The chemical reactions inside lithium-ion batteries slow down at low temperatures, and that slowdown reduces how much current the battery can deliver instantly. As a result, a phone can shut off suddenly in cold weather even when it technically still has usable energy left.
This is usually a temporary performance dip rather than permanent damage; once the phone warms back up, the battery percentage reading typically returns closer to the real value. That said, batteries repeatedly left in extreme cold can see a small permanent capacity loss over time.
Charging in cold weather deserves caution too; fast charging below freezing can stress the battery's internal structure, since lithium ions have a harder time settling properly onto the anode at low temperatures. This is why some phones automatically cap charging speed in very cold conditions.
Practical tips include carrying the phone in an inner pocket rather than an outer one, not leaving it outside for long stretches, and letting the device warm up before charging when possible. The same logic applies to power banks carried in winter; keeping one in extreme cold temporarily reduces its output.
Charjett's chargers are tested across a wide temperature range to stay stable in cold conditions, but since no charger can override a battery's own chemical limits, taking simple precautions on the user side is still the most effective approach.