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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.  

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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.

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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.

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