Direct drive Cree XM-L questions

I was just planning on making a homemade led lantern and have been messing around with the direct driving the xm-l led. I used 4 eneloop nimh batteries totaling 5.6 volts. I measured 3.3 amps of current going to the led. Is this too much for the xm-l led to handle? I know that the xm-l is rated to handle 3 amps of current. Also what is the led Vf? Can anyone explain that to me and how it would effect my direct drive setup? Also the led is mounted on a cpu heat sink so I am pretty sure that should be adequate cooling even though I have not done any long term test.

Well, 4 Eneloops powering an XM-L directly means that the LED is getting 4.8V, but, the battery voltage would sag under load down to a safe level, as you are seeing. I've seen an XM-L turn blue (white LEDs turning blue means that you're seconds away from frying it) from 3 NiMH cells, but ran fine on 3 alkaline batteries due to the fact that alkalines have a high internal resistance and that limits the current. Is 3.3A safe on an XM-L? Probably. As long as you don't see blue light you're fine, but, keep in mind that you are over driving the LED and that you probably will not get the rated lifetime out of it as a result. Then again, the rated lifetime of that LED is far longer than most of us will use up in our lifetimes so have fun with it.

Vf (forward Voltage) is the voltage of the emitter at a given current. As you increase the current the forward voltage rises. In a direct-drive setup, the voltage sag of the batteries under current and the Vf rise with current reach an equilibrium and you end up with what you end up with. With your DD setup, measure the voltage at the LED when it is running at 3.3A and you get the Vf of the emitter at 3.3A. Remove one cell and try it again and you'll get a Vf of that emitter at whatever resulting current you get from it.

Thanks for helping in clarifying what Vf meant. I was under the impresion that if Vf for a xm-l was 3.3 volts at 3 amp then if my power source/batteries supplied 3.3 volts then the led would be driven at 3 amps. I measured the current again when the batteries were half discharged and I got 2.5 amps. I also measured the voltage of the led at 2.5 amps and got 3.1 volts. So the Vf of my led would be 3.1 volts at 2.5 amps right?

I think your thread will do better in H&M, so I'm moving it there now...

daguy80 said:

Thanks for helping in clarifying what Vf meant. I was under the impresion that if Vf for a xm-l was 3.3 volts at 3 amp then if my power source/batteries supplied 3.3 volts then the led would be driven at 3 amps. I measured the current again when the batteries were half discharged and I got 2.5 amps. I also measured the voltage of the led at 2.5 amps and got 3.1 volts. So the Vf of my led would be 3.1 volts at 2.5 amps right?

Click to expand...

That sounds about right. Looking at the chart that Justin Case posted, your numbers don't line up, but, keep in mind that all LEDs are manufactured with some variance from unit to unit and that the chart is a guideline. Your particular LED looks like it has a lower Vf than what Cree specs. Likewise, there are XM-Ls out there with a higher Vf than what appears on that chart. The only way to know for sure what you have is to measure it like you did.

fV goes down by -3mV for each °C.

If you are interested i can measure and create a discharge diagram for the eneloop @ 3 amps.
At the moment i just got one @ 10 amps: Klick

samotronta05 said:

If you are interested i can measure and create a discharge diagram for the eneloop @ 3 amps.
At the moment i just got one @ 10 amps: Klick

Click to expand...

Yes I would be very interested in that. Can you also do a discharge diagram for 4 eneloops at 3 amps also. Thanks.

daguy80, SilverFox's NiMH Battery Shoot Out thread is an excellent resource for investigating the voltage drop under various loads of many NiMH cell brands/models. Below is his graph for the AA Sanyo Eneloops:


I've personally instaflashed an XM-L with only 3 NiMH cells hot off the charger. If you're measuring only 3.3A with four Eneloop AAs, then you must have a ton of just the right resistance in your build.

Discharge diagram @3A: click
I had only one Eneloop already soldered to 4mm gold connectors.
But since all important values are included in the diagram, it's no problem to draw your conclusions for 4 cells.