Tesla Model S in Albuquerque's 'snowstorm' during NY-to-California road trip [photo: David Noland]
Last month, my wife and I took my 2013 Tesla Model S on a day trip from our home in New York’s Hudson Valley to Brattleboro, Vermont, a distance of 180 miles.
We picked up our daughter, charged the 85-kilowatt-hour battery up to 98 percent at the Brattleboro Supercharger, and returned home via a longer, more scenic route of 210 miles.
To my surprise, the return trip took almost every electron the battery could muster. We pulled into the driveway with only 5 percent capacity remaining—roughly 15 miles of range.
At first I attributed the close call to a headwind on the way home. But the efficiency readout for the return leg had showed a respectable 302 watt-hours per mile.
That was only a bit worse than the 290 Wh/mi of the outbound leg, and well within my normal range for summer Interstate cruising.
Diminished battery capacity
Looking more closely, what caught my eye was the dashboard readout for total energy used for the 210-mile return leg: 63.5 kWh.
I had started with 98 percent battery and finished with 5 percent. Thus the 63.5 kWh amounted to 93 percent of the total battery capacity.
That suggests the 100-percent capacity was 68.3 kWh.
Wait a minute: Wasn’t this supposed to be an 85-kWh battery? What the heck had happened to the other 17 kWh?
Too much Supercharging?
There has long been speculation that Supercharging--and DC fast charging of any electric car--can cause long-term loss of Model S battery capacity.
And there is credible research to support the general idea.
A 2014 study by Idaho National Laboratory of two 2012 Nissan Leafs concluded that, after 40,000 miles, the Nissan Leaf that used 50-kW fast charging exclusively had 3 percent less battery capacity than the one that used standard 6.6-kW charging.
At 120-135 kW, the Tesla Superchargers are more than twice as powerful as the Leaf fast-chargers. It stands to reason that its effects on long-term battery capacity might be even greater.
Was my apparent loss of battery capacity due to too much Supercharging?
Tesla Model S electric-car road trip, Route 66 Museum, Elk City, Oklahoma [photo: David Noland]
As it happens, I’ve done a lot of Supercharging in the past six months.
In January, I drove my Model S to California, using Superchargers most of the way. During a two-month stay, we made several Supercharged road trips along the West Coast.
Then I drove the car back to New York, Supercharged all the way along I-70.
And for the past few weeks, I’ve done some local Supercharging at a new station that recently opened near me.
Overall, I’d estimate that of the last 10,000 miles I’ve driven the Model S, 8,000 of them have been Supercharged.
No problem, Tesla says
Has this Supercharging frenzy come back to haunt me?
Solar panels at Supercharger in Barstow, CA, during Tesla Model S road trip [photo: David Noland]
Tesla says no. (In fact, one Tesla tech rep I consulted almost shouted, “Absolutely not!”) The official company line is that Supercharging has no deleterious effect on the battery, period.
But a funny thing: all three Tesla reps I talked to, including the shouter, hedged their bets.
After assuring me there was absolutely no problem, each one advised me that--all else being equal--slower charging was better for the battery in the long run.
“It’s fine to Supercharge,” one of them told me. “Just don’t do it too much.”
When I pointed out the thundering contradiction in that statement, she just shrugged.
So can I just Supercharge up to the point where I start to lose range?
Sudden capacity loss
Before I left for California, I’d done a couple of quickie battery capacity tests during long drives. I carefully noted the kWh used in running the battery from 90 percent to 40 percent capacity, and from 80 percent to 30 percent.