I've noticed my iPhone 15 loses like 20% battery just walking to work in winter, but in summer it lasts way longer. Is this normal or is my battery dying? I'm trying to figure out if I should get it replaced or if it's just physics.
It's totally normal physics. Lithium batteries don't like cold - when temperature drops, the chemical reactions inside slow down, which means the battery can't deliver power as efficiently. Your phone's reporting lower capacity than it actually has, and it drains faster because it's working harder to keep up with your usage while fighting the cold. Once you get it back inside where it's warm, the battery voltage bounces back and things stabilize again.
That said, there's a difference between temporary cold-weather drain and an actually dying battery. If your iPhone is newer and this only happens when it's freezing outside, you're probably fine. But if you're seeing crazy drain even indoors at normal room temperature, or if your phone shuts itself down at like 30% battery regularly, then yeah, the battery might actually be degrading and you should look into a replacement. Most phones can handle a few seasons of winter before the battery starts genuinely failing.
The practical move: keep your phone close to your body when you're out in the cold - pocket, inside jacket - and charge it indoors before heading out. That'll make a noticeable difference. If you're still seeing those big drops even when you keep it warm, then consider getting Apple to check the battery health. They can run diagnostics and tell you if it's actually worn out or just being a typical lithium battery in winter.
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