Fenix L2D Q5 Premium Turbo mode warning?

The instructions for my new Fenix L2D Premium Q5 congratulate me on the wise decision of purchasing this precision flashlight capable of having a light output as high as 180 Lumens in Turbo mode. Then it warns me not to use this mode for more than 10 minutes even though it is capable of running as long as 2.4 hours in this mode. Should I run it for more than 10 minutes, it warns me that I may either damage the flashlight or the batteries.

While I doubt that I would ever need to run my light in Turbo for 2.4 hours, let alone more than 10 minutes, I do worry should I forget and leave the light unattended in Turbo mode, that I will do irreversible damage to the light. Is this warning by Fenix one that is very likely to happen regardless of the ambient temperature? Or is it only going to happen if the flashlight is left on in Turbo in a place where the light can't cool itself, such as in a backpack or a coat pocket?

If the flashlight can be that easily damaged, I wonder how wise my decision was to purchase it.

It is always bad for a high output light to be left in a place "such as in a backpack or a coat pocket" while it is on. Well, sometimes it's also bad for the stuff that is in there with it. These lights can really heat up and cause damage to itself and others.

I believe the warning is their due to the fact the the L2D (I have a non Q5) is relatively small, which limits it's heat sinking ability. On turbo mode the LED will generate quite a bit of heat, dumping it into the body of the light. If you are holding it, your hand will draw some of that heat away, so you could actually run it longer than 10 min without damage. But turn it on and leave it sitting (candle mode for example) that heat will not radiate away easily.

Some lights, like a P60L module, has a has a thermal limiter to drop output if heat builds too fast.

USM0083 said:

I believe the warning is their due to the fact the the L2D (I have a non Q5) is relatively small, which limits it's heat sinking ability. On turbo mode the LED will generate quite a bit of heat, dumping it into the body of the light. If you are holding it, your hand will draw some of that heat away, so you could actually run it longer than 10 min without damage. But turn it on and leave it sitting (candle mode for example) that heat will not radiate away easily.

Some lights, like a P60L module, has a has a thermal limiter to drop output if heat builds too fast.

Click to expand...

I have not seen these warnings elsewhere with Fenix non-Q5 lights, or with other manufacturers using the Q5 LED. Are Fenix lights not heat sinked well enough to handle these new emitters?

Has anyone really damaged their lights using them more than 10 minutes in regular use, not in backpacks or pockets? How were the runtimes in Turbo obtained by reviewers or the manufacturer without harming their lights? They had to run them continuously for more than 10 minutes to determine those times. Did they run them with fans blowing on the lights?

I can't speak authoritatively to your question, but there doesn't seem to be much difference between the high setting and turbo.

I wish they would just go with low, medium & high settings and be done with it. Most folks around here don't really care about SOS, turbo and such.

That being said, I think Fenix lights are well made.

BabyDoc said:

I have not seen these warnings elsewhere with Fenix non-Q5 lights, or with other manufacturers using the Q5 LED. Are Fenix lights not heat sinked well enough to handle these new emitters?

Has anyone really damaged their lights using them more than 10 minutes in regular use, not in backpacks or pockets? How were the runtimes in Turbo obtained by reviewers or the manufacturer without harming their lights? They had to run them continuously for more than 10 minutes to determine those times. Did they run them with fans blowing on the lights?

Click to expand...

With the Fenix, it wasn't until the Q5s came out did they worry about thermal management. I think if you ran your light for extended periods, it won't kill the LED, but drastically shorten it's life. Surefire switch the bezels on the G2L and G3L to aluminum to provide better heatsinking, but it's less of an issue with a P60L because of their modest output and thermal regulation.

Malkoff has this warning for the M60:
Note: This drop-in module was intended to operate at maximum brightness for tactical usage similar to the original setup. Operating the unit for more than 15 continuous minutes in plastic lights may cause damage to the drop-in module. This is not an issue in metal lights.

Most metal bodied lights are fine with high output LEDs. Fenix's with Q5s have that warning simply because of their modest size also restricts their heatsinking ability.

USM0083 said:

With the Fenix, it wasn't until the Q5s came out did they worry about thermal management. ...
Most metal bodied lights are fine with high output LEDs. Fenix's with Q5s have that warning simply because of their modest size also restricts their heatsinking ability.

Click to expand...

Well, perhaps I am missing something with Fenix lights. They apper to be all metal and glass, except for the circuit boards and the housing for the emitter. Have we just reached a critical light output, at 180 lumens or more, where no matter what the housing of the emitter is made of, it just can't handle the heat?

