# Electronics, Lighting, & Gadgets - Calculating runtime?

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View Full Version : Calculating runtime?

BikeManDan
02-23-08, 12:35 AM
I don't know why I have so many problems with these calculations but I cant get my numbers straight

So say that I want to run 3 Cree LEDs at 350mA from a 14.8V 4.4AHr, what kind of run time is that?

Appreciate the help

znomit
02-23-08, 12:56 AM
Roughly 15 to 16 hrs:

Method 1
LEDs are around 10.2v total.
14.8v/10.2v = 1.45 * more volts than needed
4.4Ah/0.35A = 12.6 hours
12.5 * 1.48 = 18.2hours

Minus 20% for driver losses, around 15 hrs.

Method 2
At 350mA a led is burning around 1.1w... total 3.3W
Your battery has 14.8V * 4.4Ah = 65Wh(watt hrs).
65Wh/3.3W = 19.7h
Again minus 20% for driver losses = 16hrs.

BikeManDan
02-23-08, 03:02 AM
I appreciate you taking the time to explain something so simple. I think I got it now, thanks

Seems like I should step it up to 750mA

3.7V per LED * 3 = 11.1V driven
4400mAh / 750mA = 5.87hr
5.87 * (14.8V / 11.1V ) = 7.82hr
15% loss = 6.65hr

Not bad at all. Can finally put my halogens to rest