I run that on a semi-regular basis. No issues for 3 or months on the same set of cells. I recharge when they start to dim, if that matters, probably never quite flat. I have maybe 20-25 cycles on mine, I use 17500's most of the time with a TL-3, 50+ cycles no issue.
How many "cycles" do you figure you have on yours ?
check cell voltage with a DMM, see what you get.... we'll proceed from there...
other possibility is that the bulb finally blew out. keep in mind that a pair of Li-Ion cells drive the lamp HARDER than it otherwise would have been driven on a set of 3xCR123s. Li-Ion doesn't sag as much under the 2.5+A load of the MN16.
They only had about 4 or 5 cycles on them, but it usually got drained until it doesnt light up anymore, I never noticed any dimming, it just stops working after about 20 minutes and they recharged and work again fine for several more cycles, until now. The lamp works with 4 primary cells. Both the 17670s will work in my L4. I dont have any way of testing them other than putting them in the DSD charger, I've done them separately and they should both be fully charged. Theyre both protected, one of them is from several months ago, and AW just replaced one of the two for me thinking that would fix the problem. One battery must be a little different than the other in some way, thats got to be the reason they wont work together?
I am more of a li-on user than guru but a few things I have learned include:
1. The MN16 is a big draw for the 17670, near the limit of what you want to run them at.
2. Matched sets are the way to go. I "matched" mine when new and they stay that way. I recharge both at once in the DSD and have never had an issue. Mismatched lions are a bad thing IIRC, maybe as bad or worse than mismatched primary cells.
3. I suspect you ment to say 3 cells in your last post. If it is the 4 cell/12 V lamp ( MN20 or MN21 ) then I suspect the 17670 do not have enough juice and the protection circuit is kicking in right away.
4. Have you tried "double tapping" to get them going. Two presses of the button in rapid sucession. This can "trick" the protection circuit but I do not think it is a good thing.
Double check that you are running the MN16 bulb, does the MN15 work - it draws much less juice ? If you are using the M4 bulb(s), then IIRC the Lions will not drive them - not enough voltage.