Are there other small lights out there with 180 lumens or more made by other manufacturers, that can be safely run for extended periods of time? In the case of Fenix L2D, the Q5 is an uprgrade to old design. (I believe it is the 4th revision of the original L2P V1, with each revision stepping up the output.) Perhaps new designs made specifically to handle the heat output of the Q5 will allow you to have your cake and eat it, too. At the very least, you would hope they would add a thermal sensor or a timer, to limit the heat buildup and possible damage.

BabyDoc said:

Are there other small lights out there with 180 lumens or more made by other manufacturers, that can be safely run for extended periods of time? In the case of Fenix L2D, the Q5 is an uprgrade to old design. (I believe it is the 4th revision of the original L2P V1, with each revision stepping up the output.) Perhaps new designs made specifically to handle the heat output of the Q5 will allow you to have your cake and eat it, too. At the very least, you would hope they would add a thermal sensor or a timer, to limit the heat buildup and possible damage.

Click to expand...

Olights are similar in size to Fenix's, but appear to be built a little heavier, and there are no operating restrictions that I can see.

I think you're worrying a bit about nothing. Keeping your hand on the light will help act as a heatsink.

Marduke said:

I think you're worrying a bit about nothing. Keeping your hand on the light will help act as a heatsink.

Click to expand...

That's exactly what I am trying to figure out. Why am I worrying about nothing, when Fenix says I should worry if I run my light in Turbo too long.

Furthermore, does it make sense that my hand which is at 98.6F can serve better as a heat sink for a hot flashlight, than ambient room temperature which is less than body temperature? For example, why shouldn't a flashlight in candle position surrounded on all sides by cooler air than hand temperature, be better able to disipate heat than a warmer hand? Furthermore, when doing runtimes on this light, I doubt anyone holds this flashlight for 2 hours to avoid damaging it in Turbo mode. Or did the testers sacrifice their lights to get us the runtimes?

It would be nice to hear from someone, who has safely run their L2D Q5 at highest output for extended periods without hurting it. It also would be nice to know, if other than putting a turbo lit flashlight in an insulated backpack or pocket, that you couldn't damage this light running it in Turbo as long as you wanted. If the latter were true, why didn't Fenix simply say don't use this flashlight in Turbo mode without adequate ventillation or cooling around it? Instead they say, don't use if for more than 10 minutes, limiting the usefulness of the "improvement" they made in this model.

BabyDoc said:

That's exactly what I am trying to figure out. Why am I worrying about nothing, when Fenix says I should worry if I run my light in Turbo too long.

Furthermore, does it make sense that my hand which is at 98.6F can serve better as a heat sink for a hot flashlight, than ambient room temperature which is less than body temperature? For example, why shouldn't a flashlight in candle position surrounded on all sides by cooler air than hand temperature, be better able to disipate heat than a warmer hand? Furthermore, when doing runtimes on this light, I doubt anyone holds this flashlight for 2 hours to avoid damaging it in Turbo mode. Or did the testers sacrifice their lights to get us the runtimes?

It would be nice to hear from someone, who has safely run their L2D Q5 at highest output for extended periods without hurting it. It also would be nice to know, if other than putting a turbo lit flashlight in an insulated backpack or pocket, that you couldn't damage this light running it in Turbo as long as you wanted. If the latter were true, why didn't Fenix simply say don't use this flashlight in Turbo mode without adequate ventillation or cooling around it? Instead they say, don't use if for more than 10 minutes, limiting the usefulness of the "improvement" they made in this model.

Click to expand...

Your body's circulatory system is like a radiator, taking away all the excess heat generated by a high power LED.


Also, running the light for more than 10 minutes in turbo won't necessarily hurt the light, it's just not the best idea. Also, if you read the fine print, it says "...in high ambient temperatures". The testers often have fans cooling their light boxes also. The previous P4 generation had the same warnings, so this isn't just a "Q5 thing".


The reason not to consistently run the light in turbo exclusively for long periods is a decreased life of the LED (which happens to any LED if it's not cooled properly), and it can be hard on your batteries.


Just use it when you need it, there are 3 other modes there if you need them.

Marduke said:

Your body's circulatory system is like a radiator, taking away all the excess heat generated by a high power LED.

Click to expand...

I am not sure if Babydoc is a real doc, but I am. Thinking of the body as a radiator, just because there is blood flowing in a circuit is quite misleading. The epidermis normally has very little actual blood flow to it, and below that are several more layers of skin, then finally a nice layer of adipose tissue. Adipose tissue (fat) is quite a nice insulator, ask any marine mammal. Actually, this has much more to do with fluid dynamics. Air can be thought of as a fluid (remember those PCHEM lectures?), thus a flashlight surrounded by nothing but air will have much better heat dispersion than one grasped in the hand (insulating it). Convecting currents will rise above the flashlight, bringing in more 'cool' air (at least less than 98.6 degrees).

