Originally Posted by
rumrunn6
was reading safety warnings for a car battery charger I got yesterday. one of the warnings was never charge a frozen battery. they didn't say why but there's probably a good reason. was gonna take it out of my kids car, bring it inside to charge it overnite. but the warnings about gas clouds & explosions, etc had me doubting the idea. eventually my mechanic said he'd go check it out & it was probably the starter anyway
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.