Battery Capacity Kerfuffle

Mark D Larsen

Your Mileage May Vary
(Click to enlarge)

In early 2012, some early adopters in Phoenix noticed that their Leafs had lost their first capacity bar prematurely, supposedly because of the hot climate in Arizona. This worrisome concern soon snowballed into well over 400 pages of discussion in the “My Nissan Leaf” forum.

Nissan should have immediately addressed the issue, but it took the automaker several months to finally announce that they would analyze several of the affected Leafs in the lab at their Arizona Testing Center. Nonetheless, with mounting frustration over the delays, a few of those owners decided to contact the news media, and the Arizona CBS affiliate, KPHO, aired a segment with their complaints on July 18, 2012. Two days later, the station ran a follow-up story. I had to groan upon viewing these broadcasts. The news reports turned the issue into another “runaway Prius” news blitz in front of the public eye. Consequently, any potential car buyers who viewed these segments probably crossed the Leaf off their list of vehicles to consider.

Nissan eventually completed its analysis, but —most lamentably— also delayed in releasing the data to the public. Nonetheless, Executive Vice President Andy Palmer hinted at the results in a statement to the Australian automotive website Drive on September 4, when he claimed that the hot weather in Arizona was causing a “faulty battery level display” rather than an abnormally accelerated drop in capacity.

A handful of the owners objected to Palmer’s statement. To thus prove to Nissan that their complaints were legitimate, they decided to conduct their own experiment. Afterwards, they posted the results in the “My Nissan Leaf” forum. On the one hand, the testers admitted that “Andy Palmer was right... they have poor instruments.” Yet they also insisted that “he was wrong about the batteries.” When I examined their data, I could not understand said conclusion.

For example, as a benchmark they used Nissan’s own range estimates according to its Technical Bulletin NTB11-076a. That bulletin states that, at 4 miles-per-kWh, in 70° temperatures, a fully charged, brand new Leaf should achieve 76-to-84 miles of range. The owners therefore attempted to duplicate these same parameters in their experiment: 4 miles-per-kWh speeds in Leafs charged to 100%.

However, to interpret the results, they then compared them to the very highest range of Nissan’s estimate (84 miles). I deemed this a mistake. Nissan gives a range of the range for a reason, i.e., your-mileage-may-vary. To be as fair and accurate as possible, it is thus necessary to compare the results to the full scale of projected ranges —not just the top number.

Moreover, it is also crucial to factor in the mileage of each Leaf tested. Nissan had made it clear from the beginning that battery capacity will diminish with time and miles driven. The automaker estimates that, if owners drive an average of 12,500 miles per year under normal conditions, the capacity will drop to around 80% after 5 years, and about 70% after 10 years. It is therefore possible to extrapolate from the declared benchmarks of 100% (0 miles), 80% (62,500 miles), and 70% (125,000 miles) the corresponding drop in capacity from month-to-month over time. The following graph plots the polynomial curve that intersects those three benchmarks:

The very long table down the right side shows the results when applying, over time and miles, the above polynomial curve to the 76-to-84 mile range projected by Nissan at 4 miles-per-kWh in its technical bulletin.

Using the extrapolated calculations in that table, we can now compare the actual ranges achieved in the test vehicles with their corresponding percentages according to mileage. The following lists the results side-by-side, from the lowest mileage vehicle to the highest.

Below is a scatter plot that illustrates those results along the polynomial curve (in green) for the projected range loss according to mileage. We can see (in blue) that 7 of the 12 Leafs actually achieved ranges within their projected scales. Of those, one is actually higher than its scale, three are in the middle, and three toward the bottom. Of the remaining 5 Leafs, three (in purple) are in such close proximity to the low end that the differences are practically negligible. However, two vehicles (in red) fell notably below the predicted spread: Blue494 and White272 . Oddly, this latter vehicle had the same mileage on its odometer as White626 which fell well within its projected scale.

In general, the results for these Leafs seem to show that, contrary to the testers’ conclusions, most of them actually fell within “normal” parameters for their mileage. As for the two glaring exceptions to the rule, I would hypothesize that there must be additional factors involved beyond hot temperatures. For example, it would be informative to know those vehicles’ usual driving speeds, D vs. ECO mode, the use of climate control, locations and temperatures when charging, 100% vs. 80% charging, the number of charges per day, and/or Level 1 vs. Level 2 vs. Level 3 charging.

We can also assess how accurate the capacity bars were in those Leafs. The Nissan Leaf Service Manual reportedly clarifies that the first bar represents a 15% loss, and the remaining bars a 6.25% loss each. By applying such percentages to the number of capacity bars in each of the vehicles tested, we can compare the range they are supposed to represent with the miles actually achieved:

The two newer Leafs (Black782 and Blue842) still had all 12 capacity bars visible, so there was no need to include them in the above table. We can see that all of the other Leafs with missing capacity bars actually drove farther than their gauges indicated would be possible. This would suggest that Andy Palmer was right: the Leaf has a faulty battery level display.

