Tesla Model S Customer Blog: Regenerative Braking And Its Discontents

It’s been 27 months since I wrote a check for $5,000 to Tesla Motors, my deposit on a Model S sedan. As owner number P717, I’ve gotten some modest bennies to keep me interested till the expected delivery date of mid-2012: a test drive in the Roadster, an invitation to the opening of the New York Tesla store, and some nice promotional swag (T-shirt, coffee mug, and, most recently, a cool little remote-control toy Roadster) .

Last week I was invited to an owners-only preview before a Model S promotional event in Greenwich, Ct. Set in the posh clothing store Richards, just across the street from an Apple store, the event featured a sinuous dark red early proof-of-concept prototype of the Model S. Unfortunately, we weren’t allowed to drive, sit in, or even touch the car (“It cost more than $2 million to build,” we were told). But the black-clad Tesla reps on hand offered some intriguing technical info about the car that, to my knowledge, had not been previously revealed. Among the more interesting tidbits:

• The Model S will not have the blended regenerative/friction braking that is standard in the Nissan Leaf, Chevy Volt, Toyota Prius and virtually all other mainstream electric and hybrid cars. Like the Tesla Roadster, the brake pedal in the Model S will operate only standard friction brakes. Regen braking, which turns the electric motor into a generator and sends power back into the battery, will occur only when the driver backs off the accelerator. From the driver’s point of view, the feeling is precisely the same as engine braking in a standard car.The question is, how much regen? Hopefully not as much as the Roadster. Back off the gas pedal at 60 mph in that high-performance two-seater, and you’ll be thrown against your shoulder harness by the aprupt deceleration. The feeling is like driving a high-revving gas-powered sports car stuck in first gear. That may be fine for racing or high-performance driving on twisty roads, but in my opinion it’s highly annoying in normal everyday driving.Tesla is currently mulling over whether to include driver-adjustable trailing-throttle regen on the Model S, and if so, what form it should take. There’s currently no industry consensus about driver-adjustable regen. Nissan doesn’t include it on the Leaf, but the Volt has a simple and very effective system: the driver can increase regen by shifting to “Low” on the gear selector. (There’s no actual change of gear, of course; it just feels like it when you back off the gas pedal.) Volkswagen’s Golf Blue e-motion goes a step further with a paddle shifter on the steering wheel that offers three levels of trailing-throttle regen, plus a “coast” mode.

  According to a Tesla rep in Greenwich, the current Model S Alpha prototypes have infinitely adjustable driver regen. Perfect! I fervently hope production cars retain this feature. But I got the feeling that the company may be leaning toward no driver-adjustability at all for the Model S, along with jarringly strong built-in regen like that of the Roadster. “You’ll get used to it,” the rep told me.

  Bad move. Instead of telling us what we’ll have to get used to, Tesla should be asking us, “What would you prefer?” I , for one, would prefer blended pedal braking and infinitely adjustable trailing-throttle regen.

• Longer-range versions of the Model S will have better battery chemistry, not just more cells. Tesla’s battery packs are arrays of thousands of standard Model 18650 laptop computer batteries. (The Roadster battery pack, for example, has about 6,800 18650 cells.) The Model S will come with three available battery packs offering nominal ranges of 160, 230, and 300 miles. But instead of merely adding more cells to increase capacity, the longer-range versions will also have more advanced cells being developed by Sanyo. As a result, the longer-range versions will weigh only a few pounds more than the basic 160-mile model. But each jump in battery capacity will cost an additional $10,000 or so. “The more advanced chemistry is expensive,” the Tesla rep said.Actual everyday range numbers will be about 20 percent less than the quoted numbers. According to the Tesla rep, the 160, 230, and 300-mile numbers assume the use of 100 percent of the battery capacity. In practice, however, the batteries will be charged only up to 90 percent. And when the battery charge drops to 10 percent, it will revert to a limited “limp-home” mode to conserve power. Thus for normal driving, the range options of the Model S will be 128, 184, and 240 miles, plus a small limp-home reserve..

• Those killer 21-inch wheels that we’ve drooled over in all the Model S photos and displays will be extra-cost options. The standard car will come equipped with 19-inch wheels and low-rolling-resistance all-season tires.

• Beta prototypes of the Model S, virtually identical to the production cars, will be available for test drives by owners (and presumably the press) beginning this fall. Where do I get in line?