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November 1, 2016
Paul Gipe

Battery Degradation Two-Year Status Report: 2015 Nissan Leaf


We recently passed the two-year mark on our 2015 Nissan Leaf, a battery electric vehicle (EV). The event was a good time to take stock of how much the Leaf’s traction battery has degraded during that time.

The amount of time EVs set on a dealer’s lot unused can affect later degradation of the traction battery. Our Leaf was manufactured in September 2014 and we took delivery in October 2014. Our car did not sit idle long on the dealer’s lot before it was shipped to us.

My wife Nancy is retired and I work at home. Thus, we drive less per year than the average North American family. In two years we’ve put about 14,000 miles (23,000 km) on the car. That’s as much as some Americans drive per year.

Traction Battery Capacity & Range

The 2015 Nissan Leaf was built with a 24 kWh traction battery. Not all that capacity is used to propel the car. Some 22 kWh are usable, giving the car an official EPA range of 84 miles (140 km) when the car is new.

All batteries degrade with time and use. Anyone with a smart phone knows this. Unfortunately, the Leaf developed a reputation for accelerated battery degradation in hot climates such as Arizona and parts of California. Not all EVs on the market have suffered the same fate as the Leaf. GM’s Volt and Tesla’s Model S have shown little or no degradation. At least for now, accelerated degradation is unique to the Leaf’s traction battery.

It is thought that Nissan began introducing new battery chemistry in the 2015 model Leaf to address the accelerated capacity degradation from high temperatures. Denizens of mynissanleaf.com have called this the “lizard battery.”

We live in Bakersfield, California. It’s hot here in the summer--very hot. Temperatures above 105 F (40 C) are not uncommon during the summer. Our use of the Leaf in Bakersfield presented an opportunity to monitor the lizard battery’s degradation under real-world conditions in a hot climate.

It’s reasonable to expect that our Leaf would experience some battery capacity degradation after two years of use in this climate even with the lizard battery and that’s what we found.

Caveat: Seasonal Capacity

Temperature affects battery capacity. According to measurements by Tony Williams, Nissan’s traction battery reaches its rated capacity at a standard temperature of 68 F (20 C). Lower temperatures reduce capacity. Higher temperatures, up to a point, increase capacity. Thus, the traction battery’s capacity varies with the season. It reaches its lowest capacity during the winter and it reaches its maximum capacity during the summer. Capacity is not constant throughout the year.

Caveat: Charging & Driving History

Early in the Leaf’s introduction, Nissan recommend only charging the traction battery to 80% of its capacity as a means of extending the battery’s life. For various reasons, Nissan abandoned this recommendation in the 2015 Leaf.

Some drivers continued to follow this recommendation. We did not. We deliberately chose not to “baby” the battery. We drive the Leaf in the manner we think any other driver would. We charge to 100% every time we charge at our home station. We drive the car like we would drive any other car. On road trips, we will use DC Fast Chargers to raise the state of charge (SOC) to 90% and sometimes more when we felt it was necessary.

Using multiple fast charges on road trips raises the battery’s temperature, sometimes dramatically. On one road trip we used fast chargers nine times. On another trip during the summer, I drove the car hard across the Mojave Desert, charging at fast chargers along the way. On this particular trip I inadvertently pushed the temperature display just into the red zone. See Star-Crossed Trip: Bakersfield to LAX & Return.

Leaf Spy

There’s no way to know how much capacity the traction battery contains from the car’s dash display alone. The dash displays SOC, but that is only a measure of the percentage of the capacity remaining, not the amount itself.

Fortunately, the popularity of the Leaf has a spawned a whole subculture of hackers and software developers. One product to result from this is Leaf Spy, an app that works on both Android and Apple phones. It even works on my BlackBerry 10.

Leaf Spy communicates with a dongle that plugs into the car’s data port found under the dash. Hackers have determined what the various signals from the car mean and Leaf Spy interprets them.

