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The Ford Fusion Hybrid and its sibling, the Mercury Milan gas-electric, have failed, on numerous occasions when driven in cold weather, to return gas mileage results that match their EPA rating of 41 mpg city/36 highway. Likewise, the Mini E's usable range, when driven by Dr. Lyle Dennis, dropped substantially as the mercury dipped down to 23 degrees Fahrenheit. Heck, even General Motors admits that the Chevy Volt's electric-only range will likely degrade as winter sets in and we're fairly certain that the Nissan Leaf is not immune from bitter cold temps. So, what causes a battery's performance to degrade as the mercury drops?

Bryan Johannsen of The Car Electric, a pro-plug-in vehicle site, summarizes the chemistry that causes batteries to suffer in the cold:

All batteries deliver their power via a chemical reaction inside the battery that releases electrons. When the temperature drops, the chemical reactions happen more slowly and the battery cannot produce the same current that it can at room temperature. A change of ten degrees can sap 50 percent of a battery's output. In some situations, the chemical reactions will happen so slowly and give so little power that the battery will appear to be dead when, in fact, if it is warmed up, it will go right back to normal output.

Johannsen concludes with this bit of advice:

Cold has a negative impact on all aspects of battery operation. Keep this in mind if you're planning an electric car purchase; we don't want you finding out the range of your car has been halved when it's five below zero and you're fifteen miles from home.

While gasoline-fueled vehicles are not invulnerable to the effects of cold weather, electric-drive cars are where the decreased efficiency is most noticeable when the bitter temps set in.

[Source: Washington Post]