So, it's been cold in the midwest. Actually it's really never been consistently this cold outside of Duluth and this means when jumping a dead car it's not a bad idea to consider if the battery is frozen.

So what are the best steps to check for this? Around what temperatures is the to be expected at a reasonable charge, eg the car started when it was 15*f warmer? After an actual freeze can the battery be used again, or must it be replaced?

  • A battery with lower pH will have a lower freezing point, so there could be a lot of factors, such as state of charge (lower state of charge means higher pH), battery technology (AGM batteries use lower pH), and possibly other factors I'm unaware of. – Paul Feb 20 '15 at 23:25
  • I don't know exactly at what temperature a battery will freeze at, but we had winter weather in Montana where it was -30°F for about a week. Our batteries never froze in that weather. For reference though, this website says, "A 100 percent fully charged battery will not freeze until approximately minus 76 degrees Fahrenheit. A fully discharged battery can freeze at or around 32 degrees" so I guess it depends on the state of your battery charge. – Pᴀᴜʟsᴛᴇʀ2 Feb 21 '15 at 0:56

Frozen batteries can pose a significant hazard. As @Paul has stated the freezing point varies with the state of charge and battery type. A frozen battery can explode with considerable force spraying acid and shrapnel quite a distance. The explosion is caused by the expansion of the gas from charging. The gas is trapped by the ice and unable to vent. The battery case may also be weakened by the expansion of the ice. Some clues the battery is frozen are a distorted or bulging case. You can also remove the covers and see the ice on the top of the lead plates. If you suspect the battery is frozen remove it and allow it to thaw. If the battery has frozen, the damage may be slight enough to be usable after a thaw and recharge or it may be destroyed. The only way to tell is to thaw, charge and test it.

  • Thanks for the details on how the explosion is caused. I had not though of actually opening up the covers on top of the battery to check for ice crystals. – QueueHammer Feb 21 '15 at 21:41

If the battery is frozen and the plastic case is bulging, do NOT charge it under any circumstances. Let the battery thaw out completely and then check for cracks. If cracks show in the casing, do not charge or use it. My battery exploded after the airport parking guys charged it when it was frozen.

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