio you can find.
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« Reply #4 on: June 06, 2008, 05:37:53 PM »

one additional note:
maybe the one telling you they should last longer was just confused by comparing battery lifetime of e.g. remote controls and led lights/lamps.
he was misleaded then. typical ir led uses like remote controls emit light in modulated signals and not a continuous stream (typically 36-40khz square/digital signals), resulting in a significantly lower current drain.
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« Reply #5 on: June 06, 2008, 09:16:38 PM »

2. use leds with the best mW/mA ratio you can find.

Be careful to look at mW (total radiated power) rating, and not mW/sr.  The later is a measurement of angular energy density, and can vary dramatically based on measurement technique and viewing angle of the LED.  Unfortunately few datasheets actually publish mW ratings, only mw/sr.

Another useful metric is quantum efficiency.  electrical power * quantum efficiency > optical power.  For example, a 100mA 1.3V 70% efficient LED would have a total radiant a bit below 91mW (less due to optical properties of the package).  It's rare however to find a datasheet that lists QE but not TRP.
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« Reply #6 on: August 14, 2008, 03:36:25 AM »

Its simple, 9v batteries usually have 450-600mA capacity. Using your led at 100mA (like most used leds at this forum) that means that you should use a 68/82ohm resistor.
If your led is runing at 100mA, then your battery should last 4.5/6 hours.

Many people think that using grater voltage batteries should last longer than smaller ones... thats NOT true
if your led needs 1.6v and you give him 9v, that means that you will loose 7.4v in HEAT. how? --> on the resistor. if you dont place a resistor your led will die VERY quickly, and the battery wont last much.
i personaly use 2 AAA rechargable batteries 1100mA. that means 2.4v, with a resistor 6.8ohm resistor. this batteries will give me 11 hours of use. the cost and size will probably be the same or less than a 9v alcaline bat...
bye!
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