Cell Cycle Lifetime

When we consider the lifetime of a battery cell we normally look at the cell cycle lifetime and the cell calendar lifetime.

The cell cycle lifetime is the number of charge and discharge cycles that a battery can complete before losing performance. This cycle lifetime is affected by a number of parameters:

  • Temperature
  • Usable SoC Window
    • upper and lower SoC limits
  • Charge Rate
  • Discharge Rate
  • Temperature Gradient
battery cycle ageing curves

Initially tests are run from 0% to 100% to 0% with a charge/discharge c-rate typically of C/3.

Note: the upper and lower SoC limits are a better way of characterising the cycle as it matters where the usable window is positioned with respect to the upper and lower voltage limits. An NMC cell is likely to age faster if it is cycled between 70 and 100% compared to 40 to 70%.

Battery Capacity versus number of cycles showing the Knee Point

Knee Point

The knee point describes a sudden change in the gradient of a cell cycling curve.

The impact of this nonlinear degradation in cell capacity depends to some extent on the architecture and application, but often results in an early failure of the pack.

References

  1. Selcuk Atalay, Muhammad Sheikh, Alessandro Mariani, Yu Merla, Ed Bower, W. Dhammika Widanage, Theory of battery ageing in a lithium-ion battery: Capacity fade, nonlinear ageing and lifetime prediction, Journal of Power Sources 478 (2020) 229026

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