Calendar Aging
Calendar aging is battery degradation caused by the passage of time and storage conditions, including periods when an EV is parked and not being driven.
What drives calendar aging
Chemical reactions continue inside lithium-ion cells while the battery is at rest. Their rate depends strongly on temperature, state of charge, cell chemistry, and time. Long periods at high temperature or high state of charge can accelerate capacity loss and resistance growth, although the exact response differs among cell designs.
Calendar aging is not the same as Battery Degradation from charge-discharge use. A low-mileage battery can still age while stored, and a frequently used battery experiences calendar and cycle aging at the same time. Real-world battery health therefore reflects both mechanisms rather than a clean split between them.
For the underlying degradation mechanisms and protective operating limits, see Battery Degradation.