EV performance records document real-world battery degradation rates

A vehicle fleet website (geotab.com/blog/ev-battery-health/) has compiled 6000 vehicle performance records on EVs to assess battery durability of differing models in actual usage. You can use their free EV Battery Degradation Tool (geotab.com/fleet-management-solutions/ev-battery-degradation-tool/) for your EV, or any other model that you might consider buying. They found an average of 2.3%/year range loss for the EVs on the road today. This means that after 10 years, range will be 23% less than what it was when it rolled off the assembly line. They conclude, “If the observed degradation rates are maintained, the vast majority of batteries will outlast the usable life of the vehicle.”

This data set was large enough to quantify the effect of various performance enhancers or degraders. The largest degrader was temperature: “A major distinction is if the battery pack is cooled and/or heated by air or by liquid.” The air-cooled Nissan Leaf has an average degradation rate of 4.2%, whereas the liquid-cooled Tesla Model S is 2.3%. I was surprised to see that the Model S was only average, whereas the Chevy Volt was much better, at 1.4%.

One reason for the Volt’s better battery longevity is that the Volt has very conservative (i.e., large) state-of-charge buffers; the battery is not allowed to be fully charged (top buffer) nor fully discharged (bottom buffer). Some EVs allow the user to adjust the size of these buffers (within limits), but shrinking the state-of-charge buffers reduces battery longevity. Other observations:

  • Hard use (many miles driven per month) reduces longevity per unit time, but only by a minuscule amount. Using an EV is almost always better than getting out the non-EV car.
  • Hot weather significantly shortens battery longevity (unfortunate for Phoenix EV drivers; relatively beneficial for Colorado EV buyers).
  • Much of the instantaneous cold-weather range loss is due to using the climate control for the passengers and the battery; use heated seats (75 watts) instead of heated cabin air (3000-5000 watts) whenever practical.
  • Level II (220 Volts) charging is the best; Level I charging (110 Volts) is ever so slightly worse. The difference, however, is too small to be concerning.
  • Fast-charging shortens battery life appreciably; don’t’ fast charge (DC chargers) unless necessary.

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