Why your phone battery drains faster when the weather turns cold

It happens every winter: you step outside with a phone at 40 percent, take a few photos in the snow, and the screen suddenly goes black. The battery is not broken. It is simply cold.

Smartphones run on lithium-ion batteries, which store energy by moving lithium ions between two electrodes through a liquid electrolyte. When temperatures drop, that electrolyte becomes more sluggish and the chemical reactions slow down.

Why the phone switches off early

As the reactions slow, the battery’s internal resistance rises. When the phone asks for a burst of power, for example to run the camera or the screen at full brightness, the voltage briefly drops. If it falls below a safety threshold, the phone shuts down to protect itself, even though the percentage on the screen said there was charge left.

The energy is still in the battery, it just can’t get out fast enough. Warm the phone up again and much of that missing charge reappears.

Dr. Sofia Reinholt, battery researcher at Elmbridge University

Below about zero degrees Celsius, the effect becomes much stronger. In real-world tests, phones lost a quarter or more of their usable capacity after an hour outdoors at minus 10.

The real danger: charging in the cold

Using a cold phone does no lasting harm. Charging it does. At low temperatures, lithium can build up as a metallic layer on the electrode instead of being stored inside it, which permanently reduces capacity. Many phones block fast charging when they detect cold, but it is still best to let a frozen phone warm up before plugging it in.

Tomas Varga, who runs a phone repair shop in Greywater, sees the results every winter. “Skiers charge their phones in a freezing car, then wonder why the battery is weak by spring,” he said.

How to keep your phone going

• Carry it in an inside pocket, close to your body.
• Use a case: even a thin one slows heat loss.
• Turn on low power mode and lower the screen brightness before long trips outside.
• Let a cold phone warm up gradually at room temperature before charging. Never put it on a radiator.
• Keep a small power bank in a warm pocket for long days in the mountains.