Originally Posted by cyccommute

First it's damned difficult to freeze a car battery. The electrolyte in the battery drops the freezing point significantly. The solution in the battery is about 25% sulfuric acid which drops the freezing point to -25°C (-4°F for the metrically challenged). Most of the stuff you find on the web says that a discharged battery freezes around the freezing point of water but I suspect that just Internet plagiarism at work. A fully charged battery doesn't freeze until around -60°C (-75°F).

For a frozen battery, I suspect that the explosion risk is due to a higher charge being forced into a solid rather than a liquid. This probably starts gas formation...water splitting...and results in increases in pressure in the battery which can probably be highly localized.