Manufacturer error or power consumption vs lumen misunderstanding.

Hi, I'm new to the forum. candlepowerforums has been my "go to" for a long time when researching flashlights. I originally found CPF when researching headlight options for my pickup and have been lurking ever since.

Anyway, I'll jump to the question.

I own a Klarus ST 11, which I think is great general purpose light for most everything, but I am looking for options for distance applications. I started browsing Klarus's website to see what options they had for distance applications, and came across this conundrum. Here are 3 flashlights I was comparing

ST11:
Battery: 1x 18650
Lumens: 900
Candela: 7817 cd
Runtime: 2.3 Hrs

XT11:
Battery: 1x 18650
Lumens: 820
Candela: 8217 cd
Runtime: 3 Hrs

XT30:
Battery: 2x 18650
Lumens: 820
Candela: 29000 cd
Runtime: 1.8 Hrs

All of these lights use the CREE XM-L2 emitter. The XT11 & XT30 us the U2 LED, and I do not see if the ST11 also uses a U2 or not.

If I understand, lumen & candela measurement correctly, lumen output is total light output, and cadela is peak beam intensity. It seems that power consumption should be directly related to lumen output on any particular LED. The ST11 with 900 lumens last for 2.3 hours. Now the XT11 making 820 lumens goes up to 3 hours of run time. This makes sense in my understanding. What confuses me, is how can the XT30 with the same lumen output as the XT11 (820) only last for 1.8 hours. With not one 18650 battery like it's counterpart, but 2 18650 batteries.

Is there something going on here that I am missing , or is this a typo? How can this flashlight consume so much more power with the same LED and light output?

I think that you might be over thinking this? To me it looks like a typo.

A couple things. First the most obvious, some companies like to skew the results for runtime in their favor by bending the rules in regards to advertised runtime. Related is that a light that has perfect flat regulation at 800 OTF lumens will have less runtime to 50% than a light that puts out 900 lumens but has poor regulation, as it only has to put out those 900 lumens for a short period of time before it drops below the output of the other light. There is also the possibility that one light may have really, really bad driver efficiency but this is something most companies have worked out these days.

One possibility is that the XT11 uses a step-down: maybe it's 820 lumens for a few minutes, then reduces output considerably (many manufacturers seem to quote the run time in this scenario as the run time under the maximum lumens produced for just the first few minutes - misleading at best). I honestly know nothing about Klarus lights, so this is just an idea I'm throwing out there.

As luceat said, there is also the possibility that it's simply a typo.

According to Selfbuilt's review of the XT30 the LED is more heavily driven, has more ceiling bounce and more candela than the XT11 (If I read his review correctly). I would expect more light from this flashlight and less runtime.... For me runtime is not an issue (assuming good batteries). If I need more time I bring back-up cells to power the light (whatever it is). I don't own either light --- so I would not bet $$$ on these numbers.... They all look like great flashlights!

You can back into the likely run times. It is very hard to utilize battery energy that doesn't exist!

Figure that output is about 100 lumens per watt. The typical capacity of a 18650 these days is about 3100mAh, so the total energy capacity of the battery system is roughly 3.7volts x 3.1 amp hours or about 11.5 watt hours. Basically 3 hours at 820 lumens (XT11) is impossible. 8.2 watts x 3 hours= 24.6 watt hours, and the battery is only 11.5 watt hours. 2.3 hours at 900 lumens (ST11) is equally impossible. 9 x 2.3 = 20.7 watts hours,from a 11.5 watt hour battery. The 1.8 hours at 820 lumes (XT30)from a pair of 18650's is in line with what the batteries are capable of. 2 x 18650's =23 watt hours, 1.8 hours at 820 lumens is about 15 watt hours, so even with sizeable losses in the driver, that value is within the realm of reality.

This is a good question that has an easy answer. Look at the run-time graphs from Selfbuilt. See the gigantic drop in output after just a few minutes on the XT11 (red line on graph).

The Turbo Mode initial output is only suppose to be a very short but very bright burst. By definition, Turbo Mode output should drop down to High Mode output after a few minutes. If Turbo Mode never drops down in output, then by definition it would simply be High Mode. Manufacturers are allowed to take the ANSI lumen measurement 30 seconds after activation.







Notice how a lot of the lights similar to the XT30 in this run-time graph do not have a drop in output. They don't have a Turbo Mode, they start out in High Mode.





The big drop in output is because the smaller lights cannot dissipate the heat, so it is a thermal management output drop. The bigger lights can dissipate the heat, so it is not necessary to drop the output.

Excellent information. Thank you very much.