Calendar ageing

Calendar ageing is the capacity a battery loses simply with the passing of time, even while the car is parked. It is driven mainly by temperature and by the state of charge the battery spends its time at.

Ageing while standing still

Most cars spend the large majority of their life parked, so for many drivers calendar ageing is a major share of total degradation. Even with no current flowing, slow side reactions at the electrode surfaces continue, mostly growth of the SEI layer on the graphite anode. Each bit of SEI growth binds lithium that can no longer be used to store energy.

Accelerated storage tests show that calendar capacity loss typically slows down over time. A common approximation is that loss grows with roughly the square root of time, because a thicker SEI layer partly shields the electrode from further reaction.

Temperature sets the pace

The rate of these side reactions rises with temperature, a relationship usually described with the Arrhenius equation from chemistry. In practice, a battery stored warm ages noticeably faster than the same battery stored cool. The effect is not linear: each additional degree weighs more heavily at higher temperatures.

This is why climate and parking location matter. A car parked in direct summer sun or in a hot region spends more hours with a warm pack than a car parked in a cool garage.

State of charge matters, in steps

Storage studies on NCA, NMC and LFP cells found that calendar ageing depends strongly on the SOC at which the cells are kept. The relationship is not a smooth line: ageing rates show plateaus and steps that line up with stages in the electrical potential of the graphite anode. Below certain SOC ranges ageing is relatively slow; above them it steps up.

The exact position and size of these steps differ per chemistry and cell design. LFP cells generally show a weaker SOC dependence than nickel-based cells, but higher SOC and higher temperature still tend to accelerate their calendar ageing.

How Voltgevity models it

Voltgevity models the time your battery spends at each SOC level and at its estimated pack temperature, then applies a calendar ageing rate that depends on both. Pack temperature is estimated from your climate, parking situation and the vehicle's thermal management. Because individual cells vary, results are shown as ranges rather than single precise numbers.

Key takeaways

Calendar ageing continues while the car is parked and is a large share of total ageing for many drivers.
Calendar loss usually slows over time, roughly following the square root of time.
Higher temperature and longer time at high SOC both accelerate calendar ageing.
The SOC effect comes in steps linked to the graphite anode, and its size differs by chemistry.