like ri chevy said, 3 x NiMH AA fresh of the charger will be closer to 1.45*3=4.35 v. 3 alkalines would be worse. That probably could have fried it since it is out of the given range.
Yup. Just checked it. Runs like gang busters on 18650. Sux A$$ on 2 AA’s.
This pisses me off. 3 1.2v cells in series should not have done this. Maybe I am not understanding the rules right. When running cells in series what happens to voltage and amps?
And what is the rule for parallel?
Ok so this drop in is only good for one or two if these cells?
The only thing you are missing here is that 1.2v is total BS. It’s “nominal voltage” and means nothing. A NiMH cell ranges from 0.8v when mostly dead up to the voltages we’ve posted above when fully charged.
Edit:
Yes.
series voltages add, you are right. what you fail to realize is that NiMH have a nominal voltage of 1.2 V. fully charged they can be as high as 1.5 V and then discharged to around 0.9V. They just happen to stay around 1.2V for most of their discharge and hence their nominal voltage. Alkilines can be over 1.6V fresh and then go down from there in a roughly linear fashion. Sorry you fried it.
FYI, Regular Li-ion (LiCobalt) is fully charged at 4.2 but has a nominal voltage of 3.6-3.7 V. LiFePO4 is full at 3.6 with nominal at 3.2V.
EDIT: ditto what wight said
Ok. So i screwed this up. The unit is working on fully charged 18650. This drop in will run up to 4.2v. What is the voltage of the average battery pack harvested 18650?
Sure would hate to finish the thing off for good!
4.2v or less when fully charged. You have little to worry about from that corner.
I knew when i got into this P60 thing with all these cell types and different dropins, that it was a matter of time before i burned one up like a dumbA$$.
For the record it took 16 months.
Now that i know the .08-4.2v will run 2 AA’s or one 18650, i will just buy more of these.
Just in case, how many CR123’s can this thing take?
One. Use two with a SF high voltage dropin.
So the voltage of the 123 is what? Obviously lower than 4.2v.
Also, i see people using dimes and pennies as spacers between cells when the host is a bit longer than the cells.
Correct my thinking. Wont the spacer material short out on the inner wall of the host body and cause problems?
The inner wall is anodized and therefore non conductive. As long as that's still in place, there will be no short. I'm not really sure what the resistance of coins is, probably only so-so.
HKJ says that the maximum voltage of a 123A is 3.2v, you can refer to this article of his in order to get an idea of the discharge curve: http://lygte-info.dk/info/CR123A%20and%20rechargeable%20substitutes%20UK.html
Thanks for all the help. I think i would just wrap the spacer with black tape just in case.
Cr123's are 3 volts
they are actually about 3.2 before loaded
and there's an oddball one at 3.7 that you'll probably never see .
A 16340 is the same size as a cr123... it's the rechargable version which is the same voltage as a 18650 > 4.2 volts
16 is the width ...16 mm wide
34mm length
and 0 means cylindical = 16340
same with all batteries with numbers like the 18 / 65 / 0..18650
?? Never heard of coins to make up space in a light . Seems rather lazy and dangerous .. How about at least a wooden dowel drilled and a nut and a bolt .
or a nylon bushing ...
or some rubber washers ..or
or 
or and
< a screw binding post>
Curious, but why are you needing a spacer?
If it’s an L2M you just run one tube section for a CR123a and make a stumpy light out of it.
I think it’s also worth checking the voltage range of your drop in, SF sells quite a few different types and it’s possible they sent you something else, or you ordered something different to what you thought you had.
BTW - have you ever used this drop in with 1 or 2 AA’s previously, or was this the first time?
Yes. It did work previously. The sticker on the side of the dropin says .8-4.2v.
So if these guys are right, my new cells which are marked 1.2v are actually 1.6v, then me using 3 cells for a total of 3.6v actually turned out to be 4.8v, i burned out the booster circuit myself. I can buy this. My stupidity.
The spacer is because…. Two AA cells fit very tightly in the L2M with the 1 extension that it comes with. If you add another extension, AA’s slide maybe 1/4”. So a 1/2” spacer would take up the space and provide adequate tension on the springs to make good contact and eliminate rattle.
The whole purpose of this project is to have a light that can run off a variety of cells in many configurations. Here in the US, the AA is extremely prevalent. There is a pair in every tv remote control on every coffee table in every living room in the country.
Even though 123 or 18650 is far better than AA, it is important to be able to run on AA for that moment in time when the only thing available may be AA. Think EOTW type scenarios. I know it may sound silly. But i do take comfort in knowing there is a variety of options for cells.
Well, now that you have all of this figured out, you would probably be better getting a couple of different drop ins to satisfy your needs. 
I happen to like my Surefire 9P (Similar to your setup), as it does offer me a variety of different battery options with my different drop ins. I have a Malkoff M31N that I use with 2-AA’s. I have a host of other Malkoff’s that I can use with 3-CR-123’s, and 18650’s (1 or 2). I can also use 2-14500 Li Ions as well. You just have to be cognizant of the drop in that yoor using, and only use the batteries that “fit” the drop in.
The Malkoff M31 is for “LOW VOLTAGE” use only. 2-AA’s max
The Malkoff M61 is for 3.4-9v. Any combination of batteries that I described above. 3-CR123’s primaries, 2x18500’s, 2x18650’s with A19 adapter. Even 2-AA’s. You just get a much lower light output due to the battery voltage and the drop in running out of regulation. It runs in direct drive. DO NOT USE 3-RCR123’s or Kaaapoof!
One always has to be ready for when the SHTF or the EOTW! 
Murphy’s Law.
I recently assembled an L2M for 2AA use. I replaced the 16340/18350 extension tube which was included with the L2M and used an 18650 extension tube. This seemed to do fine. I did shove a rolled up piece of plastic drink bottle down the tube to center the cells. I did it mostly to eliminate any complaining from the end user, not because I thought it was necessary. I assumed the process would also work the other way - take an L2, L2P, etc and add the short 16340/18350 extension to fit two AA cells. I did not measure or compare the length between the two extensions.
I noticed that when you screw it all together tight, the cells are very tight up to the back of the p60 as well as tight against the tail cap. So i add another section. But then the cells are free to slide inside the tube by about .250”. This is why i was looking for the spacer.
Having the cells so tightly compressed against the module cant be good. Especially if the light is dropped on its face, all the impact will be transferred to the rear of the drop in.