How to compute runtimes for various LEDs/Batts

[jcw122]

Hi everyone,

I'm trying to figure out how to approximate runtimes for various LED and battery combinations. I'm having trouble figuring out what kind of runtimes to expect from various P60 drop-ins.

Example:
Cree XP-G @ 1.4 amps using a 18560....vs. same LED but a 17670

I don't know enough about electricity to figure this out, and I'm unsure where to look for the proper numbers for specific LEDs.

Thanks.

 

[kramer5150]

Approximate run time = (cell capacity / current draw) x.80

(2.4Ah / 1.4A)x.8 = 1.37h

 

[tolkaze]

This should probably be in the battery section, but kramer5150 pretty much summed it up.

If you wanted to get more accuracy, something like this

http://www.kenworster.com/ledcalc/

Would give you a better estimate

 

[red02]

What are some of the accepted values for the different LEDs? I think XPG was 3.2?

 

[ti-force]

This is only a rough estimate, but

Convert to watts, then calculate:

Guessing that your XP-G has a Vf of 3.2v (won't know until you measure), and you say the current is 1.4A (assuming that's at the tail). 3.2v x 1.4A = 4.48W


Now, say you're using an AW 2600mAh 18650. Calculate to WH 3.7v x 2.6Ah = 9.62WH, then divide battery capacity by emitter wattage: 9.62 / 4.48 = 2.15 hours. Reduce this by driver efficiency. Say (roughly) 85%: 2.15 x .85 = 1.83 hours or 1 hr. 50 min.

An AW 1500mAh 17670 would be: 3.7v x 1.5Ah = 5.5WH

5.5 / 4.48 = 1.23 hours. Reduce again: 1.23 x .85 = 1.05 or 1 hr

 

[red02]

How do you account for batteries in series? Just add the mAh(1)+mAh(2), v(1)+v(2)?

Im doing the trueblue mod http://www.candlepowerforums.com/vb/showpost.php?p=1205794&postcount=21, what method can I use for resistors assuming the LED is regulated from a single level driver?

Some really good info here, thanks guys.

 

[kramer5150]

How do you account for batteries in series? Just add the mAh(1)+mAh(2), v(1)+v(2)?

V1+V2 is correct. The Voltage potential of 2xRCR123 cells in series is 8.4V.

However the mAh capacity of the series pair is equal to the mAh capacity of each cell. So a pair of RCR123 cells in series is 500mAh (for example).

 

[ti-force]

How do you account for batteries in series? Just add the mAh(1)+mAh(2), v(1)+v(2)?

Im doing the trueblue mod http://www.candlepowerforums.com/vb/showpost.php?p=1205794&postcount=21, what method can I use for resistors assuming the LED is regulated from a single level driver?

Some really good info here, thanks guys.

Here's some good info to help you find needed resistance:

Click here

 

[jcw122]

Thanks for the help everyone, much appreciated!