This is kind of long but you might find it informative.
I tried reading threads to get an idea what fits and is good for this light but I can't seem to find any hard facts . Too bad manufacturers don't give size limits in their specs . Can anyone recommend a Li-ion for the Jet 1-MK. Thanks .
Probably my favorite 14500 is the Fenix 800mAh. But I've only owned like 3 different brands. Keeppower makes a nice 14500 and the one I have is a little fatter than the Fenix. Both are based off of the Sanyo / Panasonic cell. My Keeppower 14500 won't fit in the new Fenix LD12 but works fine in the Fenix LD11. But I think Fenix made the tube kind of narrow in the LD12.........so the LD12 is the issue more than the battery. I'd test it for you but its in the truck that has a cover on it and its stinkn' cold outside. Maybe tomorrow I'll dig it out. My guess is that the Jet I-MK won't have an issue like the Fenix LD12 with any 14500 battery.
Do you feel the li-ion is the way to go with this light or eneloops ?
On this particular light my vote is for the Eneloop. This is a very well regulated light and efficient with an AA Eneloop. Also the spacing is good with an Eneloop. With the 14500 there is some type of pwm or frequency varying control to the light. You don't see it with your eyes, but a data logging light meter picks it up as would an oscilloscope on the LED. As an EE and purist with the driver circuits, this sort of bugs me and so I focus on an Eneloop with that particular light.
My theory is this:
Single AA lights that are designed to run both alkaline/NiMh voltages along with the higher lithium ion voltages need to have a driver that can accommodate voltages from 0.9v on up to 4.20v. Since the LED's forward voltage drop is around 3.0v, this necessitates a boost driver circuit for the alkaline/NiMh chemistries. But when utilizing something above 3.0v (ie: 14500), you need to have some type of buck circuit. So it presents an issue for designers as to which is the best and most cost effective way to accomplish both. I have seen the Nitecore MT10A and the Jetbeam Jet I-Mk utilize some type of pwm for the 14500 power yet have a rock solid efficient current controlled drivers with the alkaline/NiMh.
The other issue I have with AA/14500 lights is that I have seen a couple of them not be as efficient with a 14500 as similar lights that were CR123A/16340........even though the 14500 typically has about 100 more mAh's. If wanting a little pocket rocket, I tend to favor the 16340 lights and when wanting something that utilizes the availability of an AA, I tend to just use Eneloops because of the pwm issue. Now all manufacturers may not be like this but I do know that the Jet I-MK is. However the Jet II-MK is not and is very well regulated. It also puts out a little more on its top end. A little bit of that may be due to the XP-L in it vs the XP-G2 on the Jet-I MK.
So, typically I prefer the Eneloop in the AA/14500 combo lights, but I do like the 14500 in a Fenix LD11 because they did a good job with their driver between AA and 14500. Fenix's new LD12, NW did not do as great of job in my opinion with balancing the mode levels, and the efficiency with an Eneloop at the highest level is terrible..........but I'm getting side tracked.
The Jetbeam Jet-I MK is one of my favorite single AA lights because of its efficiency, regulation and price.......but that is mainly utilizing the Eneloop.