Let’s talk about battery degradation

Battery degradation isn’t that big of a deal for many

[UPDATE: Here’s a link to an article about a LEAF in the UK that went 100,000 miles in under two years with no degradation.]

First, a definition. Electric vehicle (EV) battery degradation is when the battery loses capacity over time. If you’ve ever owned a laptop or a cell phone, you have experienced battery degradation. An EV just has a much larger battery than your average laptop.

A glance at the image above will indicate the level of battery degradation in our 2013 Nissan LEAF. The blue, white, and red segments on the right indicate the current battery charge and battery capacity. The 100% indicator in the middle of the meter display shows that the battery is currently fully charged. Now I’ll break down the display on the right.

77 miles is the expected range with a full charge based on the most recent previous driving experience. A very conservative driver will see more range, and a very aggressive driver will see less range here.

The twelve blue and white bars are a more coarse display of the 100% state of charge indicator in the middle of the display. As the 100% display is reduced during a driving event, the blue and white bars will also decrease in tandem. If you own a LEAF you should know that this is not a linear relationship. Each blue and white bar does not represent the same amount of battery charge remaining. If you are concerned about how much state of charge remaining, I suggest keeping the changeable center display in the state of charge mode. This state of charge display is not available on the 2011 and 2012 LEAFs.

Now we get to the part about the battery degradation. If you carefully count the small white segments on the far right, you will count nine of them, plus the two red segments on the bottom of the display. These segments represent the battery capacity, not the state of charge. A new LEAF will have ten white segments atop the two red segments for a total of twelve, to match the blue and white bars showing the state of charge. Which means that this LEAF has battery degradation of one segment, or roughly fifteen percent of its new capacity. To clarify – when fully charged, a LEAF will always show twelve blue and white state of charge bars, even if several of the battery capacity segments on the far right have been lost. The question is, how does this impact day-to-day driveability of the car. In our case – little to none.

We have accumulated 14,808 miles in just over two years of driving. This is an average of only about twenty miles per day. So even with our slightly degraded battery, we can easily accommodate our typical daily drive with a fully charged range of 77 miles. Are there days when we need to plug in? Sure. But it doesn’t happen often. Also, keep in mind that the fully charged range of 77 miles could be more or less depending upon our most recent previous driving style.

What prompted this article was a Department of Energy Lawrence Berkeley National Laboratory study showing that EVs will meet the daily travel needs of the vast majority of drivers longer than is typically assumed (found here). In other words, battery degradation is not that big of a deal for most EV owners. Certainly, some drivers will be impacted by reduced capacity more than others. Some LEAF owners have found a way to make it work. A Seattle man made a 130 mile round-trip commute each workday for three years on his 2011 LEAF, ultimately with the loss of three battery segments to battery degradation prior to buying another LEAF. He did have to stop to charge along the way with the battery degradation, but he saved over $10,000 in fuel costs during that time over his prior vehicle – a fuel efficient Honda.

On a final note, it seems that time is a bigger factor impacting battery degradation than are miles. As you can see above, we lost one battery segment at almost exactly the two year point with just over 14,000 miles. Steve Marsh, the man in Seattle, drove his car over 78,000 miles – and 24 months – when his car lost its first battery capacity segment. Time, rather than usage, seems to be the more significant factor. Much as it is on that laptop or cell phone battery.

If you wish, you can provide your own battery degradation experience in the comments section for others to benefit by. If you choose to do so, please include the year, make, and model of your EV, miles driven, length of time prior to loss of first battery segment, approximate region of the country (hot weather areas impact battery life more significantly), and any other commentary that you wish to share.