-- 18650 / CR123A
LiIon cells will self-discharge, although not as fast as normal NiMHs for example. If you're putting a light in a go-bag or car, for emergencies only, in a "throw in and forget"-way, I'd definitively consider CR123A to be a better option.
If you check it occasionally (4 times a year or more), and there's are no temperature extremes, then 18650 should be just fine as well. Note that it would be better to swap the 18650 4 times a year, to a freshly charged one. They'd still work after all, even if remaining energy isn't what you'd want.
There is a lot of room here for personal preference, and different weighting of pros and cons. My own rule of thumb is that if I'm somewhat confident that I'll check and swap batteries in a light in the next 3 months or so, I'll never consider anything but 18650. If I might forget, such as a go-bag, I'll put in CR123A, and have a fresh set of CR123As stored with the light as well. That way, even if the light has seen some use, I always have a fresh set there. The light would be good to go 3 years later, if I forget.
For passive storage of LiIons, it's best to store them with only a partial charge. Storing them fully charged causes some wear, but letting them self-discharge below empty is even worse. Lots of different numbers floating around, but storing them at a voltage of 3.92V seems to be about ideal. It's low enough to not have the bad effects of storing fully charged, there's useful charge in the cell if you need to use it without charging, and there's a long way of self-discharge before damage. Not critical to be exact, 3.8 is just fine for example. Be below 3.92V rather than above, just don't get too low. I would rather charge a cell than store it at 3.5V for example.
Most of this matters if you want long term storage with minimal loss in capacity. I'd do it for cells I don't plan to use in the next 6 months, for anything in rotation I just charge fully and put in the ready pile.
-- AA
There are thee main types of AA batteries in my book:
- Lithium primaries
Allows for log storage, non-rechargeable. Can be stored in light.
- Low self-discharge (LSD) NiMH (such as Eneloop)
These are great, and pretty reliable. Can be stored in light.
- Alkalines
Not to be stored in light, only used for emergencies.
What I'd do for go-bag type light, is similar to CR123A, of having Lithium in the light, and fresh Lithium ready outside the light. No harm having Eneloops (or similar) in the light, and as extras, but do keep some Lithium as well.
My preferred choice would probably be Eneloops in them, and then Lithium extras, that would be somewhat reserved for emergencies. Replaced with fresh if ever used.
If you're tempted to have a bunch if alkalines for extended power outage or similar, I'd much rather recommend a pile of CR123A. Lithium primary AA would be fine, but is almost always more expensive. Personally I buy 10-20 CR123A a year, and use none. Adds up.