Ask most drivers when batteries die and they'll say winter. They're half right. Cold is when a lot of batteries finally fail, but in most cases the real damage was done months earlier, in the heat.
What heat does inside a battery
A car battery makes electricity through a chemical reaction between lead plates and sulfuric acid. Heat speeds up chemical reactions, including the ones you don't want. In a hot engine bay:
- Water evaporates from the electrolyte, especially in conventional flooded batteries. Less electrolyte means less active plate area and less capacity.
- The positive plates corrode faster. That corrosion is permanent, and it steadily eats into the battery's ability to store and deliver energy.
- Self-discharge increases. A hot battery loses charge faster while the car sits, which leaves it partly discharged more often, and that does damage of its own.
Remember that the battery sits next to the engine. On a hot afternoon, after a drive, under-hood temperatures run far higher than the air temperature. That's the environment the battery lives in all summer.
Why the failure shows up in fall
Here's the cruel part. A battery that has lost a big chunk of its capacity over the summer can still start a warm engine on a warm morning, because a warm engine turns over easily and a warm battery delivers its current well.
Then the weather turns. On the first cold morning, two things happen at once: the battery's available cranking power drops because the chemistry slows down in the cold, and the engine gets harder to turn because the oil is thicker. The worn battery has no margin left. Click.
That's why battery techs in hot places stay busy in summer and then get another wave of calls on the first cool mornings of fall.
How much life does heat take?
A battery that might last five or six years in a mild coastal climate often lasts only two or three in desert heat, and around three across the humid South. Every city page on this site shows the typical battery life for that city's climate, so you can see what's normal where you live.
How to protect a battery from heat
- Park in shade or a garage when you can. Even a few degrees helps.
- Get it tested in spring, before the worst of the heat, and again in fall in hot climates.
- Keep the terminals clean and the hold-down tight. Heat plus vibration is a bad combination for the welds inside a battery.
- Consider an AGM battery if you live somewhere hot. AGM batteries are sealed, don't lose water to evaporation and generally tolerate heat better. If your car came with one, it needs another.
- Use a maintainer if the car sits for weeks in summer.
The takeaway
If your battery is past two or three years old and you live somewhere hot, don't wait for the first cold morning to find out how much summer took. A ten-minute battery test tells you where it stands, and if it's weak, you can replace it on your schedule.