I should put a different driver in my XM-L2 SK-68, but will be a standard one. Wondering how many 1735s I should have. Seems too little for x 8, in spite of the fins.
The M6 with 3 emitters and 4 cells would, in theory, dim out and die when the cells got below 3.2V or so? Whatever minimal voltage the 3 emitters require? So whether or not the driver has protection, the emitters will be the limiting factor anyway? Because the Direct Drive FET driver is essentially like running the emitter with no driver, right? Cool.
Now I’m gonna have to run the M6 til it dies to find out how/when/if that works out as the theory predicts. (another excuse to play with the light during the day. 
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Dale, the firmware you're using will start dimming the output at around 3.0v and then shut off the light at around 2.8v. An XM-L2 will run under 2.5v in my experience, although it should cut off soon after that (at around 2.5v it starts to get very dim.)
You should know this because (1) you have assembled the drivers yourself; (2) you have been compiling and programming the drivers yourself; and (3) this is the exact same voltage read system and layout that has been used on all the 105C pattern drivers.
Sometimes I think that you're getting a bit "old" for all of this new-fangled electric stuff, but I'm still glad you're here! 
(I hope you know that I probably learn more from your "old" ways than you probably do from my "young" ways.)
I don’t think they make a 7135 15mm driver for the SK68
I’ve never run a light til it shut down. Of all the Qlite’s I have, I’ve yet to see that side of the driver in play. I don’t like to take the cells down very far and I don’t leave em after using em, always recharge so the next use is on topped off cells.
I don’t want to head outside with a light that may die because I used it once or twice before and forgot how long I’d run it. A walk into the back 40 and around can have me quite a ways off from the house for a change of cells. (As if I go out with only one light, but that’s my story and I’m sticking to it!)
As far as new ways and old dogs…I’m thinking about putting some of these new LED’s in a light, see how they perform. The M6 does really well on Xenon’s…
The way that the STAR momentary is set up it would take a LONG time to run out of light completely. If you've got four cells in there I think that it will run for at last a day before it shuts off from when it starts dimming, maybe more. Also, JonnyC programmed in a nice little feature: no matter how low the batteries are if you hold down the switch the light will stay on.
I’m trying a test of runtime vs heat. So far, the heat is winning.
On my 4th run, I let it go to 138.3º and even the battery tube was getting too hot to hold. That run took 3 minutes. Previous runs up to 130º were just under 2 minutes.
I want to see how many of these runs I can expect to have on a set of 25R cells.
You can use a partially-used-up CR123 to check the low voltage behavior. I also have some old 'junk' 18650s I've kept around only to use for low voltage testing, they're kept nearly empty and charged to whatever partial voltage so it doesn't take 2 hours to see what's gonna happen.
One thread suggested putting a 17 mm. driver behind the pill. This light has length to spare in the battery space, so it should work.