However regen is not 100% efficient at recapturing the car's kinetic energy. The guy who drove his Model S for 400 miles on one charge in Florida was careful to drive slowly and not use regen when he could avoid it.
http://www.teslamotorsclub.com/showthread.php/12549-Dave-Met...
regen is not 100% efficient at recapturing the car's kinetic energy.
True, but if you don't use regen you are 0% efficient at capturing it. Any positive number is better than zero. I'll have to read through the link you gave to see what the guy's reasoning was, but regen should always increase your range.
Edit: On reading through some of the links available on Google, I suspect that he didn't specifically try to avoid regen, but just drove too slowly and with too gentle acceleration and deceleration for the regen system to kick in. Regen does require a certain minimum speed and rate of deceleration to achieve a net gain in energy; if you try it when the car is going too slowly or decelerating too gently all the energy goes into losses in the electrical system and none goes to the battery. The car's control system should be calibrated to only allow regen to kick in when there will be a net gain.
So in that sense you could say that the less than 100% efficiency means there are times when there's no point in using it. But it never causes a loss of energy; you're never worse off than you would be not using regen. It's just that the car's controls won't bother trying if there's no point.
But that alternative usually requires driving at higher speed, which more than cancels out any gains from avoiding stop and go because of the increased loss from drag and friction. There aren't any highways where you can drive 400 miles at 25 mph cruise.
Metcalf's description does say that he accelerated and decelerated very gently; see my edit to my upthread post. So he was trying to approach the ideal of driving at a steady 25 mph as close as he could. If the car's regen system had had lower losses, regen would have saved him some energy on those unavoidable decel/accel cycles.
> But that alternative usually requires driving at higher speed, which more than cancels out any gains from avoiding stop and go because of the increased loss from drag and friction.
No, this is a false choice. The choice should not be between stop-and-go driving, versus speeding along. For a proper evaluation, the tested alternatives should be (a) stop-and-go driving with an average velocity of V, versus (b) driving at a constant velocity of V. In that comparison, a constant velocity is much more efficient. The reason is that regenerative braking cannot recover more than a fraction of the energy lost to braking.
The above is in keeping with the best scientific practice, in which an experiment changes just one thing and keeps everything else the same. So we should choose an average velocity, then compare steady speed and stop-and-go driving at that velocity. In that experiment, steady speed wins.
> Metcalf's description does say that he accelerated and decelerated very gently ...
Doesn't matter. Adding a given amount of energy E to a moving object requires the same expenditure of energy regardless of how quickly or slowly it's done (although in practical examples, fast acceleration is wasteful for reasons outside the simplest explanation of the physics). It's the same with removing energy from a moving object, and it is here that the unavoidable losses in regenerative braking prevent the two cases from being equal.
> So he was trying to approach the ideal of driving at a steady 25 mph as close as he could.
That ideal is only achieved by maintaining a steady speed of 25 MPH, not by stop-and-go driving. It's not clear at this point whether Broder was actually told by someone at Tesla that stop-and-go driving was more efficient or not, but if so, that person needs an education.
> If the car's regen system had had lower losses, regen would have saved him some energy on those unavoidable decel/accel cycles.
Yes, but regenerative braking can only minimize losses, it can't recover all the energy lost to braking. Therefore a steady speed is more efficient.
For a proper evaluation, the tested alternatives should be (a) stop-and-go driving with an average velocity of V, versus (b) driving at a constant velocity of V. In that comparison, a constant velocity is much more efficient.
I completely agree, if you are trying to run a scientific experiment. But if you're driving on real-world roads, you're faced with a different set of choices. As I said, you can't expect to drive 400 miles at a steady speed of 25 mph in the real world.
Adding a given amount of energy E to a moving object requires the same expenditure of energy regardless of how quickly or slowly it's done
I wasn't saying that accelerating/decelerating more gently saves energy. I was saying that it probably prevented the regen system on the car from activating at all, meaning that none of the vehicle's kinetic energy was recaptured. Since he could not avoid stopping and starting again (since you can't drive 400 miles at a steady 25 mph on real-world roads), if it had been possible to reclaim some energy through regen during deceleration, it would have increased his range compared to stopping and starting again with zero regen. That's all I was saying, and it's completely consistent with what you're saying.
It's not clear at this point whether Broder was actually told by someone at Tesla that stop-and-go driving was more efficient or not, but if so, that person needs an education.
Not necessarily, because Broder's choice was not between stop and go driving at an average speed of 25 mph, or driving at a steady 25 mph. It was between stop and go driving in Manhattan (you are not, I trust, claiming that it's possible to drive through Manhattan at a steady 25 mph without stopping), at an average speed of 25 mph or so, and driving on freeways at an average speed of, say, 60 mph. Given that choice, it's entirely possible that the stop and go driving would give more range; the exact tradeoff would depend on details like the vehicle's drag coefficient, rolling friction, efficiency of regen, etc.
> I completely agree, if you are trying to run a scientific experiment. But if you're driving on real-world roads, you're faced with a different set of choices. As I said, you can't expect to drive 400 miles at a steady speed of 25 mph in the real world.
A red herring. Whatever speed seems appropriate, steady speed is more efficient than stop-and go driving. My only point is that the advice to intentionally engage in stop-and-go driving is mistaken.
True, but if you don't use regen you are 0% efficient at capturing it. Any positive number is better than zero. I'll have to read through the link you gave to see what the guy's reasoning was, but regen should always increase your range.
Edit: On reading through some of the links available on Google, I suspect that he didn't specifically try to avoid regen, but just drove too slowly and with too gentle acceleration and deceleration for the regen system to kick in. Regen does require a certain minimum speed and rate of deceleration to achieve a net gain in energy; if you try it when the car is going too slowly or decelerating too gently all the energy goes into losses in the electrical system and none goes to the battery. The car's control system should be calibrated to only allow regen to kick in when there will be a net gain.
So in that sense you could say that the less than 100% efficiency means there are times when there's no point in using it. But it never causes a loss of energy; you're never worse off than you would be not using regen. It's just that the car's controls won't bother trying if there's no point.