The MJLED unresistored on a 14500 is pretty risky. More than one person is running that combo with success although I'm not sure how... I tried it and fried it in under 10 seconds. The battery hadn't seen a charger in weeks, but it made no difference to that poor LED though.
The 14500, if I have this correct, is 4.1 - 4.2V (fresh off charger, no load), dropping to 3.6V under load. The Vf of the MJ, though opinion seems to vary on this, is roughly in the 3 - 3.6 range, so you're a little outside the safe territory. If you get a runaway LED, the 14500 is happy to provide the current needed to produce smoke.
So, can anyone recommend what ohm/watt resistor is good to limit a 14500 to about 100 ma, or even around 110 - 120 for a little overdrive?
I'm also wondering if just adding a Kroll introduces enough resistance to bring current down to a semi-safe range.
Davidefromitaly - just to understand, why does driving more than one LED from 3 X NiMH drop voltage? Is it because the larger load from three LED's causes the batteries to sag a little?