In Extreme Temperatures
Besides water or liquid, there are few things worse for an electronic device than heat. Once an iPhone reaches a certain temperature threshold, it will stop working properly to protect itself. But even before it hits that threshold, you could still run into problems and permanently damage your battery if you use it while it’s too hot.
You shouldn’t keep your iPhone on your dashboard, beach towel or anywhere else where it’s in direct sunlight. Keep it out of your glovebox, too. And ensure that your iPhone is never in a hot environment when it’s charging.
On a somewhat related note, it also pays to be mindful of cold temperatures, too. It isn’t the cold itself that can damage your smartphone, but subjecting an iPhone to a rapid difference in temperature (like when moving from an indoor environment to an outdoor one) that can lead to problems.