Regarding the original poster. I have a L2D Premium RB100 that I use, and until recently didn't even know it had a warning about turbo mode. I have had it on for 2 hour+ intervals, and replaced the batteries and run it for another 2 hours, all in turbo mode. It seems quite stable, and only feels slightly warm at the 2 hour battery change. However, I use it for mountain biking at night, and usually (unless its a steep hill) average 5 mph or more, no doubt helping to cool it.

If its too hot to hold, turn it down. Otherwise, I would probably run it at 100 lumens, unless you are in motion. Really, it only has to last a year, since if you are like me you will want next years model anyway (more light, runtime etc as LED's continue to improve). Either way, I wouldn't worry.

rdhfreethought said:

I am not sure if Babydoc is a real doc, but I am. Thinking of the body as a radiator, just because there is blood flowing in a circuit is quite misleading. The epidermis normally has very little actual blood flow to it, and below that are several more layers of skin, then finally a nice layer of adipose tissue. Adipose tissue (fat) is quite a nice insulator, ask any marine mammal. Actually, this has much more to do with fluid dynamics. Air can be thought of as a fluid (remember those PCHEM lectures?), thus a flashlight surrounded by nothing but air will have much better heat dispersion than one grasped in the hand (insulating it). Convecting currents will rise above the flashlight, bringing in more 'cool' air (at least less than 98.6 degrees).

Regarding the original poster. I have a L2D Premium RB100 that I use, and until recently didn't even know it had a warning about turbo mode. I have had it on for 2 hour+ intervals, and replaced the batteries and run it for another 2 hours, all in turbo mode. It seems quite stable, and only feels slightly warm at the 2 hour battery change. However, I use it for mountain biking at night, and usually (unless its a steep hill) average 5 mph or more, no doubt helping to cool it.

If its too hot to hold, turn it down. Otherwise, I would probably run it at 100 lumens, unless you are in motion. Really, it only has to last a year, since if you are like me you will want next years model anyway (more light, runtime etc as LED's continue to improve). Either way, I wouldn't worry.

Click to expand...

You may be a doctor, but you obviously have a lot to learn about flashlights and thermodynamics. Air is a very poor thermal conductor, while your hand absorbs the heat much more readily.

Your hand can absorb approximately 24 times more heat than air when surrounding a flashlight. This is why air is generally used as the primary insulator material in both natural and man-made insulators.

Marduke said:

You may be a doctor, but you obviously have a lot to learn about flashlights and thermodynamics. Air is a very poor thermal conductor, while your hand absorbs the heat much more readily.

Your hand can absorb approximately 24 times more heat than air when surrounding a flashlight. This is why air is generally used as the primary insulator material in both natural and man-made insulators.

Click to expand...

You may be true on several counts. (I do have a lot to learn about flashlights and thermodynamics) I am curious where you get hand having 24x thermal transfer than air. Any data on that? Air is obviously a good insulator, as everyone knows that trapped air in the form of styrofoam, house insulation and double paned windows, is quite a good insulator. While not as good as water (back to your radiator example), air-cooled engines perform quite well when there is air flow.

Show me data that demonstrates 24x better heat transfer into a persons hand than air, and I will be astonished and thankful for the education. Quoting thermal conductivity of air vs ... flesh(!!)... doesn't impress me much. Although, I'd love to know where the themal conductivity of a hand data comes from!!

Oh shoot, maybe I'll just look it up:

Human skin: 0.293-0.322 (W/mK)
Water: 0.627 (W/mK)
Air: 0.025 (W/mK)

http://users.ece.utexas.edu/~valvano/research/Thermal.pdf

ok, so that is more like 10x, but it still sounds surprising. But thats static thermal conductivity, which is why Marduke suggested I learn more thermodynamics. Maybe someone already learned it for me? Another way to look at it would be to calculate the delta T of the hand as it absorbs that heat, and compare that to effectively infinite cooling capacity of the atmosphere, in regards to cooling the flashlight.

What we need is for someone to hold a 100w bulb in his hand, and compare that temperature to one in the air after 5 minutes. Any takers? Marduke? Hehe

rdhfreethought said:

You may be true on several counts. (I do have a lot to learn about flashlights and thermodynamics) I am curious where you get hand having 24x thermal transfer than air. Any data on that? Air is obviously a good insulator, as everyone knows that trapped air in the form of styrofoam, house insulation and double paned windows, is quite a good insulator. While not as good as water (back to your radiator example), air-cooled engines perform quite well when there is air flow.

Show me data that demonstrates 24x better heat transfer into a persons hand than air, and I will be astonished and thankful for the education. Quoting thermal conductivity of air vs ... flesh(!!)... doesn't impress me much. Although, I'd love to know where the themal conductivity of a hand data comes from!!

