daq
Newly Enlightened
I'm considering doing direct drive of 3x Cree XR-E LEDs in series with 8x AA NiMH batteries. Is this reasonable?
How I came to think this might work...
1) Looking at SilverFox's NiMH Shootout https://www.candlepowerforums.com/threads/79302 it seems that that at 1 amp, the voltage on a NiMH AA drops from 1.4v to ~1.25 in 15 minutes. 75 minutes later it drops to ~1.2 volts.
2) Looking at Newbie's Production CREE XR-E Testing http://candlepowerforums.com/vb/showthread.php?t=138503 the Vf vs current chart shows Vf increasing from ~3.2V at 500 mA to almost 3.4V at 1000 mA. My understanding is that Vf varies widely on the XR-E (3.0 to 3.5) but I'm assuming that if I have 3 in series, I can assume they "average out" to about 3.3V at ~800 mA.
So... 8x 1.25V = 10V battery =~ 3x 3.3Vf. I'd be using Artic Alumina to attach the LED to an aluminum tube similar to what Nightrider did http://candlepowerforums.com/vb/showthread.php?t=142791 so the heat sinking should at least be "pretty good". Maybe add a 1/2 Ohm resister just for good luck? (This would also help me determine the current)
I don't mind if the LEDs "only" last ~100 hours under these conditions with ~90% lumen maintenance since I'm sure I'll want to install new LEDs in a year or two.
The alternative is to use the awesome looking bFlex http://www.taskled.com/bflex.html with a 14.8V Li-Ion pack http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=1073. I'm a little leary of Li-Ion, but running at approximately 700 mA discharge current probably isn't in the "danger zone"
Any advice on either the NiMH idea or Li-Ion alternative is appreciated.
Thanks,
Dale
How I came to think this might work...
1) Looking at SilverFox's NiMH Shootout https://www.candlepowerforums.com/threads/79302 it seems that that at 1 amp, the voltage on a NiMH AA drops from 1.4v to ~1.25 in 15 minutes. 75 minutes later it drops to ~1.2 volts.
2) Looking at Newbie's Production CREE XR-E Testing http://candlepowerforums.com/vb/showthread.php?t=138503 the Vf vs current chart shows Vf increasing from ~3.2V at 500 mA to almost 3.4V at 1000 mA. My understanding is that Vf varies widely on the XR-E (3.0 to 3.5) but I'm assuming that if I have 3 in series, I can assume they "average out" to about 3.3V at ~800 mA.
So... 8x 1.25V = 10V battery =~ 3x 3.3Vf. I'd be using Artic Alumina to attach the LED to an aluminum tube similar to what Nightrider did http://candlepowerforums.com/vb/showthread.php?t=142791 so the heat sinking should at least be "pretty good". Maybe add a 1/2 Ohm resister just for good luck? (This would also help me determine the current)
I don't mind if the LEDs "only" last ~100 hours under these conditions with ~90% lumen maintenance since I'm sure I'll want to install new LEDs in a year or two.
The alternative is to use the awesome looking bFlex http://www.taskled.com/bflex.html with a 14.8V Li-Ion pack http://www.batteryspace.com/index.asp?PageAction=VIEWCATS&Category=1073. I'm a little leary of Li-Ion, but running at approximately 700 mA discharge current probably isn't in the "danger zone"
Any advice on either the NiMH idea or Li-Ion alternative is appreciated.
Thanks,
Dale