Smartphone owners warned: 35-degree rule critical as heatwaves threaten batteries

As heatwaves become more frequent, smartphone makers are urging users to protect their devices from extreme temperatures.

Companies such as Apple highlight a simple guideline often called the 35-degree rule, aimed at preventing irreversible battery damage.

In practice, this means avoiding using or storing a phone in environments where temperatures rise much above 35 °C for extended periods. Modern phones rely on lithium-ion batteries that are highly sensitive to heat and can degrade quickly if exposed to it.

According to Apple’s support guidance, high temperatures can cause a phone to slow down, drain faster or suddenly switch off to protect its internal components. In severe cases, the battery may permanently lose a portion of its capacity, even if the device still reports 100 percent charge.

Experts explain that repeated overheating accelerates chemical aging inside the battery cells. Over time, users may notice that their phone lasts only a fraction of the day on a full charge, forcing more frequent top-ups and shortening the overall life of the device.

Heat does not come only from the weather. Charging a phone in a hot room, using it under direct sunlight, or running heavy tasks such as gaming and high-resolution video streaming for long periods can all drive temperatures up. When this happens, the device may feel unusually hot to the touch.

One of the most dangerous locations is a parked car in the sun, where interior temperatures can rise far beyond safe operating limits within minutes. If the cabin climbs above roughly 45 °C, a smartphone left on a seat or dashboard can suffer immediate and lasting damage.

To reduce risk, users are advised to keep phones out of direct sun, avoid leaving them in hot vehicles and pause intensive apps if the device becomes very warm. Letting the phone cool before charging is also recommended during hot weather.

Apple encourages iPhone owners to check battery health regularly in the settings menu. The maximum capacity indicator shows how the battery compares to its original state and may reveal early impact from heat or heavy use.

Taking simple precautions to follow the 35-degree rule can significantly extend a smartphone’s lifespan. Ignoring these warnings, however, may lead to costly battery replacements or even the need to buy a new device much sooner than expected.