Oh shoot, maybe I'll just look it up:

Human skin: 0.293-0.322 (W/mK)
Water: 0.627 (W/mK)
Air: 0.025 (W/mK)

http://users.ece.utexas.edu/~valvano/research/Thermal.pdf

ok, so that is more like 10x, but it still sounds surprising. But thats static thermal conductivity, which is why Marduke suggested I learn more thermodynamics. Maybe someone already learned it for me? Another way to look at it would be to calculate the delta T of the hand as it absorbs that heat, and compare that to effectively infinite cooling capacity of the atmosphere, in regards to cooling the flashlight.

What we need is for someone to hold a 100w bulb in his hand, and compare that temperature to one in the air after 5 minutes. Any takers? Marduke? Hehe

Click to expand...

Just below the thin layer of skin is mostly water, so it's really somewhere between 10x and 24x

People have taken both qualitative and quantitative notes on the temperature of a high power LED flashlight left to run unattended, and held in a hand. Lights held firmly in a human hand are SIGNIFICANTLY cooler to the touch, and in actual temperature with all other factors held constant (ie. battery type, brightness setting, flashlight, ambient temperature).

You have to remember, LED's and incandescents product heat very differently also. Incans radiate heat outwards, and the heat doesn't bother them in the least. LED's are like a computer chip, and generate heat from the circuit board they are formed from. They don't radiate heat outwards, but rather generate it locally, and the heat must be heatsinked away out the rear of the emitter, into the body of the light. Heat buildup for LED's actually shortens their life in the long run. They are just like regular computer chips, and prefer to be ran as cool as possible.

Since most of us don't want to put active cooling system in already increasingly complicated flashlights (although there are a very few LED lights with active cooling systems), we rely on passive cooling. This means holding the light, having a proper sized aluminum or copper heatsink, having a direct thermal path from the emitter to the outside of the light, or relying on passing airflow (such as a bike light).

I have a P3D-CE and in the instructions it is telling me to use the Turbo mode sparingly.
The light is getting pretty hot after 2-3 minutes of continuous use.
I accidentally turned on the light at work (it was on Turbo...) and after a while my pants pocket was burning hot. I think it was on for at least 10 minutes. The output look the same. I was afraid I fried the LED but it is ok.

rdhfreethought said:

I am not sure if Babydoc is a real doc, but I am. .......Regarding the original poster. I have a L2D Premium RB100 that I use, and until recently didn't even know it had a warning about turbo mode. I have had it on for 2 hour+ intervals, and replaced the batteries and run it for another 2 hours, all in turbo mode. It seems quite stable, and only feels slightly warm at the 2 hour battery change. ). Either way, I wouldn't worry.

Click to expand...

Yes, I am a real doc, too. I am a pediatrician and know a lot more about babies than I do about flashlights and thermodynamics. Yet somehow, my knowledge of babies makes me see a similarity between a hot flashlight and a hot baby. With regard to babies, who have fevers, the best way to drop that temperature, besides giving Ibuprofen or Tylenol, is to undress the baby. The babies clothing insulates and helps retain the heat. Undressing the child allows the child to disipate his heat to the room which is certainly less than his body temperature. I can imagine that a flashlight is like a warm baby, and will cool quicker if it is aired, rather than bundled in somebody's hand, even though that hand is of a lower temperature than the flashlight. (Having said all this, the comparison of a flashlight to a baby is flawed. The baby disipates his heat not only directly to the ambient room air, but also as a result of giving up heat through evaporation with the formation of sweat, or by sponging down the baby and letting that water evaporate. )
Anyway, I am glad to hear that you have tortured your flashlight and it still works fine. Will it work for 50,000 hours or has its life been shortened. At my age of 65, I am not certain my body will work for another 50,000 hours, let alone worry about a flashlight lasting that long. It's just fun to obsess over this. Why I do is a question for another kind of doctor. Now let me call my shrink.

babydoc, i have left my p2d-ce q5 on turbo for an hour and no issues with it whatsoever.

My general rule which I've set for myself is that if a metal light is too hot to touch comfortably, turn it down or turn it off. Other than that, I say run with it...and I've never fried an LED

Patriot36 said:

My general rule which I've set for myself is that if a metal light is too hot to touch comfortably, turn it down or turn it off. Other than that, I say run with it...and I've never fried an LED

Click to expand...

that sums it up!

i was out for about half an hour with my L2D on turbo constantly the other night, it was a cold night and as such the light didnt even get warm.

leave it in your pocket and turn it on it will get hot quick and eventually fry itself hence the disclaimer.