On a cold morning your phone can lose charge surprisingly quickly, even if you have hardly used it. The battery has not suddenly lost all of its stored energy. The problem is that temperature changes how easily the chemical and electrical processes inside the battery can happen.

Most phones use lithium-ion batteries. When the battery supplies energy, lithium ions move through the electrolyte inside the cell while electrons travel around the external circuit. That movement of charge produces an electric current. The reactions depend on particles moving and interacting, and colder particles have less kinetic energy on average. Ion movement slows and the reactions at the electrodes become less effective.

Cold also increases the battery's internal resistance. When the phone demands a large current—for example while gaming, recording video or using the camera flash—more of the battery's potential difference is effectively lost inside the cell. The terminal voltage can therefore fall below the level the phone needs, even though chemical energy remains stored in the battery. The phone may shut down to protect its electronics.

This connects several familiar electrical ideas. Current is the rate of flow of charge, potential difference is energy transferred per unit charge, and electrical power is P = IV. If the battery cannot maintain its voltage while supplying the required current, it cannot deliver the power the phone is asking for. Some of the energy is also transferred as heating inside the battery because of its internal resistance.

When the phone warms up, the reactions and ion movement become easier again, so some of the apparently missing capacity can reappear. Engineers have to design around this effect. Electric cars heat and cool their battery packs, outdoor cameras and drones specify operating-temperature ranges, and battery-management systems continuously monitor current, voltage and temperature. Heat is not simply 'better', though: high temperatures can speed up unwanted chemical reactions and permanently age a battery. Batteries work best within a carefully controlled temperature range.