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Offline icoms8Topic starter

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LED
« on: February 17, 2009, 07:39:10 PM »
how long will a LED run on a 6v battery?

Offline airman00

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Re: LED
« Reply #1 on: February 17, 2009, 07:42:26 PM »
check the datasheet
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Offline Wizzard

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Re: LED
« Reply #2 on: February 17, 2009, 08:19:27 PM »
There are lots of variables that you would need to know. such as the power consumption rate of the LED, how big of a 6v battery, I have a few 6v golf cart batteries that could run a LED a very long time. Much longer than a small 5 cell AAA pack.

Give more information in a question like that if you want a good answer.

Offline ArcMan

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Re: LED
« Reply #3 on: February 17, 2009, 08:23:15 PM »
Well, assuming you keep your battery charged and limit the LED's current at or below its spec it should last you for a good 100,000 hours or so.

Offline SmAsH

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Re: LED
« Reply #4 on: February 17, 2009, 11:18:39 PM »
yes, a small led normally 20ma? last like a week or two on a button cell, but to work this out just see you battery backs label where it says ??mah and seeing as most rego leds consume 20ma (see your leds datasheet to confirm) for this example well say the battery has 1600mah and the led consumes 20ma. youd get approx 80hrs.
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Offline HDL_CinC_Dragon

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Re: LED
« Reply #5 on: February 18, 2009, 12:48:46 AM »
Well, assuming you keep your battery charged and limit the LED's current at or below its spec it should last you for a good 100,000 hours or so.
This number is pulled out of the air.
For all you know that battery could have a capacity of 100mAh or 100Ah

To find out how long a battery will last you need to use the following calculation:
mAh of battery / mA draw of circuit
1000mAh battery / 20mA draw of circuit = 50 hours
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Offline Soeren

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Re: LED
« Reply #6 on: February 18, 2009, 03:34:46 PM »
Hi,

yes, a small led normally 20ma? last like a week or two on a button cell, but to work this out just see you battery backs label where it says ??mah
I beg to differ.
One week = 168 hours, two weeks = 336 hours.
Powerfull button cells
CR2032 Lithium (3.0V, actually a coin cell) is around 170..200mAh
S675 zinc/air cell (1.4V, the size of an AG13) is ~650mAh
Assuming no loss whatsoever (which is of course impossible), 2 serially connected S675 (the strongest button cell I know of in current use) will drive a 20mA red LED for less than 33 hours so one might argue that one cell might drive it for half that amount, i.e. less than 17 hours.

For fun, I tested the inner resistance of S675's by taping two together with a single red LED (directly, with no resistor) and left them to die.
At first the LED was bright, but judging by eyesight, not overly so, compared to one driven at exactly 20mA.
I did it around 2 p.m. and the next morning, around 8 a.m., the light was but a faint glow, barely visible.


and seeing as most rego leds consume 20ma (see your leds datasheet to confirm) for this example well say the battery has 1600mah and the led consumes 20ma. youd get approx 80hrs.
For a red, green or yellow LED, you'd need at least two cells in the battery (assuming 1.5V cells), for a white, blue pink, violet or UV LED you'll need at least 3 cells. If you want to be able to use the batteries up completely, you'd need one more cell in each case.
Still, lets assume no losses and pretend that 1600 mAh cells would give 80 hours, it's still much less than the 1..2 weeks mentioned.
Regards,
Søren

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Offline Razor Concepts

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Re: LED
« Reply #7 on: February 18, 2009, 04:08:57 PM »
Well, assuming you keep your battery charged and limit the LED's current at or below its spec it should last you for a good 100,000 hours or so.
This number is pulled out of the air.
For all you know that battery could have a capacity of 100mAh or 100Ah

To find out how long a battery will last you need to use the following calculation:
mAh of battery / mA draw of circuit
1000mAh battery / 20mA draw of circuit = 50 hours

He meant 100,000 hours on an infinite battery - as in how long the led lasts untill it burns out on its own, regardless of the battery.

Offline Webbot

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Re: LED
« Reply #8 on: February 18, 2009, 04:28:49 PM »
how long will a LED run on a 6v battery?
Check the LED datasheet to see the voltage required and the current required. Check the battery for its 'mAH' as that is the total number of milliamps it can supply for one hour before dying. It could supply half the current for twice as long. etc. Just do the maths.

The datasheet for the LED may also hint at the life expectancy of the LED before it goes pop. But, with the correct current limiting resistor, this should tend towards infinity.

Or put it another way: how long do you WANT the LED to last?
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Offline Soeren

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Re: LED
« Reply #9 on: February 18, 2009, 04:56:27 PM »
Hi,

Check the battery for its 'mAH' as that is the total number of milliamps it can supply for one hour before dying. It could supply half the current for twice as long. etc. Just do the maths.

The datasheet for the LED may also hint at the life expectancy of the LED before it goes pop. But, with the correct current limiting resistor, this should tend towards infinity.
Lead-acid batteries are often rated by their 20 hour capacity and eg. Nickel based batteries by their 5 hour capacity (as allways with batteries, only the manufacturer will know, so check the datasheet).
That means that a 20Ah lead acid (when new and fully charged) will provide 1A for 20 hours. It won't supply 20A for more than perhaps 20..30 minutes! Capacity depends on discharge current (and ambient factors like temperature).

LED's doesn't go pop or burn out when their rated life is reached.
Their life expectancy is simply the amount of time it is expected to last with a light intensity of more than 50% of the nominal light intensity rating. If they're down to 50% after 11 years, they should still give about 25% of nominal after 22 years. The only things that kills LED's instantly is serious overcurrent (which turns the LED into a short circuit or, in cases of really high current, might vaporize the bonding wire connecting the semiconductor crystal to the anode wire), or bad sales. The rest of them just fades slowly away.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Webbot

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Re: LED
« Reply #10 on: February 18, 2009, 05:21:45 PM »
how long will a LED run on a 6v battery?

@iconms8: Jeez - bet you thought it was a simple question !!   I repeat - how long do you want/need the LED to run? Seconds, minutes, hours, days, weeks ?
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Offline ArcMan

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Re: LED
« Reply #11 on: February 18, 2009, 06:18:08 PM »
Funny.  Why does my wife think we're a bunch of geeks  ???

Offline Razor Concepts

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Re: LED
« Reply #12 on: February 18, 2009, 06:42:23 PM »
Wives are usually right  :-X

Offline want2learn

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Re: LED
« Reply #13 on: February 18, 2009, 09:00:33 PM »
Yeah my wife's telling me right now that wives are always right ;D
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Offline SmAsH

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Re: LED
« Reply #14 on: February 19, 2009, 04:46:36 AM »
ahhh, whats wrong with being a geek? without 'geeks' we wouldnt have technology as we know it now would we?
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Offline Wizzard

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Re: LED
« Reply #15 on: February 19, 2009, 01:18:27 PM »
And Speaking of super Geeks, Did anyone take into account the normal loss of charge over time even when there is not a wired load on the Battery? I have no idea how to account for that without doing tests on the battery over a very long time span with no load attached?

oh and another thing.... I have heard it quoted from some book that "the Geek shall inherit the earth" or at least something close to that  ;)

Offline SmAsH

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Re: LED
« Reply #16 on: February 19, 2009, 02:04:17 PM »
well the battery loss would be very minimal but would vary from battery to battery. but would most likely depend on battery brand.

and yay for geeks!
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