What does Calendar Aging mean?

Explanation

Calendar aging is the slow degradation of battery cells that happens simply because time passes, even if the car sits in a garage unused. Chemical reactions inside the cells gradually reduce capacity and increase internal resistance. This process accelerates at higher temperatures and higher states of charge.

The practical implication is that storing your EV at a very high or very low state of charge for extended periods increases calendar aging. If you are going on a long vacation and leaving your car parked, setting the charge limit to around 50-60% is ideal for minimizing degradation. Storing a fully charged car in a hot climate for weeks is the worst combination.

Calendar aging works alongside cycle aging (degradation from use). Most EV batteries lose 1-3% of capacity per year from calendar aging alone. Combined with cycling, typical total degradation is 2-4% per year. After 8-10 years, a well-maintained battery typically retains 80-90% of its original capacity.

Based on data from National Renewable Energy Laboratory.

Where you'll see this

• In vehicle specifications

Common confusion

People assume battery degradation only comes from charging and driving. Calendar aging means the battery slowly degrades even when parked. This is why storing an EV at moderate SoC in a cool environment is recommended for long-term storage.

Example

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