Cycle ageing

Cycle ageing is the wear caused by charging and discharging the battery. It depends on how much energy passes through the cells, the depth and position of each cycle within the SOC window, the current and the temperature.

Wear from moving lithium

Every time lithium ions shuttle between the electrodes, the materials expand and contract slightly. Over many cycles this mechanical stress can crack electrode particles, expose fresh surface on which new SEI forms, and gradually reduce the amount of active material. Charging at high current or low temperature adds the risk of lithium plating on the anode.

Researchers usually link cycle ageing to energy throughput: the total amount of charge moved through the battery. All else being equal, a battery that is driven more experiences more cycle ageing.

Depth and position of cycles

Studies on NMC and LFP cells found that cycles over a smaller SOC range (a lower depth of discharge, DoD) generally cause less wear per unit of energy than large, deep cycles. Where in the SOC window a cycle sits also matters: cycling around the middle of the range tends to be gentler than cycling that repeatedly reaches very high or very low SOC.

The size of these effects differs considerably between chemistries. Comparative testing of commercial cells showed that LFP cells typically tolerate many more cycles than NMC or NCA cells under similar conditions, and that each chemistry responds differently to DoD, temperature and C-rate.

Current and temperature

Higher charge and discharge currents (higher C-rates) generally accelerate cycle ageing, especially in combination with low temperatures. Cycle-life models for LFP therefore express capacity loss as a function of throughput, temperature and C-rate together, rather than any single factor in isolation.

Cells that look identical can still age at noticeably different rates. Large cycling datasets show substantial cell-to-cell variation, and some cells reach a 'knee' after which capacity drops faster. Predicting exactly when such a knee occurs remains an active research topic.

Putting it in context

For many private drivers with moderate annual mileage, calendar ageing can be as large as or larger than cycle ageing. High-mileage users such as taxis and fleets cycle much more, so cycle ageing becomes a bigger share. Voltgevity estimates cycle ageing from your annual distance, energy use and charging pattern, and combines it with calendar ageing rather than treating either in isolation.

Key takeaways

Cycle ageing scales mainly with energy throughput, but depth, SOC position, current and temperature change the wear per kWh.
Shallower cycles around mid-SOC tend to be gentler than deep cycles to the extremes.
LFP generally has a longer cycle life than NMC or NCA, though each chemistry responds differently.
Individual cells vary, and late-life knees make long-term prediction uncertain.