A frozen lead acid battery is a damaged or ruined lead acid battery. But under what circumstances will a lead acid battery freeze?
Putting it simply… A dead lead acid battery will freeze at 32 degrees-F.
However, a well or moderately charged battery will not freeze. (Sort of)
A little more detail… A fully charged (lead acid) battery will freeze. But not until temperatures drop to 94 below zero (-70 °C)! That’s pretty much not going to happen anywhere here on earth!
But >> Important >> The less charge on the battery, the more susceptible it is to freezing.
I built a chart that cross references battery state-of-charge with the approximate temperature at which the battery will freeze. This is for lead acid type batteries. Car batteries, for example. Or those which typically install in lawn tractors, ATV’s, snowmobiles, etc..
As you can see, if your battery is only half charged, it could freeze solid at -4 degrees F.
Or, if it’s down to just 30% charge, the battery will freeze at 13 degrees F.
I lost two 12-volt batteries a few years ago in my 5th-wheel. I forgot to disconnect the batteries for the winter. As it turns out, there was a parasitic load constantly drawing down the batteries. Since I wasn’t charging them, they eventually went dead. And then they froze. It was an expensive mistake that I will never make again.
Since that time I installed a high current DC ON-OFF switch directly at the battery compartment location. I also make sure to FULLY CHARGE the batteries before winter sets in.
You can leave a lead acid battery installed and/or outside during the winter. But only if the battery is in good condition, there is no parasitic load, and the battery is fully charged.