Cold weather doesn't so much kill batteries as expose them. As the temperature drops, a battery's available cranking power drops with it, and at around zero degrees it can be down to roughly half. At the same time, cold oil makes the engine harder to turn. A battery that was fine in October can be helpless in January.
Six checks before the first hard freeze:
1. Get a real battery test
Not just a voltage reading. A healthy battery shows about 12.6 volts at rest, but voltage only tells you charge, not health. A load test or conductance test measures how much cranking power the battery can actually deliver compared to its rating. If it's weak in the fall, it will fail in the winter.
2. Check the age
Look for the date sticker or code on the battery. If it's four or more years old in a cold climate, or three or more anywhere with hot summers, plan on replacing it soon even if it seems fine. The test result matters more than the date, but age is a strong hint.
3. Clean the terminals
Corrosion adds resistance, and in the cold you can't spare any. White or blue-green crust on the terminals should be cleaned off, and the clamps should be tight. If corrosion keeps coming back, have the battery and charging system checked.
4. Make sure the CCA is right
Cold cranking amps (CCA) measure how much current a battery can deliver at 0°F. When replacing a battery in a cold climate, buy at least the CCA your car's manufacturer specifies. Going a little above is reasonable; going below is asking for trouble.
5. Check the block heater and battery warmer
If you live somewhere truly cold, a block heater and a battery blanket make a huge difference. Plug them in and confirm they work before you need them, and check the cord for cracks. A battery warmer keeps the battery warm enough to deliver its power, and a fully charged battery is also far less likely to freeze.
6. Drive it, or maintain it
Winter driving is hard on batteries: short trips with the heater, defroster, seat heaters and lights all running don't give the alternator much time to recharge. Mix in a longer drive each week. If the car sits through a cold snap, a battery maintainer keeps it topped up.
If it fails anyway
Even with good habits, the first sub-zero morning catches some batteries. A mobile battery tech can come to the driveway or the office lot, test the battery and charging system, and install a new one in the cold so you don't have to arrange a jump and a drive to a store. Find your city on the locations page.