Leaf Spy reports a number of parameters, but reporting on the traction battery’s capacity is one of its most valuable uses. Leaf Spy reports two measures of capacity: Gids, and kWh. There is a direct relationship between Gids, named for one of the Leaf’s early hackers, and kWh. Leaf Spy also reports a measure that the user community has described as the state of health (SOH). These three measures, then, can be used to track the traction battery’s degradation.

I didn’t begin using Leaf Spy until late in the first year of driving the Leaf. Thus, I don’t know what any of these values were when we took possession of the car.

Gids & SOH

When I began using Leaf Spy 10 months after leasing the car, the app reported that the car had 287 Gids, a 98% SOH, and 22.2 kWh of capacity. While the car may have had more Gids when it was first unloaded in Bakersfield, it probably wasn’t significantly more. (Other drivers have reported 292 Gids and 22.6 kWh for the 2015 Leaf when new.) For lack of other measurements, I assume the 287 Gids is our starting value.

As noted, capacity changes with seasonal temperatures. It also changes with charge history. Road trips where we used DCFC stations several times boosted the reported number of Gids and the battery’s state of health. Whether this was a real increase in capacity or the fast charges simply reset some internal measurement in the Leaf, I don’t know. You can see this effect on the accompanying chart.

Losing 2 kWh

The second chart displays only kWh. You can also see the upward ticks in kWh during the three long road trips with several fast charges.

We began monitoring the battery with slightly more than 22 kWh and ended the two-year period with slightly less than 20 kWh. That’s 10% degradation during the period. This isn’t out of line with what others have seen.

If we just look at the year on year loss, say from August 2015 to August 2016, we find a loss of 1.6 kWh or about 7%. Nevertheless, the average loss is about 10% for the two-year period.

Losing two kWh after two years of use effectively reduces the range 9 miles at an average efficiency over the period 4.5 miles/kWh.

Estimated Range

So, what is the estimated range now? We don’t really know for sure. As seen in the accompanying chart, the dash display of estimated range varies widely. But the gradual trend of the lowest estimates of range is visible. The lows during the fall and winter of 2014 were in the mid 80 mile range. By the spring of 2016, the low had fallen to less than 80 miles for the first time.

The loss of range may not have affected how we drive the Leaf.

Miles Driven on One Charge

We typically drive our Leaf 50 to 60 miles before recharging. This is well below the Leaf’s range then and now.

We’ve never driven the car to turtle—the point when the Leaf begins to shut down. The lowest state of charge (SOC) we’ve recorded is 8% in the spring of 2016 when we reached 74 miles on one charge.

We’ve driven it to less than 15% SOC several times, usually around town where we know the distance to our home charge station.

The furthest we’ve driven the Leaf on one charge is 80 miles. The dash displayed an estimated range remaining of 11 miles. We don’t know if those were real miles or not. With 12% SOC we know we could have driven further, but that was far enough on that one charge.

Reserve Margin

On road trips, we’ve tried to maintain a reserve margin of 20% to 25% to account for inoperative charge stations or other unexpected problems. This has served us well. We’ve never been towed.

As I’ve gained experience with the Leaf, online estimators, and the limited number of routes out of Bakersfield, I’ve reduced the margin to 20%. On a full charge when the car was new this allowed us to use 18 kWh. If we charged to only 80% at DCFC station, this would give us 13 kWh to work with.

On the other hand, with only 20 kWh available and a 20% reserve margin, we now have only 16 kWh on a full charge. On a 80% charge, we’re down to only 12 kWh.

What does that mean in working range? Assuming conservatively that we only have from 3.5 miles/kWh to 4 miles/kWh to plan with, this gives us a range of 56 to 64 miles on a full charge; 42 to 48 miles on an 80% charge. For the road trips out of Bakersfield, this is about the length of each leg.

Thus, it doesn’t appear that the loss of 2 kWh of our 2015 Nissan Leaf’s traction battery capacity will effect how we use the car either in town or on road trips. It may reduce my comfort level on the steep climbs to the Tejon Pass and to the Tehachapi Pass, but we should be able to clear the summits with a reserve remaining, just with less than I prefer.

For more on our experience with the Leaf and reports on our various road trips, see the links below.

Electric Vehicles

EV Trip Reports

 

 


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