I agree that it's a little confusing, in part because the labeling isn't very clear on things. Here's a quick summary from memory:
- L2m: Typically fits 1x16340, unless you get the "2009" edition, which is bored out wide enough so that it can accept an 18650 with an extension tube. Usually the tube is bundled together with the flashlight for these.
- L2(x/p): Unless you find a really old L2, should fit 2X16340 or 1x18650.
- L2i: Fits 3xAAA or 1x18650/2x16340 (with some mashing; it's a very tight fit). No extensions available.
- L2r: Fits 2xAA. No extensions available.
Extensions below are for L2(x/p/m) only, not for L2i or L2r:
- CR123/16340 extension tube: Large enough for one 16340. Should be wide enough for an 18650 (mine is), but I don't know if all of them in circulation are. If it's wide enough, then 2 of these are equivalent to one 18650 extension tube.
- 18650 tube: Large enough for one 18650.
Now here's the fun part: 2 AA batteries are just a little shorter than 3 16340s, so you can put 2 AA batteries in anything that is the length of 3 16340s, e.g., an L2m body with an 18650 tube, or an L2 body with a 16340 tube, or even an L2m body with 2x16340 tubes. Note that the tubes are wider than AA batteries, so things can rattle around a bit, but it seems not enough to cause contact problems.
Using the 2xAA = 3x16340 logic above, you can do a little math and come up with other combinations. For example, 4xAA = 6x16340, which is the length of one L2 body and 2x18650 tubes, but it can be achieved with other combinations as well.
If you want to add spacers into the mix, then you have even more options. A 15mm charging spacer ($2.00 from lighthound) will make an AA about the length of an 18650. Let's say you want to do 3xAA: If you add a 15mm spacer, then you can think of your third AA as having the same length as an 18650, so you just add one 18650 extension tube to the 2xAA solution above: L2 body + 16340 tube + 18650 tube + 15mm spacer = 3xAA solution.
Perhaps another way to summarize this is to explain everything in terms of the number 16340 lengths that are provided or required.
Provides:
- L2m: 1x
- L2(p/x): 2x
- 16340 tube: 1x
- 18650 tube: 2x
Requires:
- 16340: 1x
- 18650: 2x
- AA: 1.5x
- 15mm spacer: 0.5x
To figure out combination of tubes that you need for the number of batteries you want to use, compute the number of 16340 lengths you need, add a 15mm spacer if needed to round up to whole numbers, then add the number and type of tubes needed to provide that length.
Finally, please keep in mind that the discussion above is all about size, not output. Don't forget use a drop-in that's well suited to the output from your batteries.