Electronics, Lighting, & Gadgets - Calculating runtime?

Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.

PDA

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