We are approaching the two-year anniversary of leasing a 2017 Chevy Bolt EV. This is a good time to take stock of any degradation in the Bolt's traction battery capacity and how that compares with a 2015 Nissan Leaf we leased previously.
Everyone who uses a smartphone understands that lithium-ion batteries degrade over time and with use. The question is how much over what period of time.
The traction battery is at the heart of an electric vehicle (EV). It's what makes it go. Battery degradation is important because it can affect the EVs utility. Significant degradation can reduce an EVs range sufficiently to affect how the car is used. It can also affect the car's resale value.
Nissan was the first manufacturer to offer a mass-market EV. A few years after introducing the Leaf, Nissan began fielding complaints about rapid battery degradation in hot climates, such as Phoenix, Arizona. Subsequently, Nissan was forced to offer warranty battery replacements. They also began changing battery chemistry as a result. Our 2015 Leaf used what the online community called the "lizard battery" because it was presumably more resistant to high temperatures.
On the other hand, minor battery degradation is expected and may never affect how the EV is actually driven. A good example is the reported performance of Tesla's Model S. Though there's measured degradation, it's considered minor even in high-mileage vehicles that are several years old. (See Tesla battery degradation at less than 10% after over 160,000 miles
, according to latest data for details on one vehicle and Tales from a Tesla Model S with 450,000 miles: Battery life, durability, and more
Chevy introduced the Bolt in late 2016. Since its introduction, at east one driver, Eric Way of News Coulumb fame
, has driven the Bolt more than 70,000 miles and reported his findings. He concluded that the degradation wasn't significant: ~8%. It certainly hasn't affected how he drives his Bolt. He drives it hard--often charging it to 100% and then depleting it to only a few percent state-of-charge on his weekend forays. (See Way's video report at Chevy Bolt EV: 70,000 Mile Battery Degradation
Traction Battery Capacity & Range
The 2015 Nissan Leaf had a nominal 24 kWh traction battery. However, not all that capacity was used to drive the car. Some 22 kWh were usable, giving the car an official EPA range of 84 miles when the car was new. So though the car had a nominal 24 kWh traction battery, in practice drivers never saw that.
When we first began driving the Leaf, the car often estimated we had a range in excess of 90 miles and occasionally in excess of 100 miles on a full charge.
Unlike the Leaf, our Chevy Bolt's nominal traction battery capacity of 60 kWh is what was available to drive our car, giving it an official EPA range of 238 miles on a full charge.
There's some debate among the online community over whether the usable capacity of the traction battery is really 60 kWh. However, when we've run the state-of-charge down to around 10% of remaining capacity, the range indicator suggests we may have started with 60 kWh or a little bit more.
Early in our operation of the Bolt we drove it 238 miles around town here in Bakersfield. We consumed 55.3 kWh at an efficiency of 4.3 mi/kWh and had a SOC of 10% remaining. The range indicator said we had 27 miles left. In another case we drove it 232 miles, consuming 52.6 kWh at an efficiency of 4.4 mi/kWh with 14% SOC remaining. The range indicator said we had 36 miles left. (See Driving the Chevy Bolt EV--Our Impressions
The Bolt's estimate of the range available on a full charge hasn't changed much in the two years we've driven the car. Typically the mid-value of the Bolt's three range estimates has hovered around 250 miles of range on a full charge.
Charging & Driving History
We intentionally drove the both the Leaf and the Bolt in the manner we'd drive any car. We took no special precautions. We used the EVs as we'd expect most consumers would use them.
In the first year of operation of both vehicles, we often charged to 100% and just drove the cars until we needed to charge again. In the case of the Bolt, after the first year we began using "Hill-top Reserve" to charge the car up to 88% of its full capacity, but we weren't religious about it.
While Bakersfield may not be as hot as Phoenix, Arizona, it is hot here in the summer--blistering hot. It's not uncommon here for temperatures to exceed 105 F (40 C) for several days in July, August, and September. It's not unusual for temperatures here to exceed 113 F (45 C). Any EV that can survive several Bakersfield summers probably will not be susceptible to temperature degradation.
Leaf Spy & Torque Pro
There's no universally accepted way for consumers to measure an EV's traction battery capacity. In general, manufacturers prefer that consumers remain in the dark. As long as the car does what it was advertised to do, consumers needn't worry about their battery pack, or so their thinking may go.
Fortunately, pioneering EV drivers are technically savvy and some have hacked the communications from the cars' computers through the OBD (On-board Diagnostic) port.
For Nissan's Leaf, there's a commercial app, Leaf Spy, which interprets the signals from the OBD port with a compatible OBD reader. (See Leaf Spy--an Essential Tool for Serious Nissan Leaf Drivers
.) One of the parameters measured by Leaf Spy is kWh of battery capacity.
It's not as simple for Bolt drivers, but hackers have determined how to configure Torque Pro to interpret the signals from an OBD reader. (I use the same OBD reader in the Bolt as I did in the Leaf. See Peeking Inside the Bolt's Brain Reveals Valuable Secrets
We measured from a little less than 2 kWh loss of capacity in our 2015 Nissan Leaf to as much as 2.5 kWh over 8,500 miles of use in the 22 months that we monitored the EV's health. The loss of capacity ranged from 8.6% to as much as 11.5% before we returned the car to Nissan at the end of the lease. (See Battery Degradation Two-Year Status Report: 2015 Nissan Leaf
Despite the Leaf's loss of capacity, it didn't affect how we used the car. It did reduce my comfort level on the steep climbs out of Bakersfield, but we didn't change how we drove the car.
We've since downsized to only one car and that's the Chevy Bolt. We're driving it much more than we ever drove the Leaf. Nevertheless, the traction battery has degraded half that of the Leaf over considerably more miles driven. Over 13,000 miles in 12 months, the battery has degraded 3.6% from a measured high of 61.1 kWh. From the nominal capacity of 60 kWh, the battery pack has degraded less than 2% over the nearly 20,000 miles we've driven the car.
Other anecdotal data points can be tracked down on the various EV forums. For example, one forum participant bought a used Bolt with 100,000 miles and a loss of only 5.5% from the nominal 60 kWh for the Bolt as measured by Torque Pro. If this is the norm, it puts the longevity of the Bolt's traction battery on a par with that from Tesla. (The entire thread can be read at Battery Degradation after 100,000 miles?)
The estimated range of our Leaf gradually declined during the three years we leased the vehicle. The range decreased about 10 miles. At 4 miles/kWh that's a decline of about 2 kWh, which is in line with the measured capacity loss.
We haven't seen any discernible decline in the estimated range of the Bolt. We still see the high estimate of range exceeding 300 miles, depending on how the Bolt was driven before we charged the car. On one recent 180-mile round trip to the mountains where we averaged 4.9 miles/kWh, the Bolt's high range estimate reached 353 miles--the best we've seen since we leased the car nearly two years ago.
In short, the battery degradation we've measured in the Bolt is minimal and hasn't had any effect on our range or how we use the car.