Why do car batteries die in cold weather?

Most Canadian drivers know the sound: a slow, groaning crank on the first properly cold morning, or just a click. The battery that started the car fine in October gives up in the first −20 °C week. Cold rarely kills a healthy, fully charged battery outright. What it does is expose one that was already weak, old or undercharged, because it cuts supply and raises demand at the same time.

Cold cuts the power a battery can deliver

A lead-acid battery makes electricity through a chemical reaction, and cold slows that reaction down. Varta, the Clarios battery brand, puts numbers on it: taking 20 °C as the optimum, "the chemical reaction at 10 °C drops to 50% and at freezing point it is as low as 25%" (Varta).

Interstate Batteries describes it differently: at 0 °C (32 °F), "a typical battery is only 65% efficient, both at recharging and giving power" (Interstate). The two figures measure different things, chemical reaction rate versus usable performance, so they don't contradict each other. Both point the same way: at 0 °C you have noticeably less to work with, and on a −20 °C or −30 °C morning less again.

Cold also slows recharging. Varta notes that charging "is slower due to the cold, so that the battery needs more time to become fully charged" (Varta).

…while the engine needs more

At the same time, the engine is harder to turn. Varta says that below 0 °C engine oil "becomes highly viscous", so the starter needs a higher current to get the engine moving. Once it's running, heaters, fans and defrosters add load, and the alternator's maximum output is limited, so "if a large number of consumers are active, little energy remains to charge the battery."

Put the two together and the margin disappears. Add a winter of short trips of a few kilometres, which Varta says don't let the alternator fully charge the battery, and a battery can go into a cold snap already partly discharged.

Summer heat sets up the winter failure

Varta makes a point that surprises many drivers: the damage often happens in summer. It calls +20 °C optimal for a car battery and says that, as a rule of thumb, chemical reactivity doubles for every 10 °C rise. That speeds up self-discharge and grid corrosion inside the battery. Varta also notes that this wear goes unnoticed through summer and autumn, and the problems start to show in winter, often on the first morning that demands full starting power.

Can a car battery freeze?

Yes, if it's discharged. As a battery discharges, its electrolyte becomes more like water, and its freezing point rises:

The exact figures vary by maker and battery type, but the lesson is the same: a charged battery survives a Canadian winter night, and a flat one may not. Keeping it charged is the best freeze protection.

If you suspect a frozen battery: Optima says a car that has sat in extreme cold and "won't even turn on dashboard lights" may have one. Wearing eye protection and gloves, check the case for cracks or bulging; a cracked case can leak sulfuric acid. If it's cracked, Optima says it may be damaged beyond repair and should go to a battery retailer for inspection. Otherwise, move it somewhere warmer and let it thaw on its own; Optima says not to speed this up "with artificial means of any kind." Never boost or charge a battery that may be frozen. East Penn warns it can explode (East Penn).

CCA: the number that matters in winter

Cold cranking amps (CCA) is the industry's winter rating. Battery Council International defines it as the current a new, fully charged battery at −18 °C (0 °F) can deliver for 30 seconds while holding at least 7.2 V (BCI glossary). When you replace a battery, match or exceed the CCA your carmaker specifies (our advice). Don't mix it up with a "CA" or "MCA" rating on the label; East Penn tells testers to distinguish CCA from both.

Battery type matters too. Varta says vehicles with automatic start-stop need an AGM or EFB battery, and that a conventional starter battery "reacts to extreme cold and heat with a rapid drop in capacity" where an AGM copes better.

Cold also changes what a passing battery test looks like. In East Penn's load test (half the CCA rating for 15 seconds), a good battery must hold 9.6 V at 21 °C or above, but only 8.5 V at −18 °C. More on reading voltages in our car battery voltage chart.

Getting your battery ready for a Canadian winter

1. Get it tested before the first cold snap, especially if it's three years old or more. CAA says a battery between three and five years old "could fail", and offers battery tests to members (CAA). Varta recommends a workshop check at least once a year before winter.
2. Check the resting voltage. After the car has sat for a few hours, a fully charged flooded battery reads about 12.6 V. East Penn recommends charging below 12.4 V.
3. Look at the battery. CAA suggests checking for corrosion, wetness and bulges in the case. Clean corrosion with a baking-soda-and-water paste, wearing eye protection and gloves (East Penn).
4. Give it longer drives. If you mostly drive short distances, take a longer drive now and then, or use a battery maintainer with a maintenance mode. See how long you need to drive to recharge a car battery.
5. Use the block heater, on a timer rather than all night. A warmer engine turns over more easily, which eases the load on the battery. Our guide to when to plug in a block heater gives Natural Resources Canada's and CAA-Quebec's timings.
6. Don't idle to warm up. Natural Resources Canada advises driving off gently instead of idling a cold engine; the block-heater guide has its figures.
7. If the car will sit, keep the battery on a maintainer. A parked car still draws current; see how long a car can sit and parasitic battery drain.

If it won't start on a cold morning

• Check for freezing first (see above). Never boost a battery that may be frozen.
• Boost it safely. Wear eye protection and keep sparks away from the battery, which gives off hydrogen gas. Check your owner's manual first: some cars have remote jump-start terminals that must be used instead of the battery posts. East Penn's order for a conventional under-hood battery: red (+) clamp on the dead battery's positive, other red on the donor's positive, black (−) on the donor's negative, and the last black clamp on the dead car's engine block or frame, away from the battery. Run the donor at least 30 seconds, start the dead car, then remove the cables in reverse order.
• Charge it properly afterwards, then have it tested. A battery that needed a boost at −25 °C may be fine once charged, or it may be telling you it's done.