I want to keep my Jetbeam M1X (on 18650's) in my car, but here in Florida it gets insanely hot in there. Can any damage be done to the light? Or should I just completely avoid it?
Just don't leave it on the dashboard
If the light is in your glove box or, even better, under a car seat, it shouldn't get so hot that there is any problem with either the light or the batteries.
Having said that, I agree 100% that batteries could be more of an issue in cars, due to heat hastening the chemical processes that lead to internal corrosion and ultimately to leaking.
I always used Duracell batteries for years under the mistaken impression that they were essentially leakproof no matter what.
Then I found an old flashlight that had been socked away in the bottom of an unpacked box for about 10 years, and both batteries has leaked like a sieve (I've never seen such a mess).
In fairness, both batteries were a couple years past their 'best used by' freshness date, but under those conditions I would expect them to give less juice, but NOT leak and destroy my property. I will NEVER buy Duracell alkaline batteries again for this reason (though I do use their excellent 1000mAH NiMH AAA's)
One problem that folks often have with a car flashlight is that it doesn't get used for years, and yet is stored under the worst possible conditions (due to the heat you mentioned). This not only accelerates the self discharge rate of the batteries, it also increases the rate of internal corrosion that might lead to leakage (so the problem that I saw after 10 years, might happen after only 2 or 3).
NiMH batteries are designed to deal with heat without leaking because they get actually generate heat and get hot when charging, so if you want to use low self discharge NiMH that should be fine (though putting them in the car will increase the self discharge rate so I would recharge them at least every three months)
Alkaline batteries should also be fine, if you store the flashlight in an area of the car that is lower down and doesn't see quite as much heat, and make sure that they get changed at least once a year.
Energizer's otherwise excelent L91 and L92 lithium batteries are only rated for storage and operation to 60C (140 F) and you could easily exceed that in Florida, so I would avoid them and all other Lithium batteries.
If you are locked into 18650's then I would store the light under a seat in a metal box (under a seat for the lowest temperature, metal box so that if the lithium batteries let go it won't set your car on fire).
- Luminescent