The used EV market is not yet fully established and there are many reasons for it. Most notably, the volume required for a thriving secondary market do not exist. Since there are not as many older EVs on the road, as the overwhelming majority have been produced in the last four years, fewer people are trading in or selling today. That will resolve itself naturally. 

The more complicated factor is EV valuation. The rules for buying used electric cars are totally different from buying a used car with a combustion engine. Checking the motor oil or benchmarking the odometers either are not an option or don’t matter as much for electric cars. (You might have seen that on the home page!)

The high voltage lithium ion batteries that power electric cars are the priciest and most critical component of an EV. It’s well documented at this point that batteries degrade with both time and use. Getting useful heuristics to understand how they degrade and how that affects range is an important step in breaking through the used EV market.

While we know that battery reports are one part of the solution, they do not answer the common follow-up question: What else is important to understand about battery aging? 

Understanding Calendar Aging

Recurrent and other battery scientists have long understood the correlation between calendar age and overall battery health. Our new data, however, shows the importance of calendar aging, as opposed to model year, in terms of EV range degradation. 

Model year is traditionally applied to car value. Some of that probably includes the features that would be included in more recent models, but most of the model year variable is about presumed vehicle condition. Older models have likely seen more miles, more winter weather and use. 

Now let’s examine EV range. Range degradation data – how far a vehicle can travel on a charge – is an easy-to-measure metric that can be used as a proxy for battery degradation. It is hard to observe the chemical battery degradation directly while a battery pack is in the car. Using range is a way to compare current performance to original (or new) performance. 

In the charts below, you see the same range degradation data plotted on the left and on the right. However, on the left hand side, the x axis uses “model year” to represent vehicle age while on the right hand side, manufacture date is used. You can see how much smoother and more continuous the degradation curves are in the right hand plots - this illustrates the importance of calendar aging in EV range predictions. Range degradation is a factor of calendar age, not model year. 

In this data, “model year” is defined as January 1 of the year associated with the vehicle. The manufacture date is estimated from proprietary vehicle data or verified from photos of the manufacture date sticker on the car. 

This also underscores an important takeaway in terms of evaluating used EVs: model year and odometer are less important than calendar age. 

A great example of calendar age being the most important “age” in terms of EVs is the range prediction data we’re seeing on Chevy Bolt in the Recurrent community. The chart below shows the same cars a year apart, but with new batteries under the GM recall. Even though these cars have higher odometers and older model years than they did in 2021, they are getting 13% more range than last year!

This is only the beginning. We have a lot more to share about calendar age. If you want to follow along, sign up for our monthly newsletter in the web page footer!