In the UK, 2 to 3% capacity loss per year is to be expected from a Nissan LEAF 24kWh.

cropped-myfirstfastcharge.jpg

There’s lots of myths surrounding batteries in EVs, many relating to battery degradation. “What about the cost of replacing the battery then?” is a common question. This is something that has interested me both as a buyer, then an owner, of EVs but also professionally. I work in materials science, and long-term performance of any material is important. I’ve worked on the lifetime prediction of metal, asbestos and plastic pipe materials;  fibreglass for wind turbines, and more recently the lifetime of materials subject to radiation in big physics experiments and fusion energy. What all of these applications have in common is that any experiment designed to predict the lifetime needs to take measurements over some time. There’s always a lot of variation as it’s difficult to control variables and real materials never truly obey the mathematical predictions. So you end up with large “error bars” and much hand waving, and by the time you’ve got any answers, the materials technology has moved on anyway!

The LEAF is kind of a worst-case application for an EV, because of the small battery, there’s no thermal management and most of the battery capacity is used. Many drivers use a full charge every weekday I imagine. Tesla owners with much larger batteries might well not! These factors contribute to fairly fast “wear” or degradation of the battery.

Ideally you’d keep the battery at around 20C and limit the extent of the charge and discharge as far as possible, since extremes are bad news.  However when the Leaf was introduced, batteries were expensive and heavier than today, and so Nissan had little choice but to use most of the capacity. The lack of thermal management is rather less excusable in my view.

Data here was collected from an app “LeafSpy” from real users on the Speakev forum, in a thread that I started when I got my Leaf (so hopefully nobody is too upset that I also report it here!) It now resides in a google “sheets” format. I reproduce it here acknowledging the input of these users on Speakev.

Leaf-SOH-age

There’s 3 data sets here, the red is “Gen 1” or Japanese-built Leafs, from 2011-2013. The blue are Sunderland-built, 2013-on Leafs. There’s a few green points which are from a taxi company using a Leaf 24, up to 170,000 miles.

The earlier Gen 1 cars look like they’re degrading a bit more quickly than the Gen 2. The 170,000 mile taxis, with lots of rapid charging and many more charging cycles than the other cars, are also degrading more than the others per year – as you’d expect. The average is 2.7% per year.

Although there is a lot of scatter, the message is that you can expect 2 to 3% capacity drop per year in normal use, according to the “SOH, State of Health” that the battery BMS reports. I’ve noticed that this does wander about quite a bit, also it rises in winter and drops in summer (which is the opposite to expectations since Li-Ion chemistry operates optimally at 25C, a warm summer temperature in the UK). Overall, the long term trends seem reliable though.

This means that a 5 year old Leaf will have lost around 15% capacity, so will do around 85 miles per full charge, rather than 100 when it was new. Only you can decide whether this makes a Leaf a useful commuter for you.

Other aspects of battery aging are minor; rapid charging takes slightly longer since the battery internal resistance is higher. Regen braking takes longer to fully implement after a full charge, so for example is only available below 92% charge on a 5 year old car. This can be disconcerting and is an aspect I wish had been engineered out.

The cost of a battery change is actually rather academic since it’s cheaper and easier to change your car and probably get a bigger battery. In practice, more widespread charging availability makes battery degradation even less of an issue.

 

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