Calendar Ageing

Calendar ageing is the capacity loss of the battery with time and without cycling. This is a fundamental difference to the more mechanical systems that we deal with respect to lifetime predictions. In most traditional mechanical systems such as an engine the lifetime is proportional to the number of hours it is operational. When the engine is switched off there is no ageing except cosmetic.

The battery chemistry ages even when not being charged and discharged. At elevated temperatures >35°C and at elevated state of charge >70% the ageing can be significant. This ageing can be reduced to a minimum by storing the cells at 10-15°C and at <50% SoC.

Whilst be transported in a container on a ship the cells are likely to be at elevated temperatures for some time. Hence they are likely to have lost some capacity.

Lifetime

The standard method used by industry has two fundamental test regimes as inputs:

  1. calendar ageing – where the cells are held at different fixed temperatures and SoC.
  2. cycle ageing – where the cells are cycled through a complete charge discharge at different fixed temperatures.

At regular intervals the capacity of the cells are measured. This data is then used as inputs to the ageing predictions, a simple summation of these two parts gives the overall ageing model [1].

References

  1. T. M. N. Bui, M. Sheikh, T. Q. Dinh, A. Gupta, D. W. Widanalage and J. Marco, “A Study of Reduced Battery Degradation Through State-of-Charge Pre-Conditioning for Vehicle-to-Grid Operations,” in IEEE Access, vol. 9, pp. 155871-155896, 2021