Actually, it has to get pretty darn cold for a lead acid battery to freeze - it's not water, it's acid. That's why cars can sit outside in sub-zero temps and not burst their batteries.
Quote:
Can my batteries freeze?
If your battery is partially discharged, the electrolyte in a lead acid battery may freeze. At a 40% state of charge, electrolyte will freeze if the temperature drops to approximately -16 degrees F (-26 deg C). When a battery is fully charged the electrolyte will not freeze until the temperature drops to approximately -92 degrees F (-68 deg C).
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I added the Celsius conversions.
http://www.progressivedyn.com/battery_basics.html (That's one source, but there are others that say essentially the same thing).
It's good advice to keep your battery fully charged to avoid premature sulfation, so it shouldn't be sitting around in a discharged state.