Consumer Reports: Q and A: Why do car batteries die in winter?

Q: Why do car batteries go dead in winter?

A: Cold weather is often fingered as the culprit when car batteries die, but actually warm temperatures do the most damage to them. High temperatures quicken corrosion of internal plates and vaporize the electrolyte faster. But car batteries usually go dead in cold weather mostly because damage done during the summer doesn’t show up until the battery is more taxed. A cold battery has reduced cranking power, and cold temperatures thicken motor oil, making it harder to turn the engine over. Modern electrical and fuel systems can mask a weak battery by starting an engine with a minimum of cranking, but when a weak battery is further compromised by cold conditions, it’s more likely to fail.

For more information on how to choose a car battery, see our car battery buying advice, ratings and recommendations.

Source;
http://blogs.consumerreports.org/cars/2009/11/qa-car-batteries.html