Time at state of charge

For calendar ageing, what matters is not whether your battery reaches a high SOC, but how long it stays there and how warm it is. Charging to 100 % shortly before a trip adds little time at high SOC; leaving the car at 100 % for days adds much more.

Why high SOC adds stress

At high SOC the graphite anode sits at a low electrical potential, which makes side reactions with the electrolyte, and thus SEI growth, more likely. Life models that link SEI growth to anode potential capture this effect, and storage experiments show ageing rates stepping up in SOC ranges tied to graphite's potential plateaus. In nickel-rich cathodes, high SOC can additionally promote structural changes and oxygen release, especially at elevated temperatures.

Studies that limited the maximum SOC in use found that avoiding long periods at high SOC can extend battery lifetime. How large that benefit is depends on chemistry, temperature and how much time is actually spent there.

Reaching 100 % versus sitting at 100 %

A persistent myth says that 'charging to 100 % is bad'. Calendar stress accumulates over time, so a battery that reaches 100 % and is driven soon afterwards spends only a short time in the high-SOC zone. The same battery left at 100 % in a warm garage for a week accumulates far more high-SOC hours.

The opposite myth, '80 % is always best', is also too simple. Holding 80 % around the clock is gentler than holding 100 %, but if you only need a high SOC occasionally, a charge that finishes close to departure achieves a similar effect. For some drivers a lower daily target, such as 60 or 70 %, adds further benefit; for others the convenience cost outweighs a modest gain.

Charge timing as a tool

Many cars and wallboxes let you set a departure time or charging schedule. Finishing the charge shortly before you leave, instead of right after plugging in, reduces time at high SOC without reducing the range you start with. This is often a battery-preserving alternative with little practical cost.

Voltgevity models every hour of a representative year at each SOC level, based on your charging routines, targets, plug-in times, departure delays and driving. Combined with the estimated pack temperature, this gives the calendar stress contribution of each charging strategy.

LFP and calibration

LFP cells are less sensitive to high SOC than nickel-based cells, although higher SOC and temperature still tend to increase their calendar ageing. Because LFP has a very flat voltage curve, the BMS can find it harder to estimate SOC accurately. LFP vehicles are therefore often advised to reach a full charge periodically so the BMS can recalibrate; check your vehicle's documentation for the specific guidance.

Key takeaways

Time at high SOC, combined with temperature, drives calendar stress, not the act of reaching 100 % itself.
Charging to 100 % just before departure adds little high-SOC time.
'80 % is always best' and 'charging to 100 % is bad' are both oversimplifications.
LFP is less SOC-sensitive and often benefits from periodic full charges for SOC calibration.