What's the best battery to LED brightness, and life?

[mardi]

Hi,
I'm an industrial designer - purely aesthetic, no electrical background (yet). I am working with a factory that doesn't seem to offer a lot of input on my lighting project, so I thought I would reach out to the CPF community.

I am building a small, high powered LED light that should run for 20 hours or more. It only needs to project 4 inches. Here's the 4 scenarios I'm looking at (below). Can you guys/girls give me an idea which would be the brightest, and approximately how long the LED will burn?

Honestly, I don't know if there are other factors that play a role, like resisters, type/size of LED, etc.
Thank you very much for your thoughts!
Mardi

1. two AG10 1.5v, 72 mAh batteries with 2 LEDs
2. two CR1220 3v, 40 mAh batteries with 2 LEDs
3. one CR1632 3v, 130 mAh battery with 2 LEDs
4. two CE1620 3v, 79mAh battery with 2 LEDs

 

[jason 77]

What LEDs are you planning on using, how bright "lumens" are we talking about?

 

[mardi]

Hi Jason,
Regarding the lumens, this is where I get lost. I'm undecided on the LED type.
Smaller physical size is always good, but ultimately the brightness of light is the most important.

Do you have any suggestions?
Thanks.

 

[TEEJ]

If "brightness" is the MOST important aspect....the thing to consider carefully is that the brighter it is, generally, the more power it takes to BE brighter.

Think of a gar and a gas pedal....if brighter is like faster....to go faster, you floor the gas pedal, and, it goes faster, but, the miles per gallon drop.

If you make it as bright as possible, you are running the LED "floored", and it uses up your battery faster.


To try to pin this down a bit more, lumens are how an LED or other emitter's output is described....think of photons, the total amount sprayed out by the light.

Now, you can't SEE lumens, what you CAN see are the photons that hit something and bounced back to your eyes...so, an object that reflects the light back to you, say a dog...is visible because light bounced off the dog and into your eyes.

The brighter the dog is illuminated, the higher the LUX. Lux is how the brightness of the TARGET is described. Its how bright it LOOKS to you.


One lux = one lumen per square meter.


So, the larger the area you are lighting up, the more lumens you need to make the larger surface illuminated to the same brightness.


The same number of lumens can light up a very large area very dimly, or, a very small area very brightly....depending on how you concentrated them, or, spread them out.


Some cells are better if the draw is heavy...analogous to running the throttle wide open, they can support a high amp draw longer. Some cells are better at low draws. Either can outlast the other depending on which draw is in effect.


So, what did you need this light to DO?

 

[mardi]

Awesome insight TEEJ!
I only need it to light up an area within 4 inches, so that's a good thing. Think of a tea light that sits on your table top at events and parties.
I'm not sure how to measure the LED output to battery power, and life expectancy. Is there a formula for this?
Then I need to figure out what type of LED to use. I was reading that DIPs cast more lumens than SMD LEDs. Correct?
M

 

[RoGuE_StreaK]

You can look at watt-hours for the purest 100% theoretical (ie. not really achievable) runtimes. Watts are current (A) x voltage (V), and are the amount of energy required; this negates all the different voltages etc so everything is being rated on the same scale.

Let's say a theoretical 5mm LED (just for the hell of it) needs 3.4V @ 20mA to give the desired light output. That's (3.4 x 0.02 = ) 0.068 Watts required. Running it for an hour requires 0.068Wh (Watt-hours)

Now let's take your third battery options, 1x 3V 130mAh (milliAmp-hours). 3V x 0.13 = 0.39Wh. So it can run your 0.068W LED for 0.39 / 0.068 ~= 5.75hrs

That's assuming a 100% efficient conversion of energy, which ain't gonna happen; guestimate at say 80% at most, depending on which method you use.


Some SMD LEDs can output thousands or even tens of thousands of lumens. Not many DIP LEDs out there capable of that...

 

[mardi]

Rogue,
That's great info. I'm a neophyte, so please bear with me.
If I have two batteries at 3v each... does this remain at 3v and I add the mA's together?

 

[RoGuE_StreaK]

If I have two batteries at 3v each... does this remain at 3v and I add the mA's together?

If they are in parallel (+ to +, - to -). If connected in series (+ to -) then the voltage doubles and the mAh stays the same. Either way results in the same Watt-hour capability, but the "processing" requirements would be different.
Do be careful and do your research before trying to connect batteries up though, some don't play nicely if they aren't a matched.

 

[kmossman]

For just a plain circuit....................yoiu want the highest mAH at a voltage exceeding the Vf [forward voltage] of the LEDs.
I didn't see the color, but I'll presume white, which have a Vf between 3 and 4V [assuming a single LED].
So 3V wouldn't be enough.

If you want it to last 20 hours, then either find a LED of sufficent brightness at N current, where N = battery mAH / 20
or
find a battery that will last LED current * 20

However battery voltage for most types of batteries does drop so
for the 1st method use a lower mAH figure
and for the 2nd, the battery mAH will have to be significantly higher than calculated