Life With Tesla Model S: Battery Degradation UpdateLife With Tesla Model S: Battery Degradation Update

Last summer, I wrote in this space about an apparent loss of battery capacity in my 2013 Tesla Model S after a three-month Supercharging binge. 
I wondered whether, despite assurances from Tesla Motors, the two events might have been related.
My doubts had started when I noticed that, after a 210-mile trip, the car’s data readout told me I had used 63.5 kilowatt-hours and 93 percent of my battery capacity.
DON’T MISS: Life With Tesla Model S: Does Supercharging Cut Battery Capacity?
Those numbers implied a 100-percent capacity of only 68.3 kWh—6 percent below the 72 kWh I’d measured just three months previously, before a Supercharged round-trip to California. 
And the apparent 68-kWh capacity was 12 percent less than the nominal 76 kWh that Tesla guru Nick J. Howe estimates as the usable capacity of a brand new 85-kWh battery.
(Tesla, unfortunately, has not revealed the actual figures for what portion of its packs are usable.) 
Tesla Model S lithium-ion battery pack in rolling chassis [photo: Martin Gillet via Flickr]
A study last year of Model S batteries in the Netherlands showed an average capacity loss of 6 percent after 50,000 miles. Here I was, apparently with double that loss after only 34,000 miles on the battery.
Was my Supercharging binge to blame?
 Leaf comparison
A 2014 study of the Nissan Leaf showed a three-percent range loss after 40,000 miles in Leafs that used 50-kW DC fast charging compared to those using the standard 6.6-kW AC charge rate.
ALSO SEE: Tesla Model S Battery Life: How Much Range Loss For Electric Car Over Time?
In last summer’s article I speculated that, because Tesla Superchargers are more than twice as fast (120 to 135 kilowatts) as the Leaf’s CHAdeMO DC fast chargers, any resulting capacity loss in the Model S might be more pronounced than the Leaf’s 3 percent.
But I neglected to account for two things: 

The Model S battery is almost four times larger than the Leaf’s, so it can absorb a twice-as-fast charge with only half the strain; and
The Model S has a sophisticated thermal-management system that keeps the battery at the optimum temperature for charging, but the Leaf does not.

2013 Tesla Model S owned by David Noland, Catskill Mountains, NY, Oct 2015
On balance, these two factors would seem to make the Model S less susceptible to Supercharging effects than the Leaf, not more.
And since the Leaf showed only a minuscule 3-percent capacity loss after a lifetime of fast charging, I now doubt that Supercharging caused my apparent 12-percent capacity loss.
So what is causing it?
MORE: Nissan Leaf Battery Capacity Loss: Covered By Warranty, Now (Dec 2012)
Balancing the pack
My Tesla service center advised me that I might get some of that apparent lost capacity back by “balancing the pack”—draining it to near zero and then slowly charging it back up to 100 percent.
They were right. I tried it, and subsequent tests put my capacity at about a higher 70 kWh.
But that was still a bit less than my pre-Supercharging level of 72 kWh, and down about 8 percent from the presumed new capacity of 76 kWh. 
Solar panels at Supercharger in Barstow, CA, during Tesla Model S road trip [photo: David Noland]
I ended that article last summer and mused about whether there was still something amiss in my battery.
Contactor update
It turned out there was. But it had nothing to do with capacity.

2013 Tesla Model S owned by David Noland, Catskill Mountains, NY, Oct 2015
My early Model S (serial # 3662) had a first-generation main battery contactor that had proved troublesome in some cars.
As a precaution, Tesla asked me to bring in my car to my local service center to replace it with an updated contactor.
But during the replacement process, the Tesla tech found some corrosion and a couple of other things he didn’t like.
RELATED: Battery Life In Tesla Roadster Is Likely Better Than Predicted (Jul 2013)
So my battery was sent back to the factory for a complete refurbishment. In its place the service center installed a brand-new loaner battery (or so the invoice said).
New capacity
Here was my opportunity to get more precise information on the nominal 100-percent capacity of a new pack. 
Tesla Model S packed for road trip, upstate New York to southern California [photo: David Noland]
Before embarking on a planned 150-mile trip, I charged the new pack to 100 percent. Then I carefully noted the number of kWh used as the capacity passed through convenient capacity percentages—90, 80, 75, 67 and 50 percent.
At exactly 50 percent capacity, I had used precisely 37.5 kWh–which implied a full capacity of 75 kWh. The other partial kWh figures agreed quite closely with this number.
So I’m prepared to say that, as measured by the car’s own system, the full usable capacity of a new 85-kWh Model S battery pack is 75 kWh.
My original battery is now back from the factory, freshly refurbished. (At no cost to me.) The capacity of its cells was pronounced to be within the normal range for its age and number charge cycles. 
Tesla Model S electric-car road trip, Route 66 Museum, Elk City, Oklahoma [photo: David Noland]
So it seems I’m stuck with a useful capacity of 70 kWh, a 7-percent loss that’s a bit worse than the statistical average. 
But I suppose, unlike the children of Lake Woebegon, we can’t all be above average. Battery capacity-wise, it appears my fate is to be one of the guys on the low side. 
I can live with it.
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