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>I exited the freeway, hoping that the lower-speed rural road driving would allow more regenerative braking and would extend the range. This is two articles no

>I exited the freeway, hoping that the lower-speed rural road driving would allow more regenerative braking and would extend the range. This is two articles now where the authors, or technicians they're talking to, seem to think the model S has found away around the laws of thermodynamics. It has not. Regenerative breaking will not charge your battery any more than you drained it getting up to that speed in the first place. In fact, by definition, it will charge the battery less. So anyone who thinks the key to getting more range out of an electric vehicle is to abandon smooth and steady driving and find a way to slam on the brakes is in for a rude surprise.

moultano 14y ago
I think the source of this is that going at slower speeds is more efficient in the Tesla. It has a fixed gear[1], so you don't gain anything by going faster, you just lose more energy to air friction. If the regenerative breaking performs well enough, it's possible that driving off the highway will be more efficient. [1]http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission
MRSallee 14y ago
In a car without regenerative braking, city driving will return worse gas mileage than highway driving. In a car that does have regenerative braking, city driving will often return higher mileage. The regen braking isn't the sole source of efficiency gain -- aerodynamic drag, gearing and engine optimizing are responsible for the "true" gains -- but regen braking makes it more possible to realize the "true" gains because you're no longer throwing away all of the true gains on braking.
- lutusp 14y ago
  > In a car without regenerative braking, city driving will return worse gas mileage than highway driving. In a car that does have regenerative braking, city driving will often return higher mileage. The above statement is false. Regenerative braking in the Tesla (which only has regenerative braking on the rear wheels) only recovers about 20% of the energy lost in stopping the car. Accelerating again requires 100% of the car's final kinetic energy to be provided by the battery. Therefore city driving is always less efficient than driving at a steady speed unless you're planning to drive at 90 miles per hour. All regenerative braking can do is minimize the losses caused by stop-and-go driving, it is never an improvement over driving at a steady speed at the same average speed or, for that matter, most higher speeds. I wish this myth would go away -- by way of some physics education. > ... you're no longer throwing away all of the true gains on braking. No, only 80% of them. Which means you would have to drive very fast to equal the losses of stop-and-go driving.
  - cmsmith 14y ago
    I'm no expert on this, but your 20% number seemed pretty low: According to this, the tesla regenerative braking is just as efficient as the motor is at acceleration. Result: up to 64% of the original battery energy returned to the battery. http://www.teslamotors.com/blog/magic-tesla-roadster-regenerative-braking http://www.teslamotors.com/blog/magic-tesla-roadster-regener... And this page about the Volt suggests that the efficiency might get up to 70% in some situations. http://www.designnews.com/document.asp?doc_id=235313&dfpPParams=aid_235313&dfpLayout=article http://www.designnews.com/document.asp?doc_id=235313&dfp... This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving.
    - lutusp 14y ago
      > According to this, the tesla regenerative braking is just as efficient as the motor is at acceleration. That's true but misleading. If the motor's combined electrical and mechanical losses cause only 64% of the battery's energy to propel the car forward, then by that reasoning, only 64% of the braking energy is made available for charging. Let's look at all the factors that influence the outcome: 1. Both electrical and mechanical losses must be accounted for. 2. The electrical loss creates a factor of 0.8, the mechanical loss also create a factor of 0.8. Result 0.64. 3. On the Tesla, only the rear wheels have regenerative braking, the front wheels have disc brakes. Factor 0.5, so we are now at 0.32. 4. During a stop, the car pitches forward onto the front wheels and unloads the rear wheels, this needs to be taken into account. Factor 0.8 for a total of 0.2: 26%. So I stand corrected -- this analysis concludes that the regenerative braking scheme recovers 26%, not 20%, of the original kinetic energy. A literature search reveals that the best regenerative braking systems that involve all four wheels and in which every effort is made to maximize efficiency, can produce about a 40% energy recovery: http://www2.hesston.edu/physics/201112/regenerativeenergy_cw/paper.html http://www2.hesston.edu/physics/201112/regenerativeenergy_cw... A quote: "Since regenerative brakes can only recover about 40% of the energy we take the recoverable energy and multiply by .4 (Everett, Michael)." The linked paper goes on to show that 40% is actually overly optimistic for various practical reasons. But notwithstanding that, we can take the original 40% figure and apply it to the Tesla. By taking into account that only two of four of the Tesla's wheels have regenerative braking, we arrive at a figure of 20%. > And this page about the Volt suggests that the efficiency might get up to 70% in some situations. No reputable source makes claims this high for a full round trip from battery stored energy to battery stored energy (the linked article doesn't try to do this). Also, the linked chart reports energy recovered in a four-wheel test and measured before being applied to the battery, not energy delivered to the battery as stored energy. > This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving. That's true, but that doesn't result from regenerative braking. It results from lower average speeds and reduced wind and mechanical resistance. Remember that regenerative braking cannot possibly improve on driving at a steady speed. That would violate some very basic physical principles.