When I shared these findings in the “My Nissan Leaf” forum, the testers’ reactions were so surprisingly hostile that I am now considered a persona non grata on that site. Sadly enough, some of those forum members even started to follow me around to other blogs on the web to inform others that I was “a fan of fiction, not fact," and that my “[my] self-indulgent narcissism [is] even more annoying than [my] ignorance.” To bolster that accusation, the blogger provided a link to a comment that one of the AZ owners had posted about me. Since that comment is now circulating around the internet, I suppose that I might as well also provide the DM response that I sent to that owner —to which I never received a reply.

Regardless, the capacity issue continued to swell even larger. KPHO aired yet a third and then a fourth segment. Even worse, another sation, KPNX, then broadcast a fifth report announcing the discouraging news that in California a group had in fact filed a class action lawsuit against Nissan. Again, such tactics discouraged me, because they likely sullied the automaker’s reputation and contributed to a subsequent slow down in Leaf sales. For example, Time Magazine then published an article questioning if the Leaf is a flop —despite the fact that it won the 2011 World Car of the Year award.

Not long after this, Plug-In America board member Tom Saxton conducted a survey to gather more data on the issue from Leaf owners. Out of approximately 450 owners in Arizona, only 13 completed the survey, likely the same individuals who filed the complaints, which would skew the results for that state. Despite that caveat, Saxton’s preliminary report shows some intriguing results. For example, consider the cluster of responses in his graph that I have marked with an orange circle:

It is curious that, after driving a comparable number of miles, and in equally hot temperatures, two Leafs had lost 1 capacity bar, four Leafs had lost 2, yet five more still had all 12 bars intact. Also intriguing is that, in the blue circle, one Leaf had already lost 1 bar, after about 22,000 miles, in 85°F temperatures, yet it is eclipsed by multiple Leafs with all 12 bars, some of them in climates more than 10°F hotter. It seems to me that such results corroborate that additional factors besides heat must be causing the instances of premature battery capacity loss.

Subsequent research seems to substantiate that such is the case. Professor Jeff Dahn at Dalhousie University has shown that the longer a charge takes, the thicker a parasitic coating accumulates on the electrode —which deteriorates a battery’s capacity. Yes, heat exacerbates the problem, but Professor Dahn asserts that “time of exposure is really the bad actor here.” If true, contrary to earlier assumptions, QuickCharging would actually cause the least amount of battery capacity loss, while the worst case scenario would be to use the onboard 120V cableset. In fact, the original 3.3 kW charger in the original Leafs would theoretically deteriorate a pack twice as fast as the 6.6 kW chargers in the newer models.

What disturbs me most about the entire kerfuffle is that, even if the Phoenix complaints were entirely true, to buy a Leaf customers have to sign a four page disclaimer which includes this specific paragraph [my boldface]:

  • Gradual loss of battery capacity. Like all lithium ion batteries, the 2012 LEAF battery will experience a reduction in the amount of electricity or charge it can hold over time, resulting in a reduction in the vehicle’s range. This is normal and expected. The rate of reduction cannot be assured, however, the battery is expected to maintain approximately 80% of its initial capacity after 5 years of normal operation and recommended care, but this is not guaranteed. This number may be higher or lower depending upon usage and care. Factors that will affect and may hasten the rate of capacity loss include, but are not limited to: exposure to very high ambient temperatures for extended periods of time, driving habits, vehicle usage, and charging habits (Quick Charging the vehicle more than once per day).

I cannot imagine that those owners were able to take delivery without signing their acceptance of those terms. I most certainly had to sign my form, a copy of which I still have in my files. Consequently, if accelerated battery capacity loss were to occur in my Leaf, I could not in good conscience complain about it: I had been duly warned that the rate of reduction “cannot be assured” and “is not guaranteed.”

Even though said disclaimer clearly places the onus on Leaf owners, Nissan has nonetheless bent-over-backwards to “please the customer”:

Nissan deserves to be commended for going the extra mile with these efforts to “please the customer.” On the other hand, I have to admit that it irks me to see that the squeaky wheeler-dealer gets the greased palm. The solution basically rewards those who abuse their batteries, and punishes those who care for their packs according to guidelines. The implication is that you should overtax, overcharge, and overcook your battery before the 5 year/60,000 mile capacity warranty expires so that you can get a spanking new, more heat-resistant replacement for free. If, on the other hand, you care for your battery so that it retains 9 or more capacity bars beyond that threshold... you’ll be S.O.L. and have to pay for a new battery yourself someday. The good news is that, in such instances, the price is more than reasonable —and will likely drop even lower in the future.

Regardless, I just hope that these latest developments have finally put an end to the capacity kerfuffle, once and for all. I will, of course, continue to periodically test and report the health of my own Leaf’s battery pack as the months and miles are left in the rear-view mirror.