Why do smartphones lose battery capacity so quickly in the cold?

That's completely normal - lithium-ion batteries lose capacity at low temperatures because chemical reactions inside slow down. In the cold, electrons move more slowly, so the battery can't deliver energy as quickly as it does in warmth, and your phone shows a lower charge level even though the actual capacity hasn't disappeared. When you get back to warmth, the charge will come back - it's a temporary effect, and there's nothing bad for your battery's health in that.

It's physics of lithium batteries, yeah, but here's the thing: capacity loss in the cold isn't permanent damage, it's just a temporary drop in performance. Once your phone warms up indoors, the battery will "recover" most of its charge. That's why an iPhone shows 20% in freezing weather and shuts down, but then you turn it back on inside - and suddenly it's at 40%.

But there's an important threshold. Subzero temperatures freeze the electrolyte inside the battery, viscosity increases, ions move slower, and internal resistance skyrockets. In the cold, it physically can't deliver the current it needs to - that's why it dies in an hour. If you regularly put the battery through deep discharge cycles in freezing temps (when your phone shuts down because it "thinks" it's dead), that'll cause real damage over time. One or two walks in the cold - no big deal, but if it's every day - that's different, the battery degrades faster.

Practical tip: if you spend a lot of time in cold weather, keep your phone under your jacket close to your body or in an insulated case. And don't let the battery drain all the way to zero in the cold - better to head home with 30% than kill it outside.

That's exactly right - cold temporarily freezes the electrochemical processes in lithium-ion, and the battery can't discharge all its energy even though the charge is still there. The big warning: if you regularly use your phone in freezing temps, it'll actually speed up battery degradation in the long run. The battery can handle the cold and then bounces back, but constant stress cycles from temperature swings eat through its lifespan way faster than normal aging would.

I've had the same thing happen - when it's below freezing outside, my iPhone quickly shows low battery, and then once I'm home it seems to recover. From what I've read, it's exactly what people already mentioned above: in the cold, the chemical reactions in the battery slow down, so it just can't deliver all the energy that's actually there. But as soon as it warms up - the charge comes back, it's not wear and tear, it's just a temporary thing.