Mileage equals Wear

Mileage equals Wear

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Authors: Dr Kotub Uddin & Nigel Taylor

Mileage equals Wear
Figure 1: Nissan Leaf – measured SOH in the field. Note, the 24kWh model is warrantied for 5 years/60k miles while the 30kWh model is warrantied for 8 years/100k miles.

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?

We now know that the SoH algorithm for the 30kWh Nissan Leaf was re-calibrated [2], correcting what appeared to be very fast degradation of the pack. Another demonstration as to just how hard it is to predict SoH and validate any algorithm.

Now we look at the Tesla Model S/X data collected and published by the the Dutch-Belgium Tesla Forum [3]. Firstly, we need to note that Remaining Range as a percentage of original range is equivalent to SoH.

Tesla Model S/X SoH vs mileage
Figure 2. Tesla Model S/X Remaining Range vs Mileage

What we immediately see is that the Tesla Model S/X batteries are degrading less against mileage than the Nissan Leaf. Interesting, but the Tesla has a much bigger range, in fact more than 3x the range of the Nissan Leaf. This means to achieve the same odometer reading the Leaf has to be cycled 3x more than the Model S or X.

There are other significant factors in the design of the Tesla:

  • It is actively cooled. The Nissan Leaf has no means to reduce the pack temperature after a hard drive or a fast charge event, meaning the cells remain hotter for longer.
  • In normal driving we all accelerate, decelerate and drive in a similar fashion, no matter how fast the car is, power levels are then very similar. Average speeds in inner London are just 11.6mph [4]. Hence, in this city type driving the larger battery of the Tesla is likely to see less stress in terms of C-rate.

The answer: beyond mileage, other ‘stress-factors’ such as cell temperature, state of charge (SOC), depth of discharge (DOD), charging/discharging rates, usage path also impact the health of the battery. Plus we are still facing challenges in predicting SoH with any accuracy or consistency.

References

  1. Daniel Myall, Dima Ivanov, Walter Larason, Mark Nixon, Henrik Moller, “Accelerated reported battery capacity loss in 30 kWh variants of the Nissan Leaf“, PrePrints
  2. Flip the Fleet, 30 kWh Nissan Leaf firmware update to correct capacity reporting
  3. Tesla Motors Club – Dutch-Belgium Forum
  4. Average Traffic Speeds in London, London Assembly
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