Freezing Lead Acid batteries

 Freezing a Lead Acid battery, whether it be a sealed type or not, depends on the state of charge. The electrolyte is mostly water, and the expansion of frozen water is what damages the battery when it freezes.

When a Lead Acid battery charges and discharges the properties of the electrolyte change. When the specific gravity is high it is harder to freeze. The sulphuric acid becomes much less dense in the water as the battery discharges. And the acid is at its highest density, and the electrolyte at its highest specific gravity when the battery is fully charged. As the electrolyte gets closer to the specific gravity of water it's easier to freeze.

So a fully charged Lead Acid battery probably won't freeze most places on earth, while a fully discharged battery could freeze just a little bit below 32 degrees Fahrenheit. And freezing doesn't just potentially crack the case, but there can be internal damage to the plates and mounts that can ruin the battery.

So the best thing to do with Lead Acid batteries is to keep them charged when there is a potential for temps to get below 32 degrees F.

And if you think a Lead Acid battery has ice in it but hasn't fully frozen, warm it up before you charge it or further damage can occur. This is because the ice will not combine with the acid, which is part of the charging process. Also, the water that isn't ice will gather a higher concentration of sulfuric acid that is far too high for the materials in the battery. And further, the battery has a risk of overcharging because without the right amount of active electrolyte, the charge cycle will end prematurely. And then the battery will be left in a somewhat discharged state even after the charger has completed its cycle which can set up more problems later.