Can I leave lithium batteries in a hot or cold car?

Most portable electronics these days use lithium-ion or lithium-polymer batteries. Is it safe to leave these in a cold or hot car? Right now it’s cold, in the teens to thirties. In the summer it might get into the nineties, and hotter inside a car of course. Would either of these damage a battery, or reduce its performance temporarily?

I’ve never noticed problems with my cellphone being left in the car, but that’s not as demanding or expensive an application as something like a GPS unit or a netbook.

You can, it’s not a great idea, and prolonged or repeated exposure to extreme cold or heat will cause the cells to prematurely degrade.