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  1. #1
    a.k.a. BicycleKicks
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    LED w/ Lith Ion... light dims slightly during ride?

    I have Dinotte LED lights (which I like a lot) with the Lith Ion batteries and I'm getting a sense that the light dims slightly after being on a while (long before the battery is anywhere near being discharged). It also seems that if I stop for a bit and turn the lights off, then start them after a few minutes they're back to the original power and the slight fading starts all over again. This is very slight, not some dramatic change like 50% power or anything... I would guess more like a 10% drop in power.

    For the record, I have never discharged the batteries or even come close to the time limit. Also for the record, this isn't bad enough to make me not like the lights or think something is wrong with them... I am just wondering if the batteries sort of loose their "oomph" after a while.

    This may seem silly because it could just be my eyes adjusting to the light over time, but I really don't think that's the issue.

    Has anyone else noticed this? Is it normal and expected?

    FWIW, I also have an HID Niterider and I do NOT get the same sense of the light dimming over time.
    I read that on the internet.

  2. #2
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    Maybe termo protection or overheating? How hot are lights when start to dimm?

  3. #3
    a.k.a. BicycleKicks
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    not thermal protection

    Quote Originally Posted by frozen
    Maybe termo protection or overheating? How hot are lights when start to dimm?
    I had that issue previously on warm nights, and this is not a thermal protection issue. The dimming is very slight and is sort of only "perceived" at this point.
    I read that on the internet.

  4. #4
    Experienced drunk cyclist
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    Sounds like the LED die warming up in use and output dropping accordingly. LED data sheets show graphs of relative output vs. die temperature. A 10% drop is fairly typical for a temperature increase you experience in normal use.

    Cloggy

  5. #5
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    Quote Originally Posted by 40hills
    ...This may seem silly because it could just be my eyes adjusting to the light over time, but I really don't think that's the issue.

    Has anyone else noticed this? Is it normal and expected?

    FWIW, I also have an HID Niterider and I do NOT get the same sense of the light dimming over time.
    I think it's just your eyes getting used to the bright output. Very bright LED's certainly can effect night vision especially if there is a lot of bright side spill. In order to be sure you could do a run test and take pictures with a camera over the course of the test. The only problem with doing this is that some cameras are not that reliable.. ...still, unless you have a light meter it is probably the only way to know for sure.

  6. #6
    aka RossC
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    Quote Originally Posted by Cat-man-do
    I think it's just your eyes getting used to the bright output. .

    Quote Originally Posted by Mr CLOGGY
    Sounds like the LED die warming up in use and output dropping accordingly
    I would suggest a little from column A and a little from column B

  7. #7
    a.k.a. BicycleKicks
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    interesting!

    Quote Originally Posted by CloggyAbroad
    Sounds like the LED die warming up in use and output dropping accordingly. LED data sheets show graphs of relative output vs. die temperature. A 10% drop is fairly typical for a temperature increase you experience in normal use.

    Cloggy

    That is very interesting! If that is indeed what's happening, then I'm not seeing things, there really is a small drop in output. And this makes total sense since it happens over time as the lights warm up.

    Do you have any insight into HID's and whether they have the same issue? My take is that they don't.

    FWIW, I still like the LED technology better.
    I read that on the internet.

  8. #8
    a.k.a. BicycleKicks
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    Since my original post, I upgraded to 2x400L lights with the 4-cell battery for each. I still am noticing fade during a 1-1.5 hr ride. A riding buddy also has these lights and agrees that they dim slightly. We've been riding in cold weather (freezing) so I don't see how this could be a heat issue. I think the batteries lose some oomph... I will compare side to side with my HID tonight and see if I can tell a difference over time.
    I read that on the internet.

  9. #9
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    It doesn't matter if it's cold or not. There is always thermal resistance between the LED die, the slug in the LED package, and the case of the light itself. The LED die will always get hotter after it's been on for a few minutes until it reaches a steady state temperature. That steady state temp will be a little less in cold weather, but not enough to prevent slight dimming.

    HID lights do not show the same behavior. It's unique to LED technology.

  10. #10
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    Quote Originally Posted by 40hills
    Since my original post, I upgraded to 2x400L lights with the 4-cell battery for each. I still am noticing fade during a 1-1.5 hr ride. A riding buddy also has these lights and agrees that they dim slightly. We've been riding in cold weather (freezing) so I don't see how this could be a heat issue. I think the batteries lose some oomph... I will compare side to side with my HID tonight and see if I can tell a difference over time.
    In addition to what MtbMac said, this might also be caused by colder weather. Li-ion batteries don't like really cold weather. I run Dinotte lights as well and when the weather is cold the batteries don't act the same as they do in warmer weather. More than likely the cold increases the internal resistance of the battery which might change the voltage drop across the leds. This could explain why my batteries trigger the low voltage warning much faster when things are cold. I remember reading a while back that when you store Li-ion cells in a refrigerator it was recommended that you let the cells warm up before using. Supposedly using the cells before letting them warm was not a good idea. According to the Battery University website ( http://www.batteryuniversity.com/partone-15.htm ) Li-ion batteries still function well down to -20 degrees C. ( -4'F ). Regardless, this doesn't mean that the discharge characteristics of the battery don't change with temperature. In fact I would tend to think it means the opposite. Since I'm not an Electrical Engineer this is just my own theory. I have no real way to know if this is true but I definitely get shorter run times when I run in cold weather. Personally though, I've never noticed a diminished output with my Dinotte. Doesn't mean it doesn't happen, I just might not have ever noticed.
    Last edited by Cat-man-do; 01-15-2010 at 01:44 AM.

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