LED brightness question


Newly Enlightened
Apr 14, 2012
I'm sort of new to these high powered LED flashlights and have a question about total LM output. If the flashlights have a driver board in them that I think regulates the output current to the emitters then why does the total output vary when you go from 2 cells to a 3 cell battery. I just purchased one of those TrustFire 9XT6 flashlights to add to my collection just to see if it was any good at all for the price and when using 2 26650 cells it is pretty dim but when going to 3 26650 cells it must be at least 20 times brighter.


Apr 15, 2010
Brisbane, Australia
In short, it shouldn't; unless for some reason two cells in series aren't capable of supplying the current required, then there should be no difference in brightness whatesoever when you go to three cells. Only difference should be longer runtime and probably more heat generated in the driver.
I just had a brief search for that torch, and one page said it was regulated at 8.5Amps :thinking: No idea how that's supposed to be deployed across 9x XM-Ls, from a series source of 7.4V or 11.1V, it's morning and my logic drive isn't engaging. Maybe there's some layout that does make sense and can support your findings, and still be a true constant current source, but I can't think of anything offhand.


Feb 10, 2014
I don't know why but just today I was reading up on this torch, and it has real potential if the driver was upgraded.

But anyways this is a known problem that was highlet in a review on this website, (Sort I can't rember who did it but he had some fancy beam shape grids and was a very good review)

He ran this light through his light sphere to find on two cells its Max output is only <250lm but on three cell it jumps up to its full power of 3000+ lumens, (Not the rated 9500) which it would never be capable of but with an upgraded driver might easily be able to boost the current and get over 4000 lumens easily.

So its not a fualty light it is just some odd quirk.

Also to note is 3*26650 can be a tight squeeze in this light so might be worth cheakcing that so you don't damage your battery's....aka don't fuoll tighten the end cap.

Personly I think the SRK is the best LM/£$¢€¥ as a lot of these multi XML lights seem to never be driven very hard, so I think the SRK 3*XML is the sweetspot, I think I will likely soon pick one up maby.

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