Does extreme summer heat affect car battery life?

Yes. Summertime heat can affect your battery worse than the winter cold. Most batteries fail in the month of July than in January, because of the heat.

Heat is a killer of all batteries and high temperatures cannot always be avoided. This is the case with a battery inside a laptop, a starter battery under the hood of a car and stationary batteries in a tin shelter under the hot sun. As a guideline, each 8°C (15°F) rise in temperature cuts the life of a sealed lead acid battery in half.