What voltage does a NCR18650B have to be at, to DD an XML at 3A.

I have some parts here, and I want to direct drive an XML from a Pana NCR18650B. How low does the voltage on the battery need to be, to direct drive the XML at 3 amps?

Again, this is just for a test, and not for use.

Here are the numbers for the battery NCR18650B I want to use.

At 3 amps draw, it takes 16 minutes and 805 mah to get to 3.60v under load.


368mah to get to 3.50v
297mah to get to 3.40v
337mah to get to 3.30v.

How low does the 3 amp draw voltage have to be, in order to direct drive an XML at 3 to 3.5amps, which is about the upper limit of the heat my setup can take.

The Vf will vary from led to led, even those from the same batch. Temperature also has a big influence……

Sorry :slight_smile:

You normally need around 3,3-3,4V at the emitter in order to give it around 3-3,5A.

You want to be quite exact, because 0,1V too high or too low will give you output well below or above what you are seeking. Best way is to measure amps at the emitter in the setup you are using, then you can check battery voltage once you have reached the desired amps.

Easiest way to direct drive an XM-L emitter at 3-3,5A, is to go with an XM-L2 on copper and green protected NCR18650B fully charged. Peak output is usually around 3,5A (if you do not have much resistance in springs and button), output will drop quite soon.

What is the point of this test? And if its just a test and not for use like you say, why are you asking others for the answer to the test? Just test it yourself in your own test setup. That is the best way to get the correct answer.

Just curious, what is it for, to be able to test XML’s after you reflow them?

You are right, I should just test it myself, except I don’t want to fry this XML, while getting a measurement. I guess the best way I should do it, is to start at a low enough voltage to get a safe reading, and then keep charging the battery a little, and keep on measuring the emitter draw, and keep charging the battery until I reach my 3A emitter draw.

Would some of the ideas in this Insructable be of any use in this project to help prevent frying the emitter?

Because of the variables, the ONLY way to get 3.0A is with a 3A current source (driver). That is why these lights use current regulators, not voltage regulators. I have reaffirmed this myself in one of my own builds.