moultano 14y ago

I think the source of this is that going at slower speeds is more efficient in the Tesla. It has a fixed gear[1], so you don't gain anything by going faster, you just lose more energy to air friction. If the regenerative breaking performs well enough, it's possible that driving off the highway will be more efficient. [1]http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission

MRSallee 14y ago

In a car without regenerative braking, city driving will return worse gas mileage than highway driving. In a car that does have regenerative braking, city driving will often return higher mileage. The regen braking isn't the sole source of efficiency gain -- aerodynamic drag, gearing and engine optimizing are responsible for the "true" gains -- but regen braking makes it more possible to realize the "true" gains because you're no longer throwing away all of the true gains on braking.

lutusp 14y ago
> In a car without regenerative braking, city driving will return worse gas mileage than highway driving. In a car that does have regenerative braking, city driving will often return higher mileage. The above statement is false. Regenerative braking in the Tesla (which only has regenerative braking on the rear wheels) only recovers about 20% of the energy lost in stopping the car. Accelerating again requires 100% of the car's final kinetic energy to be provided by the battery. Therefore city driving is always less efficient than driving at a steady speed unless you're planning to drive at 90 miles per hour. All regenerative braking can do is minimize the losses caused by stop-and-go driving, it is never an improvement over driving at a steady speed at the same average speed or, for that matter, most higher speeds. I wish this myth would go away -- by way of some physics education. > ... you're no longer throwing away all of the true gains on braking. No, only 80% of them. Which means you would have to drive very fast to equal the losses of stop-and-go driving.
- cmsmith 14y ago
  I'm no expert on this, but your 20% number seemed pretty low: According to this, the tesla regenerative braking is just as efficient as the motor is at acceleration. Result: up to 64% of the original battery energy returned to the battery. http://www.teslamotors.com/blog/magic-tesla-roadster-regenerative-braking http://www.teslamotors.com/blog/magic-tesla-roadster-regener... And this page about the Volt suggests that the efficiency might get up to 70% in some situations. http://www.designnews.com/document.asp?doc_id=235313&dfpPParams=aid_235313&dfpLayout=article http://www.designnews.com/document.asp?doc_id=235313&dfp... This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving.
  - lutusp 14y ago
    > According to this, the tesla regenerative braking is just as efficient as the motor is at acceleration. That's true but misleading. If the motor's combined electrical and mechanical losses cause only 64% of the battery's energy to propel the car forward, then by that reasoning, only 64% of the braking energy is made available for charging. Let's look at all the factors that influence the outcome: 1. Both electrical and mechanical losses must be accounted for. 2. The electrical loss creates a factor of 0.8, the mechanical loss also create a factor of 0.8. Result 0.64. 3. On the Tesla, only the rear wheels have regenerative braking, the front wheels have disc brakes. Factor 0.5, so we are now at 0.32. 4. During a stop, the car pitches forward onto the front wheels and unloads the rear wheels, this needs to be taken into account. Factor 0.8 for a total of 0.2: 26%. So I stand corrected -- this analysis concludes that the regenerative braking scheme recovers 26%, not 20%, of the original kinetic energy. A literature search reveals that the best regenerative braking systems that involve all four wheels and in which every effort is made to maximize efficiency, can produce about a 40% energy recovery: http://www2.hesston.edu/physics/201112/regenerativeenergy_cw/paper.html http://www2.hesston.edu/physics/201112/regenerativeenergy_cw... A quote: "Since regenerative brakes can only recover about 40% of the energy we take the recoverable energy and multiply by .4 (Everett, Michael)." The linked paper goes on to show that 40% is actually overly optimistic for various practical reasons. But notwithstanding that, we can take the original 40% figure and apply it to the Tesla. By taking into account that only two of four of the Tesla's wheels have regenerative braking, we arrive at a figure of 20%. > And this page about the Volt suggests that the efficiency might get up to 70% in some situations. No reputable source makes claims this high for a full round trip from battery stored energy to battery stored energy (the linked article doesn't try to do this). Also, the linked chart reports energy recovered in a four-wheel test and measured before being applied to the battery, not energy delivered to the battery as stored energy. > This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving. That's true, but that doesn't result from regenerative braking. It results from lower average speeds and reduced wind and mechanical resistance. Remember that regenerative braking cannot possibly improve on driving at a steady speed. That would violate some very basic physical principles.