There are many documented cases of cell phones spontaneously catching fire, so I can see your concern about your cell phone. And manufacturers have worked hard on that problem, so I’m not sure how much of a risk it is at this time.
However, I have been unable to find any cases of car flashlights spontaneously combusting.
I found one instance of a battery catching on fire while being recharged in a car, but I’d never advocate anyone charge their Li-ion batteries unattended - much less in a car! :~
Anyone concerned about leaving the flashlights in the car should avoid buying any hybrid or electric vehicle. There are many more cases of car battery fires than of car flashlight fires.
Of course, this may be a case where statistics lie. The number of people with car flashlights may be much smaller than the number of people owning hybrid or electric vehicles. Moreover, the people who experience a car flashlight fire might be less likely to report the fire than people who experience their car battery catching on fire. Lastly, it is much more likely that the car battery fire would rise to the attention of the media to publish the report.
Heat is not the friend of electronics devices or batteries. Li-Ions may not explode solely due to heat in a car, but it’s not good for them nonetheless.
I store a LOT of lithium ion powered flashlights in my van. I’m just careful where I place it and I seldom park my van under direct sunlight. Been lucky to be incident free (i.e. lithium ion problems) so far but if the experts here could chime in and explain why I should not do it then I’ll be very willing to oblige.
I however have an experience with battery venting and this happened to my TrustFire X100 while displayed inside my Flashlight Curio. No I did not hear any explosion nor seen any smoke come out and no, my curio is not hotter than my van parked under the sun.
I just observed a few days ago that my X100 was dim when I played with it prior to modifying it. When I opened to check its batteries (3 King Kong 4000 mAh 26650s), I noticed that the middle battery had white discoloration (as if left there by thick white smoke) in its anode and its plastic cover surrounding the anode have partially melted. The cathode of the battery in front of it also has the same marking and I could only speculate that the middle battery vented silently while the X100 was on display.
My speculation was reinforced when I took the DMM readings of the 3 batteries. The first and the third batteries each read 3.95 volts and the middle battery read 0.00 volts. When I made a quick test using my Opus BT-C3100, the said battery won’t register anything - it was totally dead.
So I suppose my King Kong 26650 vented - and nobody even noticed it. It just vented silently and except for the white markings on the battery in front of it (which was easily wiped out with damp cloth) it did not cause in damage to the other batteries nor to the X100 nor did it cause fire or explosion. So if the experts here could chime in and explain why I should not place flashlights with lithium ions inside my van (I could just be very lucky so far) then I’ll be very willing to oblige.
Lithium-Ion Batteries Hazard and Use Assessment Link
From page 12:
A number of studies have attempted to rate the “safety” of different positive electrode materials.15,16 These studies are based on thermal stability measurements of the cathode materials with electrolyte at full-charge voltage conditions. These tests show that cathode materials begin to react exothermically with electrolyte at a range of temperatures from approximately 130 to 250°C (270 to 480°F). Safety rankings based on this data have been strongly criticized in the industry because they relate to only a single aspect of cell safety: the reactivity of the cathode. They do not take into account the many other factors that contribute to cell safety such as the reactivity of the anode (which usually begins to react exothermically at much lower temperatures), cell construction details that may affect the likelihood of developing an internal short within the cell, the probability of manufacturing defects to cause internal shorting, etc.
Page 14
As temperature increases, reaction rates between the electrolyte and lithiated carbon increase exponentially (following Arrhenius behavior). Thus, lithium-ion cell capacity fades and internal impedance growth accelerates with increased ambient temperatures; most lithium-ion cells are not designed to be operated or stored above approximately 60°C (140°F). Many soft-pouch cell designs exhibit swelling if operated or stored at 60°C or above, due to gas generation from reactions similar to those responsible for SEI-formation.
For most commercial lithium-ion chemistries, the SEI layer itself will breakdown when cell temperature reaches the range of 75 to 90°C (167 to 194°F; exact temperature depends upon cell chemistry and SOC). Accelerated rate calorimetery (ARC) has shown that commercial lithium- ion cells exhibit self-heating behavior if brought to a temperature of about 80°C (176°F).22 If cells are then maintained in an adiabatic environment (e.g., if they are well insulated), the cells can then self-heat to thermal runaway conditions (this process requires approximately two days for an 18650 cell tested in an ARC). Note that United Nations (UN) and Underwriters Laboratories (UL) tests for lithium-ion batteries discussed below require cells exhibit long-term thermal stability in the range of 70 to 75°
Another google search turned up research indicating cabin temps can get to 20C above ambient when parked in the sun so I guess risk level depends on where you live and park the vehicle.
There’s a lot of disussion about the chemistry of the cells, but little in the way of empirical testing or case examples to prove the theories.
The only test cited where overheating caused a problem was under “State of Charge” heading on page 70: “In testing one commercial cobalt-oxide cell model, the researchers found self-heating onset occurred at 80°C (176°F) for cells at 100% SOC, and at 130°C (266°F) for cells at 0% SOC.”
The lack of supporting documentation can be taken in two ways, (a) it hasn’t been a problem or (b) not enough research has been done on the problem.
And I’m not sure what to make of the testing criteria on pages 38-39:
Similar temperature tests are listed for IE Standards. I’m not sure if these are special tests for high endurance qualifications, or whether all batteries are so tested. But the test criteria are quite high (266F!).
Most LiIon batteries are run through this type of test (Cheap Chinese probably not), just be aware that the battery do not have to survive the test, pass criteria is usual "no explosion and no flames", i.e. venting is accepted.
Which means that venting does not necessarily lead to explosion and/or flames?
In that case, the ‘explosion proof’ features of aircraft grade aluminum flashlights should be enough to prevent fire inside cars caused by lithium ions. I guess the best thing to do then is be very careful where you place your lithium ion flashlights while inside your car and where you park your car? :~
I’ve kept lights with li-on batteries in my cars for years in Southern California heat and I’ve never had problems with the batteries. Not sure if I’ve just been lucky so far.
Most of the time a cell would vent not explode, I think an actual explosion needs a lot of other factors (closeby material to ignite, enclosed space to trap pressure that fails, etc), venting is what the cell is meant to do under pressure as a safety feature.
:bigsmile: 
