Meaning
Electrochemical process of driving current into a battery cell when the internal temperature is below the freezing point of water. Performing subzero charging without specialized heating circuits leads to the formation of metallic lithium on the surface of the anode. This condition occurs because the diffusion of lithium ions into the graphite lattice becomes extremely slow at low temperatures.
Lithium Plating
Instead of moving into the electrode, the ions receive electrons at the surface and turn into solid metal. This byproduct of subzero charging can grow into needle-like dendrites that eventually pierce the separator. A short circuit caused by these growths presents a fire risk and permanently degrades the health of the cell.
Thermal Management
Modern battery packs include heaters that raise the temperature of the cells before the charging cycle begins. Systems that detect subzero charging conditions will automatically limit the current to a very low value to minimize the plating effect. This control logic is essential for vehicles operating in cold climates to prevent damage during overnight charging.
Capacity Loss
Repeated exposure to these conditions reduces the amount of active lithium available for energy storage. Evidence of subzero charging is often found during a teardown analysis as a white film on the anode surface. The resulting increase in internal resistance makes the battery less efficient and reduces the driving range of the vehicle.
Monitoring the cell temperature during the start of the charge cycle is the only way to avoid these irreversible chemical changes.
