  Ageing of lithium-ion batteries results in irreversible reduction in
performance. Intrinsic variability between cells, caused by manufacturing
differences, occurs throughout life and increases with age. Researchers need to
know the minimum number of cells they should test to give an accurate
representation of population variability, since testing many cells is
expensive. In this paper, empirical capacity versus time ageing models were
fitted to various degradation datasets for commercially available cells
assuming the model parameters could be drawn from a larger population
distribution. Using a hierarchical Bayesian approach, we estimated the number
of cells required to be tested. Depending on the complexity, ageing models with
1, 2 or 3 parameters respectively required data from at least 9, 11 or 13 cells
for a consistent fit. This implies researchers will need to test at least these
numbers of cells at each test point in their experiment to capture
manufacturing variability.
