PowerSource

You've probably heard about the unseasonably cold weather currently affecting normally-balmy California. If you're a Californian unused to dealing with freezing temperatures, here's an interesting word of caution from the excellent Battery University website: It's hazardous to charge lithium-ion batteries at temperatures below freezing.
At temperatures below 32°F – for example, when you leave your cell phone on the battery charger in your car – the battery's anode picks up a metallic plating. The plating is non-reversible and cumulative, so after several sub-freezing charge cycles, the pack's safety is compromised: A sharp impact or an aggressive charge (typical of many of today's charging circuits) invites the infamous lithium-ion thermal runaway condition. Or, less dramatically, your cell phone battery may simply stop working.