It’s Not Just You, Heat Is Making Our Smartphone Batteries Worse. Here’s Why.

Updated Aug. 9, 2023
By guest author Joanna Stern from the Wall Street Journal.
Smartphone gen 1 Heat is enemy No. 1 for smartphone batteries and can lead to irreversible damage. But at what temperature will you get a warning? And what can you do to limit the impact? WSJ’s Joanna Stern heated up an iPhone and a Samsung Galaxy S to get the answers. Photo illustration: Nayon Cho for The Wall Street Journal
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This just in: iPhone and Android phone temperatures are soaring into the triple digits in some regions, causing nonstop heat-warning pop-ups and irreversible battery damage. With scorching weather and everyone looking to fast-charge their devices, global smartphone warming shows no sign of abatement. Back to you, Joanna.
Thanks, Chuck.
Sure, it’s frustrating to get an alert that your phone is too hot to use after it has been baking in a car cup holder or lounging at the pool. But the longer-term impact is worse: It can damage your battery, and your phone won’t hold a charge as long.
Yes, heat is enemy No. 1 for batteries, according to every phone maker and battery expert I’ve ever spoken to.
It isn’t just sweltering summer temps that cause heat damage. Fast-charging and processor-intensive tasks such as gaming or recording video can make your phone feel hot. If your phone starts overheating, it will tell you and limit some functions.
But how hot is too hot? At what temp are you likely to get the alert that your phone needs to chill? Is there anything you can do to prevent a phone from getting so, in the words of everyone’s least favourite wedding song, “hot, hot, hot”?

Apple’s heat warning prevents you from using the phone until it cools. Samsung lets you use the device, but features will be limited.
There was only one way to get answers: a buffet food lamp, of course. It stood in for the summer sun as I heated up an iPhone 13 Pro and Samsung SSNHZ 0.00 % increase; green up pointing triangle
Galaxy S23 Ultra numerous times. I also tested some heat-prevention methods, including a phone fan. (Yes, that’s a thing.)
What did I find out? Recording video while in the sun turns your phone into a hot plate, some thermal bags will melt like a grilled cheese, and a whole lot more.
What does heat do to batteries anyway?
I’ll spare you the chemistry class, but there are complicated reactions that occur in a lithium-ion battery. As you increase the temperature, the speed of that chemical reaction also increases. That can cause the battery to drain faster and impact the battery’s capacity.
You can think of battery capacity like a gas tank in your car, said Michael Pecht, a mechanical-engineering professor at the University of Maryland. (An ironic analogy given that batteries are starting to replace gas tanks, but let’s go with it.)
Smartphone 3
So, what can I do?
Simple: You become a shut-in, maintain a perfect 70 degrees in your home and always charge very, very slowly. What, not doable? Life happens—and so does heat. Here are some more practical ideas:
• Thermal bag: If you’re going to the beach or pool, store your phone in a thermal bag. But not just any thermal bag. One I bought on Amazon hit 320 degrees under the heat lamp and started smoking. (The company that made it didn’t respond to my requests for comment.) The USD 30 Phoozy Apollo pouch, on the other hand, hit around 105 degrees and kept the phone cool.

Smartphone 4 Is that a rave on the back of your phone-or an LED-decorated phone fan from Razer? Photo: Kenny Wassus/The Wall Street Journal
• Phone fan: The downside of a bag is that you can’t actually use your phone. Hence the phone fan, like the USD 60 Razer Phone Cooler Chroma that attaches to the back of your device. Even after 15 minutes under the heat lamp, the iPhone with its fan-backpack (fan…ny pack?) was still cool to the touch—with no sign of a temperature warning. The downside? The fan itself needs some of that battery juice. Plus, you’re now that person with a fan on your phone. If you’re in a car, you could always crank up the AC.
• Remove cases: I mean, do you go to the beach wrapped in rubber and leather? Certain types of cases can trap heat during charging, Apple says. I did try a specially ventilated case under the heat lamp but…it melted.
• Limit fast-charging: I’d like to tell you not to charge fast, especially in the hot weather, but the phone is going to limit that for you anyway. Both Apple and Samsung use software to balance fast-charging with battery longevity. And realistically, most of us will probably opt for the convenience of fast-charging over spending our lives babying our batteries. Still, if you want to play it safe, Samsung lets you turn off fast-charging in its battery settings. With an iPhone, you’d have to go back to using the tiny charging cube.
Stay cool, phones. Stay cool.