# determine how long led will stay on w/ bettery

Discussion in 'Electronic Basics' started by SklettTheNewb, Mar 29, 2005.

1. ### SklettTheNewbGuest

I would like to calculate how long an LED will stay lit given a
specific power source. I need to know what the smallest battery I can
use in a project and still get n hours of operation.

For example:
if I have a typical LED and a 1.4v battery 130mah (hearing aid
battery), how long will it stay lit to the point where you can't
visually determine it's dimming..

What is the formula?

Also, does a flashing LED consume a non-linear amount of power? In
other words, if I had an led that was on 50% of the time, could I
double the duration value from the above said formula?

As you can tell, I don't really know anything. I just have an idea for
something and am trying to determine how feasible it is...

Thanks for any help!
Steve

2. ### Francisco GayáGuest

SklettTheNewb escribió:
Batterycapacity(mAh)/Led courrent(mA)=hours of operation

3. ### Lord GarthGuest

The answer will depend upon how much current you pass through the LED
and the voltage drop associated with that particular LED.

Pulsing the LED will save the battery but don't forget that the flasher
circuit
consumes power as well so a 50% duty cycle won't double the battery life.
Battery life would be greatly extended if you had a very short on time and a
long interval between repetitions. There are advertising buttons packaged
into products as attention getters. I would guess that many get thrown into
the trash.

4. ### SklettTheNewbGuest

great, thank you for the response.
So with that calculation and a 1.5v, 140mAh battery and a 1.5v, 20mA
LED, I would be looking at roughly 70 hours of constant operation.

That is pretty good

Thanks again!

5. ### SklettTheNewbGuest

I understand what you're saying about circuit draw and duration. for
my needs I would just need a quick "tick" on, then 1500 milliseconds
off.

I found these circuits using 555 chip
http://home.cogeco.ca/~rpaisley4/LM555.html#4

I guess I will need to learn how to use a multimeter so that I can
determine how much a circuit like this will draw, but it looks like I
can set the frequency to what I want using of these circuits.

If there is a more effecient way, I would like to know about it. I
don't intend to develop the circuit myself, I just need to determine if
this project makes sense before I think about speaking to a pro and
possibly putting out \$\$, you know?

I appreciate your help so far.
Steve

7. ### Byron A JeffGuest

Missed a decimal point. 140mAh/20mA = 7 Hrs, not 70.

The other problem is that the LED requires a specific forward voltage.
The battery's voltage will most likely drop during its use. At some point
before the 7 hours, the LED will go out.

Finally you may not have enough forward voltage even at 1.5V

BAJ

8. ### SklettTheNewbGuest

yes I did, typo ;(

I was thinking I could boost it up w/ transistors, at least things I
have read in the last hour make me think that is possible.

9. ### Rich GriseGuest

This is one of the worst, most rampant misconceptions among electronic
newbies.

A(An?) LED needs a constant _current_. (or at least controlled, or
regulated.) The forward voltage "spec" is merely the voltage that happens
to appear across its diode junction when the specified current is passed
through it. It's OK to apply an LED to, for instance, a watch battery,
because the internal resistance of the battery itself will limit the
LED's forward current. But put a LED across a 1.2V NiCd cell, and you
will most likely let the magic smoke out.
And depending on the battery, the LED might go out so quickly that you
don't even see the flash.
There's a huge thread on either this NG or s.e.design or
alt.binaries.schematics.electronic about eking the last possible
electon out of an AAA cell by clever electronics. I recommend at least
giving it a look-see. I don't recall the exact subject line, but it's
one of those that sounds unlikely. Kind of a pleasant surprise,
actually.

Good Luck!
Rich

10. ### RayGuest

What are you trying to light? Maybe you can use a couple AAA instead of
watch batteries and recharge them. I use hi power LED's when the power
goes out. We live in an area that if the wind blows too much we loose
power. I use rechargeables for most things.

11. ### Byron A JeffGuest

Rich,

The LED still requires a minimum voltage to conduct.

The OP gave a current specification, so it's clear he understood about
current limitation.

BAJ

12. ### SklettTheNewbGuest

Hi Ray,

I'm not trying to light so much as indicate or get attention. This
device will need to be roughly the diameter of a sharpie and about 1/2
as long, so dueling AAA are out, plus they are relatively heavy for
what I envision. It really seems like series of tradeoffs thus far, I
can utilize more power up front and have a cost/weight/size hit or I
can have "booster" circuits(if I can call them that...) at the cost of
# of components, complexity, cost....

Lots to think about, lots to learn...

thanks for the post!

13. ### JamieGuest

batteries/Cells have Amp Hour Ratings.
that means it will produce X number of Amps
in that hour before it goes cah-put!!
at least that is the way i have always understood it.
the units actually in most cases will deliver more amps
than the AH rating, but the unit will die before its rated
time of course.!