Accelerated Testing - Battery Design

Accelerated testing is how we get a useful read on a cell or pack’s multi-year field life without waiting multi-year field life to see it. One or more stress factors – temperature, current, depth of discharge – are pushed harder than the intended application, the resulting degradation rate is measured, and that rate is then related back to the milder conditions the product will actually see.

Stress Factors Commonly Increased

Temperature (storage or operating)
C-rate / current during charge or discharge
Depth of discharge (DoD) and the SoC window cycled
SoC held during storage (calendar ageing tests)

Temperature is the most common lever because the dominant side reactions behind calendar fade (SEI growth in particular) follow something close to Arrhenius behaviour – degradation rate roughly doubling for each ~10°C rise. That relationship is only a safe extrapolation tool within a limited window, though: push storage temperature much past the mid-60s°C and cells tend to pick up additional, non-Arrhenius degradation mechanisms that don’t occur in the field, which will skew the acceleration factor and any lifetime prediction built on it [1][2]. Good accelerated test design keeps stress levels inside the range where the same failure modes as real-world use are still active, not beyond it.

Risk: additional / non real-world ageing mechanisms is a real concern when accelerating the ageing of battery cells. The result can be very non-linear behaviour and hence models that are not representative of the real world ageing. However, waiting 10 years and testing thousands of cells in different conditions is also likely to result in the wrong decisions being made. There is a balance that needs to be made.

Calendar Ageing vs Cycle Ageing

Accelerated protocols usually separate calendar ageing (capacity/resistance drift while the cell just sits at a given SoC and temperature) from cycle ageing (drift driven by charge/discharge throughput). Real-world use is a mix of both, so a full accelerated test plan generally runs calendar-only and cycling-only cells in parallel and combines the results, rather than trying to infer one from the other. Cycle-driven ageing is also where Energy Throughput becomes the more useful tracking metric than raw cycle count, particularly for duty cycles with a lot of partial cycles.

Sub-topics

Performance Testing – the capacity/power/impedance checks typically run before and after an ageing campaign to quantify the drift
Fast Charging limits – high C-rate charge is one of the more aggressive accelerants, and one of the most likely to trigger lithium plating rather than the ageing mechanism you’re trying to characterise
Ageing – a dedicated page on ageing mechanisms and models is still to be written; this page will link to it once it exists