how to power 4 X-REs from li-ion batteries

[GTRio]

I am building a light using 4 CREE X-REs and want to power them at 1.4 amps each from a Li-Ion battery pack. The pack voltage should be high as possible for the largest power density, I want about 4 hours burn time.

I bought the DX 1.4 amp AMC7135 1400mA Regulated Circuit Board and planed to put it in series with all the LEDs and 4 Li-Ion cells but then I realized the max input voltage is only 4.5V and Vf of the led is around 3.5, so in this configuration the driver would be faced with a much higher voltage than it is rated for.

I am not worried as much about cost as I am building a robust light, can someone please suggest the best driver and configuration to drive 4 XREs from LiION cells?

Thanks,
Brandon

 

[Wiggle]

You could get 3 more of these and run a driver to each XR-E. Run your 4 Li-Ions in parallel and feed all 4 drivers also in parallel.

...Actually you may be able to get away with fewer drivers, there is a trick involving using an LED on the input side as well to take advantage of currents being balanced I think.

Ok found the picture, rehosting it:

I think if you add one more Li-Ion to this example and add one more XR-E before the regulator it could work. This works by reducing the voltage seen by the regulator as each XR-E will roughly balance off the inline voltage of a Li-Ion. So the regulator sees more like 4V, rather than around 12V for 3-Li Ions. And those 3 XR-Es before the regulator are still pulling the proper currents. Oh and you have the 4 x 7135 driver instead, but it should still work.

I'd wait to see if someone else can confirm this first.

 

[dchao]

I would recommend LuxDrive's PowerPuck (1A version) from LED supply.

http://ledsupply.com/powerpuck.php

You'll find the data sheet on the web site.

 

[GTRio]

Thanks for the reply Wiggle, what you are describing is exactly what I was trying to describe. I believe that if the Vf of the LEDs is 3.5V, and the Li-Ions are fully charged, at 4.2V? then the driver would actually be seeing 4X4.2-3X3.5=6.3V, too much since it can only handle 4.5V.

Or am I wrong?

 

[Wiggle]

Thanks for the reply Wiggle, what you are describing is exactly what I was trying to describe. I believe that if the Vf of the LEDs is 3.5V, and the Li-Ions are fully charged, at 4.2V? then the driver would actually be seeing 4X4.2-3X3.5=6.3V, too much since it can only handle 4.5V.

Or am I wrong?

I had 6V somewhere in my head for 7135 for some reason. I don't think 6.3V would be exactly a disaster.

Edit:Just confirmed 6V for AMC 7135. Also, Vf will be higher at 1400mA, it's already 3.7 at 1000mA on avg, probably close to 4V at 1400mA.

 

[Justin Case]

The AMC7135 recommended limit is 6V. The absolute max spec is 7V.

 

[GTRio]

If the input is 6v then cool, I got 4.5V from the DX description.

Vf on these R2 X-REs is only 3.5V at 1.4A, I measured it myself.

I will try building it with these drivers and see what happens,

thx.