  Li-ion batteries gradually lose their capacity with time and use; therefore,
ageing forecasts are key to designs of battery powered systems. So far,
cell-type-specific studies without standardised testing practices have lead to
a variety of ageing models in which generality, simplicity, and accuracy seem
exclusive. Previous studies hint to an interplay of multiple mechanisms leading
to capacity loss, which depend on cell chemistry and are affected by
temperature, state of charge, and cycling rate. Here we show that, despite this
complexity, the time dependence of the actual capacity follows a unique master
curve, for several cell types aged under various different conditions. We
discuss the statistical origin of this common behaviour, and the testing
practice required for the characterisation of a model. The master curve is a
stretched exponential that describes many other phenomena in nature and is
theoretically justified within a diffusion-to-traps depletion model. These
findings provide a simple and broadly applicable framework for accurate
life-time predictions.
