2013 Tesla Model S electric sport sedan [photo by owner David Noland]Enlarge Photo
In the eight months I've owned my 60-kWh Tesla Model S, I've never had occasion to drive it more than 120 miles, comfortably within the car's EPA-rated 208-mile range. The phrase "range anxiety" was not in my lexicon.
But last week, I got an invitation from Don Sherman, Technical Director of Car & Driver magazine, to meet him in Danville, Pennsylvania. It's 168 miles from my home in New York's Hudson Valley.
Sherman was making a dry run for an upcoming article about a 700-mile cross-country race in a Model S--and Danville, just off Interstate-80, was going to be a charging stop. We'd e-mailed back and forth about Model S driving and charging strategies, and this would be a chance to meet and compare notes.
The journey seemed short enough to be eminently doable--yet long enough to engender, if not range anxiety, then at least acute range awareness.
Fast and hilly
Although I supposedly had a 40-mile cushion in the 208-mile EPA range, I knew very well that my mileage may vary.
2013 Tesla Model S [photo: David Noland]Enlarge Photo
I was planning to drive fast, from 70 to 75 mph along the Interstates that made up most of the route. The terrain was hilly. There would be a prevailing headwind. And the advancing fall season promised cooler temperatures.
All of these factors would eat into range.
In preparation for the trip, I set the battery charge level at 100 percent, the first time I'd done so.
Previously, I'd always used the "standard" setting, 90 percent of capacity. To get a full 100 percent, you set the charger to "range" mode, which Tesla recommends only when maximum range is necessary.
(The reason: Lithium-ion batteries degrade more quickly when charged to 100 percent of capacity.)
A couple of months ago, a software update from Tesla allowed the charge limit to be set at any level. Tesla recommends 50 to 90 percent for everyday driving. I had settled on 75 percent, which was plenty for my normal driving routine.
But for this trip, I set the charger to 90 percent the night before, then topped off the battery to 100 percent before departure the next day. The topping-off process took longer than I anticipated; charge rate dropped from the usual 10 kW to about 3 kW for the last few kilowatt-hours.
As I rolled out of my driveway, on an unseasonably warm Indian Summer day, the range meter read 199 miles.
David and Adam Metcalf beat 400 miles on a charge in a 2012 Tesla Model S (Image: David Metcalf)Enlarge Photo
The first thing I noticed was that the regenerative braking virtually disappeared with the 100-percent charge. Charge rates in max regen can approach 40 kW, way too much for a nearly full battery.
Going down a long, steep hill near my house, I actually had to use the brakes. Damn! Wasting precious energy already!
Fortunately, full regeneration came back quickly, within 10 miles.
I don't have a lot of faith in the Model S range meter. Its number is a projection based on rule-of-thumb efficiency assumptions, battery temperature, and a safety fudge factor. (New York Times reporter Jonathan Broder famously fell victim to wildly fluctuating range numbers.) I call it the guess-o-meter.
Instead, I monitor the dashboard readout of trip efficiency, measured in Watt-hours per mile. During my earlier shorter stints of highway driving, in warm weather, I'd averaged 290 to 300 Wh/mi.
I figured if I could maintain that level of efficiency, I'd make it to Danville with about 30 miles to spare. The Wh/mi reading thus became my prime focus for the trip.
For the open stretches of Interstate, I set the cruise control on 74 mph. (I figured no self-respecting cop would stop me for breaking the speed limit by single digits.
Forty miles out, along I-84, the e-meter settled in at around 310 Wh/mi. With the usual minor ups and downs for hills, the number crept steadily upward as I crossed into Pennsylvania. By the time I reached Wilkes-Barre--about halfway--it was reading 330.