This is a valid concern. Heat can be very dangerous, especially since most mods do NOT have integrated thermal protection, nor do most lower-end batteries used in them. Protected batteries, often indicated by "P" (i.e.: "AW 18650 P"), do contain a small integrated circuit to protect against thermal damage, among other things. If you leave an iPhone in the sun and it gets too hot, it will trigger a thermal shutdown to attempt to protect it from causing damage. Most mods do not have this ability (the Provari V2 does, among other variable voltage mods). And even with thermal protection, damage can still occur to both the battery, the device, and even the user. Extreme heat and lithium batteries are as dangerous as throwing water on a roaring grease fire. As stated, an insulated cooler is a good idea. Some glove boxes are actually cooled for this same reason. But lying in sun, even if the sun is coming through glass, should be avoided if at all possible. A car with closed windows in the hot sun can exceed 160 degree (F) in a matter of a few minutes...this greatly exceeds the acceptable storage temperature and even further exceeds the acceptable use temp range.
