View Single Post
  #2  
Old October 2nd 18, 02:29 AM posted to rec.bicycles.tech
Jeff Liebermann
external usenet poster
 
Posts: 4,018
Default SIX thousand and FIVE hundred lumens !!!!!!!!!!

On Mon, 1 Oct 2018 15:14:18 -0700 (PDT), "
wrote:

http://reviews.mtbr.com/magicshine-l...2018-interbike


A max actual output of 6500 lumens is cranked out by 3x CREE XHP 50.2
and 2x XM-L2 LEDs. Like the Eagle series MTB headlights, Monteer 6500
is designed with multiple beam patterns modes, the XHP50.2 will put
out a max of 5000 lumens of flood light, while the bottom row of XM-L2s
shoots into the distance with up to 1500 lumens."


Only $350.
https://magicshine.us/product/monteer-6500-mtb-headlight/
"Six cell battery pack uses super high capacity 18650 cells
for a 10500 mAh total."

Ok, let's do the math.

Cree XHP 50.2 LED specs:
https://www.cree.com/led-components/products/xlamp-leds-arrays/xlamp-xhp50-2
https://www.cree.com/led-components/media/documents/ds-XHP502.pdf
The LED can be wired for either 6 or 12V.

If I use an initial voltage of the cells at 3.85V, 3 cells = 11.6V,
which is close to the maximum operating voltage and the end of the
graph on Pg 8. At 11.6V, it draws 1.5 amps per LED and produces 180%
of rated output (Pg 9) or:
1.8 * 1000 lumens @85C = 1800 lumens
Three of these LED's will produce a total of 5,400 lumens. Close
enough to the spec methinks.

Cree XM-L2 LED specs:
https://www.cree.com/led-components/products/xlamp-leds-discrete/xlamp-xm-l2
https://www.cree.com/led-components/media/documents/XLampXML2.pdf
My guess(tm) is that these are running on one cell with a series
current regulator. At 3.85V/cell, the current could be set to
anywhere. So, doing this backwards, in order to produce 750 lumens
per LED, each LED would need to draw:
750 / 280 = 268% of rated output
From the graph on Pg 6, that requires 2.3 amps, which is getting a bit
close to the absolute max of 3.0A/LED. Total drain for both XM-L2
LED's is 4.6 amps.

Total current with all the LED's on is:
4.5A = 4.6A = 9.1 amps
Total dissipation is:
(11.6v * 4.5A) + (3.85v * 4.6A) = 71 watts

Let's see how long it will run at full power.
"Burn time ranges from 1.2H to 82H depending on modes"
I'll assume the best 2200 ma-hr cells. That would be:
3.85v * 2.2amp-hrs * 6 cells = 50.8 watt-hrs
which means the battery will last:
71 watts / 50.8 watt-hrs = 1.4 hrs
That also works as advertised.

The LED's can (probably) deliver the specified 6500 lumens total, but
the 71 watts of heat produced by the LED's will need to be radiated or
conducted somewhere. I don't see any cooling system on the package.
Building it out of mostly aluminum doesn't work without some kind of
air flow:
"To maximize heat dissipating potential, light housing has
been redesigned with hard anodized aluminum which covers
almost the entire light body."
Swell, no fins to increase surface area.

Let's see what the package can do as a heat sink.
Latent heat for aluminum is 0.900 Joules/gm-K.
The light weighs 142 grams, which I'll assume is mostly aluminum.
The light dissipates 71 watts with all the LED's turned on.
I would guess that 75C would be uncomfortably warm for both the
electronics and the bicycle rider. That's a temp rise of 50C (50K).
Joules = Watts * seconds = 71 watts * seconds. Therefo
0.900 = 71 * seconds / (142 * 50)
Time(sec) = 900 seconds = 15 min
Not too horrible. One has 15 minutes of full brightness lighting, in
still air, before the LED's burn your hand or cause a thermal
shutdown.

When there's air flow, there will be more cooling and therefore take
longer to get hot.
The dimensions are 72 * 42 * 44 mm. Surface area is roughly:
(2 * (72*42/2)) + (2 * (42*44)) + (44 * 72)
3,000 + 3,700 + 3,200 = 9,900 mm^2
I'll finish this later as I need to do something else right now.

In my never humble opinion, the specs are quite real, but I have my
doubts about the ability the aluminum heat sink to remove the heat
produced. It's going to get very hot running at full power.

I bet it even meets or exceeds that German bicycle light standard.


Probably true because StVZO only sets the minimum requirements.
However if the spec has a maximum output, so as not to blind oncoming
traffic, it would probably fail. There was a proposal to limit
brigtness to 2.0 lux at 10 meters at eye level to reduce blinding
oncoming drivers and riders, but I don't know if that was added.
A bit of StVZO analysis:
https://swhs.home.xs4all.nl/fiets/tests/verlichting_analyse/verkeersregels/de_stvzo/index_en.html


--
Jeff Liebermann
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Ads
 

Home - Home - Home - Home - Home