Why Phone Batteries Give Up In The Cold

A phone at a bus stop in winter drops from a third of its charge to nothing during a single call. Most of that charge was still there. It just could not be reached.

The chemistry slows as it cools

A lithium cell works by moving ions through a liquid electrolyte between two electrodes. That movement depends on the electrolyte remaining fluid and the ions remaining mobile.

Cooling thickens the electrolyte and slows diffusion, so ions take longer to cross and longer still to insert themselves into the electrode structure at the other side.

The stored energy has not gone anywhere. The rate at which it can be delivered has fallen, which is a different problem with the same symptom.

Resistance turns current into a voltage drop

Slower ion movement shows up electrically as higher internal resistance. Every cell has some, and in the cold it can rise substantially.

When the phone draws current, that resistance produces a voltage drop inside the cell itself. The harder the phone works, the further the terminal voltage falls.

A screen at full brightness on a weak mobile signal is close to the heaviest load a phone applies, which is exactly the situation that produces the sudden shutdown.

The gauge is inferring, not measuring

There is no direct way to count remaining charge in a cell. The percentage is estimated from voltage, current, temperature and a model of how the cell behaves.

When voltage collapses under load in the cold, the estimate follows it down, and the phone shuts off to protect the cell from being taken below its safe floor.

Bring the phone back inside, let it warm, and a large part of the charge reappears, which is the clearest evidence that it was never actually consumed.

Most of the capacity comes back

Cold-weather loss is temporary in a way that ageing is not. The cell recovers as it warms, and repeated cold discharge does comparatively little lasting harm.

Keeping the device against the body, in an inside pocket rather than an outer one, is enough to keep it in a workable range in most conditions.

Insulating cases help a little by slowing heat loss, though a phone generates modest heat and cannot warm itself back up quickly once it has cooled.

Charging cold is the part that causes damage

Discharging in the cold is largely reversible. Charging in the cold is not, because lithium can deposit as metal on the electrode instead of inserting into it.

Those deposits are permanent, reduce capacity and, in the worst case, threaten the separator between electrodes, which is why devices refuse to charge below a temperature threshold.

A phone that has been outside and declines to charge the moment it is plugged in is not malfunctioning. It is waiting for the cell to reach a temperature where charging is safe.