Pushing a Tesla Motors Model S to the edge of its range

A Tesla Motors Model S owner tests the range of his electric car on a cool, hilly 168-mile drive.  

By , Guest blogger

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    A Model S drives outside the Tesla Motors factory in Fremont, Calif.
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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.

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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. 

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.

100-percent charge

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.

Vanishing regen

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.

Efficiency readouts

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.

Initial anxiety

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.

Not good.

At this rate I'd be totally out of energy at 182 miles. Way too close for comfort.

Elevation changes

But I was pretty sure that the culprit was elevation.  From previous trips, I'd learned that elevation changes have a huge effect on the range of the Model S.

For example, when I make the 60-mile drive from my house (elevation: 423 feet) to New York City (sea level), I typically  average 270 Wh/mi in warm weather. The return trip, slightly uphill,   averages about 310.  A mere 400-foot elevation change over 60 miles alters efficiency by almost 15 percent.

I'd checked the elevation of Danville (578 feet) before I left, but hadn't paid attention to the intervening terrain. Hopefully, I'd soon begin descending.

Sure enough, as I passed Wilkes Barre,  the e-meter began to come back down toward 320 kWh/mi, then 310. I breathed a sigh of relief; I had it in the bag.

(I later determined that I'd reached a peak elevation of  about 1800 feet along I-84 just east of Wilkes-Barre.)

By the time I got to Danville, my energy usage had dropped to 295 wH/mi--right in the middle of my target zone.

The range meter read 24 miles when I finally arrived at a funky farmhouse. It was home to an affable banjo-picker named Mark Doncheski, two Corvettes, and a Tesla Roadster. Mark had agreed to make his Tesla charger available to Sherman and me.

Range meter quirks

As I'd anticipated, the guess-o-meter didn't quite square with reality. I'd started with 199 indicated miles and driven an actual 168.  That's a 31-mile difference. Seven miles got lost somewhere.

The old-reliable e-meter told me I'd used 49.4 kWh of juice for the trip. That left 10.6 kWh--enough to drive an additional 36 miles at my trip average of 295 wH/mi.  (More, at slower speeds.)

Theoretically, I had 12 more miles remaining than the guess-o-meter indicated.

This squares with an unofficial on-line Model S owner's manual compiled by Tesla fanatic Nick J. Howe. According to Howe, the 85-kWh Model S actually has 17 miles "in the tank" after the range meter reaches zero.

Prorating for my 60-kWh battery, that's pretty close to my theoretical 12-mile buffer.

Bottom line: I can still probably limp to some sort of electrical outlet or charger even after the range meter hits zero. Frankly, I never want to have to confirm that.

Tesla talk

Sherman arrived five hours later in a brown P85, accompanied by owner Fred Glomb and a support truck towing a trailer. Cruising at a steady 62 mph, they'd  covered the 251-mile leg from Ohio with 20 miles to spare.

Over a late dinner in Mark's kitchen, we talked Tesla and the upcoming race till well past midnight.

Out of the pizza-fueled discussion came a startling conclusion about Model S driving strategy for the race: Cruising speed is basically irrelevant. Any time gained by going faster between charging stops is almost exactly negated by the increased charging time.

(This conclusion assumes an 85-kWh carequipped with Twin Chargers that is charged from a 20-kW Tesla High Power Wall Connector--the fastest possible charging scenario along the race route, which had no Superchargers anywhere near.)

The breakdown: Over a typical 240-mile leg, driving 70 mph would save 56 minutes over a 55-mph speed. Based on the speed-vs-range graph on the Tesla website, the faster car would use about 18 kWh more energy.  Charging time to replace that extra 18 kWh: 54 minutes.

Of course the eventual arrival of more Superchargers will eliminate  such fascinating threads of discussion.

Next morning, Sherman's car was loaded onto the trailer for the trip back to Michigan. I topped off my car and retraced my route home, logging virtually identical numbers for the return leg.

My personal takeaway from this exercise was a practical range limit for my car: 180 miles in warm weather, 150 miles in the cold.

Bring on the East Coast Superchargers. Please.

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