Thermal transfer is going to be related to the cross section of the path, which is tiny in the thin part of the tube going down to the fins, but the pill is good sized so my guess is that most heat transfer is through the base of the pill near the fins.
I don’t see a practical way to keep the zoom functioning decently and transfer much heat to it.
OTOH high output for an extended time isn’t going to work with a SK68 or any small light, simple heat vs surface area.
In this thread Review: TrustFire Z8 - XM-L Flood-to-Throw there is discussion of thermal droop in the Z8, but more likely it was battery voltage droop. For a brighter SK68, it seems one could follow the Z8’s example by using an under driven XM-L. That reduces the throw but the larger led die won’t get as hog as as smaller die at the same power would, even though the star, pill and light do. Also, the efficiency is better. The cooling of the SK is definitely better than that of the Z8.
Why not jump to the XM-L2, more efficiency isn’t it? My first attempt will be with a XP-G2, then maybe XM-L2 if that doesn’t seem to do the trick.
Yes, I assume the XM-L2 is better. From the spec sheets it is, but I tend to always start with the cheapest possibility.
Sounds like the pill in a SK98 is the same size as the SK68, maybe a drop in mod for higher output and XM-L? SK98 clones start about $12 shipped on ebay, worth it for the solder challenged.
No swap, 98 is ~4mm wider tube. 20mm star (and probably bigger driver) instead of 16mm. Mine has hollow pill like the 68, but not all do. The battery tube takes 18mm instead of 14mm , also now wondering if the driver works with single 1.5v (and spacers)?.
The lenses on my 68/98 are close to same size, but they fit in the bezel differently - the 98 lens lip fits up into a recess in the bezel instead of being full width to the body of the tube.
I now wonder if this is a design difference between the narrow and wide bezels variants we see among the 68 clones (also for 98s?). And if this limits/enhances throw between them. I think a much wider lens would fit the 98 bezel if it was reamed out wider …
EDIT: Yes, my UF-SK98 lens fits in the 68 but the bezel of the 68 is too fat to let it seat completely. The beam focuses to the point where round becomes a smaller square with rounded corners, but the die shape is not visible. The 68 lens also easily fits in the 98 bezel. The major 3 dimensions are roughly the same, but my 98 has a wider dome and the lip is narrower; the lip looks like it was sanded down to create a press fit for the recess in the bezel.
Pic below compares SK68 (XPG2), SK98(XML), & ZY-603(XML); sorry, the body tube of the ZY is upside down relative to the SKs. The ZY already has red fins, so now it needs a driver to match. 
I have a Raysoon TD398 which is an SK98 clone or 18650 size 68 clone. Everything is scaled up about in proportion. No part would fit.
Here http://www.aliexpress.com/item/Upgrade-800Lumens-9W-CREE-XM-L-T6-Mini-LED-Torch-Focus-Zoom-Flashlight-Free-Shipping/997633297.html is one with the LM-L already in it.
Here’s a homemade AMC driver for the SK68. It has Hi/Med/”firefly” and low battery detection. The next revision will probably also include thermal protection. The XR-E is getting a bit too warm at 700 mA.
Looks very neat. Of course the production lights can’t have low battery detection because they have to work on worn out alkalines.
I bought one with similar description off ebay and it was a garden variety XR-E, but once brought to the sellers attention, via three emails quoting their listing it was a free one. I think often the seller isn’t the one who makes the listing, or knows what it says in English. That and the people doing the listings tend to copy old listings.
No worries about alkalines with protection in the driver board, just never use them and run cheap unprotected LiOns.
That custom driver board looks really nice, sounds like a great option to try.
Speaking of trying, I “might” give the XM-L sk68 a shot for $10 not much to risk, and if its not as described it says full refund. I’ve never purchased via alibaba though.
Can the low battery detection be smart, as in between low Lion 3v and high NimH 1.6v no turn on, but below 1.6v works?
The listing above says “Upgrade”. So it may be intended as written. I bought a Sipik 58 with an XR-E and three modes, so before sale upgrades do happen.
I messaged and the seller and got the reply
“Jenny Zeng:
18:29 Sep 23,2013
it is ok, we change it to XM-T6 XM-L U2 and XM-L U3”
Also, there is an image that says both SK68 and Cree XML T6 on it.
> Here’s a homemade AMC driver for the SK68. It has Hi/Med/“firefly” and low battery detection.
Oboy. Sounds perfect. Directions/pointers/instructions/offer to sell some?
I need to look at AliExpress a bit before buying anything through them, but its tempting. I did some feedback checking and the first guy that gave them 6 stars gave everybody 5 stars, so taking it with a grain of salt. The U2 is a higher output?
Link to $10 shipped xm-l sk68 clone.
I have bought things from them with no problems, after looking them up in Wikipedia. It is the retail branch of Alibaba.com which is a huge wholesale site.
Yeah, that’s a good point.
Sure, it can be programmed to tell the difference between those voltages. The only problem that stands out to me is that the microcontroller needs to use its internal 1.1V voltage reference to accurately measure the cells voltage. If it’s meant to also warn the user that a NiMH is almost dead, then there’s going to need to be some workaround for when a NiMH cell drops below that threshold.
I was planning to buy some PAM2803 IC’s from Alie Express, which is the driver chip that came in my SK68 clone. Since it’s been reported that these chips can handle the voltage of a Li Ion cell, then it could be a good candidate to test this idea on.
Arrived today. I don’t think it is glass, and have a query into FT. The lens has a burr/nub (and bevel cut on the lip) that don’t look like something with glass. I haven’t tried sanding, yet.
It seems a perfect replacement for my SK98 (though it still needs a retaining ring/method). It also fits the SK68 but since the dome is larger the bezel needs some relief for it to seat in.
It’s ~1g heavier than the SK98 lens and ~2g over the SK68. It’s relatively clear and better looking than either stock lens. Focus is very close, may vary slightly by light.
EDIT - FastTech says they checked their stock and say they are glass …
How is the focus on an SK-68? Can you still see the emitter pattern?
As above, I haven’t fitted it into the 68 bezel since the dome is a little larger than the one on my 68. In fact, I just looked again and noticed that my 68 lens sits in a recess in the slider section. This new lens is just barely wider and sits on the end of the zoom/slider section (but still inside of the screw-on bezel). That’s more likely why it’s a couple turns out from sitting flush enough for the bezel to screw in all the way, That said, it appears a pure focus will be further out. As it is right now, the die shape is almost distinguishable, but it’s more of a fuzzy square with rounded corners (which some might like better than oem).
In the 98 it focuses pretty much the same as the oem version. The new one is not a press fit on mine where the oem is, but we’re probably talking hundredths of a mm. I’d like to say it’s very slightly brighter, but realistically I would need a meter to believe it.
It may fit more naturally into a SK68s with the narrower bezel on outer end, and since I need to fix the 98 with a removable retainer it may wait till I find a 68 with the other bezel before I sand on the only one I have. The total depth on the 68 & 98 lenses is very close, but with the dimensions being enlarged my guess is the 68 might need an extension for a perfect die focus.