A car battery freezes at around -40°C (-40°F), but this is only true for a fully charged battery. The actual freezing point is directly tied to its state of charge. A severely discharged battery can start to freeze at temperatures as high as -7°C (20°F). This happens because the electrolyte (a mixture of sulfuric acid and water) becomes more like water and less like acid as the charge depletes, and water freezes much more easily.
The key factor is the battery's specific gravity, which measures the density of the electrolyte. A higher specific gravity indicates a higher concentration of acid and a lower freezing point. Here’s a quick reference table showing how the freezing point changes with the battery's charge level:
| State of Charge | Voltage (12V Battery) | Specific Gravity | Approximate Freezing Point |
|---|
| 100% | 12.6V+ | 1.265 | -60°C (-76°F) |
| 75% | 12.4V | 1.225 | -35°C (-31°F) |
| 50% | 12.2V | 1.190 | -20°C (-4°F) |
| 25% | 12.0V | 1.155 | -15°C (5°F) |
| Discharged | 11.9V or less | 1.120 | -7°C (20°F) |
A frozen battery is a serious problem. The expanding ice can physically warp the internal lead plates and crack the battery case, causing permanent, irreversible damage. If you suspect your battery is frozen, do not attempt to jump-start it, as this can lead to an internal explosion. The safest approach is to slowly thaw it in a warm garage before testing or charging.
To prevent this, especially in cold weather, keep your battery charged. Short trips that don't allow the alternator to fully recharge the battery are a common culprit. Using a battery maintainer or tender if the vehicle will be parked for extended periods is the most effective prevention strategy.
