Lifetime

For primary cells the shelf storage time or calendar ageing discharge rate is the most important factor with respect to lifetime as this will determine how long you can store the cell before using it.

For secondary cells or rechargeable cells we are interested in the calendar ageing and the cycle ageing. The calendar ageing will show how the capacity reduces with time, even when the battery cell is not being used. The cycle ageing will show you how many cycles the cell can deliver at a given charge and discharge rate. As a rule of thumb we would expect to get 1000 complete cycles for an energy cell and 3000 complete cycles for a power cell before the capacity dropped to 80% of the original Ah value when new.

Ageing of a battery manifests itself in two ways:

  1. Capacity fade – the available energy at a defined discharge rate will decrease.
  2. Increasing internal resistance – as the internal resistance increases the voltage will drop lower under load. Hence the available power before hitting the minimum voltage will decrease. Also, the I2R heating of the cell will increase.

Causes of degradation:

  • Time
  • High Temperature
  • High Voltage/SoC
  • Current Load
  • Low Temperature
  • Stoichiometry
  • Mechanical Stress
  • Low Voltage/SoC

Ageing Predictions

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.

References

  1. Birkl, Christoph & Roberts, Matthew & McTurk, Euan & Bruce, Peter & Howey, David. (2017). “Degradation diagnostics for lithium ion cells.” Journal of Power Sources. 341. 373-386. 10.1016/j.jpowsour.2016.12.011.
  2. Weilong Ai, Billy Wu, Emilio Martínez-Pañeda, A coupled phase field formulation for modelling fatigue cracking in lithium-ion battery electrode particles, Journal of Power Sources, Volume 544, 2022, 231805, ISSN 0378-7753
Tesla Model S/X SoH vs mileage

Mileage Equals Wear

When we look at this picture, we see the over-simplicity of the ‘mileage equals wear’ mindset. Figure 1 shows reported SOH as a function of miles-driven for Nissan Leaf EVs. Similar distributions are reported by Myall and co-authors [1] who analysed 1382 reported SOH data points from 283 Nissan Leaf EVs manufactured between 2011 and 2017. At 60,000 miles, the discrepancy in reported SOH is almost 30%. How is it possible that two identical EVs, with the same odometer reading have such different SOH?