The temperature at which batteries freeze involves a distinction between temporary performance loss and permanent physical damage. The primary issue most users experience in cold weather is not the physical freezing of the entire unit, but a significant decline in power output and capacity. True freezing, where the electrolyte transitions into a solid state, represents a failure point that can irreparably damage the battery structure. This outcome depends heavily on the battery’s specific chemical composition and its state of charge.
Cold Weather Slows Chemical Reactions
Before any physical freezing occurs, the electrochemical processes inside a battery slow down as the temperature drops. All batteries rely on the movement of ions through an electrolyte to generate current. Low temperatures increase the electrolyte’s viscosity, making it thicker and less conductive, which hinders the mobility of these ions.
This sluggish ion movement creates a reduction in the battery’s usable capacity and power output. The internal resistance of the battery rises, meaning more energy is wasted as heat, and less is available to power a device. For example, a lithium-ion battery might only deliver 50% of its rated capacity at \(-20^\circ\text{C}\) compared to its performance at room temperature. The battery is not permanently damaged in this state; performance returns to normal once warmed up.
Specific Freezing Temperatures for Common Battery Chemistries
The temperature at which a battery’s electrolyte physically freezes varies significantly based on its chemistry and state of charge. Lead-acid batteries, commonly found in cars, are the most susceptible to freezing when discharged. The electrolyte in a fully charged lead-acid battery is a dense solution of sulfuric acid, which has a very low freezing point, often below \(-60^\circ\text{C}\) (\(-76^\circ\text{F}\)).
However, as a lead-acid battery discharges, the sulfuric acid reacts with the plates, leaving the electrolyte increasingly diluted and water-like. A fully discharged lead-acid battery, whose electrolyte is primarily water, can freeze at temperatures as high as \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). If the battery is partially discharged, for instance to a 40% state of charge, the freezing point rises to approximately \(-16^\circ\text{C}\) (\(3^\circ\text{F}\)). The expansion of the ice inside the casing can crack the battery case, bend the internal plates, and cause irreparable failure.
Alkaline batteries, which use a potassium hydroxide electrolyte, are less affected by their state of charge. Their electrolyte generally freezes between \(-18^\circ\text{C}\) and \(-28^\circ\text{C}\) (\(0^\circ\text{F}\) and \(-18^\circ\text{F}\)). Once frozen, the expansion can still rupture the casing and cause leakage. Lithium-ion batteries, which contain organic carbonate electrolytes, typically have the lowest physical freezing points, often around \(-30^\circ\text{C}\) (\(-22^\circ\text{F}\)). However, the temperature at which they suffer serious damage is much higher and is related to charging.
Why Charging Batteries When Cold Causes Irreversible Damage
The most common cause of permanent cold-weather damage to lithium-ion batteries is attempting to charge them below \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). Many modern battery management systems prevent charging below this threshold entirely. When a lithium-ion battery is charged, lithium ions are supposed to intercalate, or embed themselves, into the graphite anode material.
When the temperature is too low, the intercalation process is dramatically slowed, and the lithium ions cannot be absorbed quickly enough by the anode. Instead of entering the anode structure, the ions deposit as metallic lithium plating on the anode’s surface. This process, known as lithium plating, is irreversible and reduces the battery’s capacity.
The metallic lithium deposits can also grow into sharp, needle-like structures called dendrites. These dendrites can eventually pierce the separator material between the anode and cathode. A puncture creates an internal short circuit, which can lead to overheating, thermal runaway, and a fire hazard. Cold charging is detrimental to the longevity and safety of a lithium-ion battery.
Protecting Batteries in Extreme Cold
To protect any battery type from cold damage, maintain its temperature above the risk threshold. For lead-acid batteries, the primary defense is keeping them fully charged, which ensures the electrolyte has the lowest possible freezing point.
For lithium-ion devices, the best strategy is to keep them insulated and warm, such as carrying them in an inner pocket close to the body. If a device has been exposed to the cold, it should be warmed to above \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) before connecting it to a charger. Some batteries designed for extreme environments include internal heating elements that automatically warm the cells to a safe charging temperature before the charging current is applied. Consult the manufacturer’s specified temperature range, especially the minimum temperature allowed for charging, to prevent irreversible damage.