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Thread: Led output reducing over time

  1. #1

    Default Led output reducing over time

    Ive had two purchases recently start off by blowing me away and then over a few weeks get much dimmer. Like 30 percent.

    Do i have fake 30q batteries. Have they worn out a little at high current or did the led burn in a bit.

    First was xhp50.2 zebralight. Started at 2400lm now looks 1800.

    Then acebeam xt80 gt was looking like 30kish now more like 18k max.

    My older xml2 lights all look similar to the day i got them but all are driven lower.

    Maybe its like when you get your first car and its amazing for a week lol. Anyway id be interested if theres anyone done sphere measurements after a year to test the drop.

  2. #2

    Default Re: Led output reducing over time

    Actually, I would look at the heatsinking of the particular flashlight as the harder you drive it, the hotter the LEDs would get - they are usually rated for between 350mA to 1 Amp from the get-go, however depending on the LEDs themselves, some LEDs like Cree XP-L2 and Nichia 219 - 319 series can easily survive the overcurrent condition providing they're cooled well enough, up to 9 Amps test on Nichia LEDs. Which IMO is insane.

    If the flashlight either don't get hot within 15 minutes, or get hot in a hurry, there's a problem with how it regulate the heat.

    Phosphor coatings are very sensitive to overheating, at over 80 Celsius (~176 Fahrenheit) the permanent efficiency losses start to really sink in. Cree XHP 70 LEDs are generally driven hard in couple flashlights, so there's that. On the other hand, permanent phosphor efficiency losses are really unavoidable, even in modestly used flashlights, as many LEDs are rated for between 5,000 to 20,000 hours before they expire, sometimes resulting in a significant color temperature shifting toward blue. I have ICON flashlight that used to be in the ~6,500 Kelvins, now it's in the 8,000s, all due to the normal wear and tear. (And tint's terrible. LOL Really wish they made the ICON LED pill easily removable so I can replace the aging LED.)

    As for the batteries, maybe but should really not be happening with the brand name batteries, as you mentioned 30Q, Samsung cells are pretty good in general.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  3. #3
    Flashaholic* DIWdiver's Avatar
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    Default Re: Led output reducing over time

    OP said he had FAKE 30q cells.

  4. #4

    Default Re: Led output reducing over time

    Right. Ok.
    ****** Custom filtered blacklight flashlight based on both Nichia NVSU233A-U365 and Convoy S2 flashlight host (Pictured in avatar icon) ******

  5. #5
    Flashaholic* DIWdiver's Avatar
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    Default Re: Led output reducing over time

    Since the two lights are so different, let's discuss them separately. First the Zebralight. I'll discuss the Acebeam in another post.

    You didn't say which model you had, but they only one they offer that claims XHP50.2 is the SC600w Mk IV.

    If the batteries are the problem, then you would notice that the light dims steadily as the batteries discharge. This would be most noticeable right off charge.

    Another possibility is that the LEDs are overheating, or are just garbage LEDs. If they were 'recent' puchases I'd assume the runtime has been on the order of hours or tens of hours. The dies would have to be pretty badly overheating to degrade that rapidly. Do you see any discoloration on the dies?

    The XHP50.2 is rated at a max drive of 3A in the 6V configuration, or 1.5A in the 12V config. Either way that's 18W. To get 2300 lm, you'd have to be pushing 18W and getting 128 lm/W, which the XHP50.2 can't quite do, especially at 80 CRI and 4500K. So to get anything like the claimed 2300 lumens even at the emitter (OTF would be considerably less), you'd have to push the chip quite hard. In a light this size, this is not sustainable, as the light would quickly become too hot to hold.

    They claim PID temperature control, which would reduce the brightness (and thus the heating) as the temperature reaches the maximum. Thus you would expect, if running it on high, that it would come on bright, stay at an even brightness (at least to the eye) for a short time, then rapidly dim quite noticably. I would expect it to take several minutes, perhaps 5 or 10, to dim to a relatively steady level. The body would over the same time warm to a relatively steady temperature.

    Zebralight picked an operating temperature, and claim to have a controller that can maintain it, and give you +/- 5°C control over it. But they don't tell you what that temperature is, or where or how it's measured (hint: it's NOT measured at the die). These are important, as high design temperature would lead to short life, and poor design or assembly could lead to significantly higher than expected die temperatures, and thus significantly less than design life. I also notice they don't publish a design life.

    It just occurred to me, another thing to consider is that if the power level (and thus the light output) is actually limited by die temperature, then then temperature of the environment would have an impact on brightness. The cooler the environment, the more power you can dissipate while maintaining a steady temperature. So maybe the difference you are seeing is seasonal.
    Last edited by DIWdiver; 03-17-2019 at 07:03 PM.

  6. #6
    Flashaholic* DIWdiver's Avatar
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    Default Re: Led output reducing over time

    Okay, let's look at the Acebeam X80-GT.

    Since this uses the same XHP50.2 family, and pushing them fairly hard, it's not likely they are getting more than 130 lm/W. To get 32,500 lm, that's 250W.

    The load on the batteries must be 250W/4 cells, or 62.5W/cell. Even at full charge, 4.0V, that's 15.6A. Relatively discharged, at 3.0V, that would be over 20A. The quality of your cells will have a big impact on the light's ability to achieve this kind of performance.

    I was going to go into a lot more detail, like the previous post, but at this point I think we have the answer.

    Acebeam does recommend that you use only their protected cells. Normally I don't put much stock in these recommendations, but not many protected cells have that high an output, and using unprotected cells in series at this power level is a much greater risk than most lights represent. I would definitely recommend using quality protected cells with at least a 20A rating. I would also do some research on CPF to see if the Acebeam cells live up to their marketing. At $14.40 each (or more), they are relatively high price, but compared to the cost of the light, if they are good cells it might be a good investment.

    It sounds like maybe you are using unprotected cells. If so, please use only quality cells, and be careful to ensure they start and stay well matched.

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