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Once you regen, you will never go back...

When I test-drove the Model S, the regeneration mode of freaked me out (like auto down-shifting). After having my car for two days now, I don't think I could ever go back. It is amazing once you get use to it, and it takes almost little to no breaking in regular city driving. This will give us a good 75% longer use of our break-pads (which can be one of the most expensive things to maintain on any vehicle, more so on a sportcar with high-peformance pads).

Mark K: I have never seen a belt driven hydraulic pump for brakes. Power steering yes. Brakes no. Every brake booster that I have ever seen has been vacuum driven. I learned something today - after doing some research I learned about brakes driven from the steering pump.

To me a scary concept since its possible to drive other cars - albeit with heavy steering - with a broken power steering pump. The idea of also losing brake assist is a worry.

Glad the model s doesn't have a belt driven pump.

@Mark K: ++1

@Brian H:
The simulated regen could only brake the rear wheels.
Most diesel-electric locomotives use dedicated electric heaters for heat dissipation of the motor-braking produced energy. But you got it, too complex for a car... with Model S maxing at 60 kW on regen, that is a lot of heat to dissipate.

@Mark K,

Thanks for the great explanation. You might think about adding this feature to Rod & Barbara's Software Enhancement List. I would do it, but you have a much better grasp of the possibilities.

@ Mark K - you wrote:
There's no efficiency hit here. With regen, pulling your foot off the accelerator means you want to slow down. If the battery is full, you'd have to manually hit the brakes anyway to slow down. The smart braking feature would just do it for you when it interprets your foot release gesture to execute the slow-down. If you want to coast, just keep your foot in the same place. Don't press, and don't release. In regen terms, that means "maintain current speed".

The problem with your concept is that the amount of braking is set by the vehicle, so there's no way of knowing whether it's correct for the given situation. Too much braking will require the unnecessary addition of power to speed the car back up, resulting in a slow-down, speed-up cycle that is wasteful. Even your "constant speed" scenario would be too hard, as any variation in foot pressure will cycle between "more current" and "braking".

I know this occurs with regen, but that's adding energy back into the system not dissipating it. The driver should control the amount of braking that is applied, not the car, because only the driver knows when and where the next obstacle or turn is.

@Getting Amped Soon,

As I read Mark's comment, he is just trying to insure a consistent regen experience, whether the car is 50% charged or 100% charged. You would release the accelerator or apply the brake in precisely the same way under both conditions. As it is now, you have a different experience at 100% than at 50%.

Owning a Roadster and having the no regen happen to me was disconcerting the first time or two. After that I was aware of it and drove accordingly. I would not want real brakes applied unless I was applying them for the reasons getamped and Douglas gave. For most of the owners I would not expect them to be fully charging the vehicle everyday so the times that you would do it you plan accordingly.

It was not hard at all to get used to. Really no different than someone who drove a stick shift their entire life and then drove a automatic for the first time. The deceleration is different between those autos.

@DouglasR - Agreed, but in my current car I have a different experience whether I stay in full-automatic or use manual downshifting, or whether I drive in Regular or Sport mode. When I let off the gas, I can get full coast or ranges of engine compression slowdown, and it's my choice. I don't want the car deciding for me!

You guys are getting too complicated trying to draw the friction brakes into this. Why not just dump power into some big resistors during those rare times when your battery is too full to accept the regen? That would be a lot simpler and safer than screwing around with the brakes.

@mrspaghetti... Like third-row seats barbecue? Tesla and fresh burgers, yum!

Up to 60 kW to dissipate...


You're making me hungry, stop that.

60kw does sound like a lot, but I'm thinking you only regen for short intervals - say 5 seconds at a shot. That works out to about .083kwh per 5 second regen, which would generate enough heat to raise the temperature of 1 gallon of water by about 19C or 66F.

Resistors might work if they were cooled by the battery thermal regulation system (not sure what the capacity of that is, but I suspect pretty high since it has to be able to handle supercharging). Or else they could add a separate battery just for accepting "overflow" regen, which could probably be pretty small. Or a combination; dissipate some of the heat in resistors and dump the rest in a "regen" battery.

Feel free to call BS if you find errors in my calculations.

@mrspaghetti: that's OK. Most diesel-electric locomotives use dedicated resistors to dissipate motor-breaking energy, since they have no batteries.

But I think it is impractical for a car, and complicates the design significantly. At max regen of 60kW, and (guessing) voltage of 400V, the current is 150A (400V*150A=60,000W). You need hefty conductors, even for 5 seconds. Then a place for the resistors, and a way to cool them (locomotives have them on the roof). Raising the temperature of 1 gallon of water by 19C (66F) in 5 seconds, that is some burst of energy - 60kW! Then if a long downhill is first thing out of the driveway, it will be much more than 5 seconds. All this to accommodate a short time driving while regen is not available.

So either do it in software and use the brakes, or do nothing and just get used to it.

Please do call BS if my reasoning is wrong...

I love the regen, and it's already changed the way I drive!


I don't see any flaws in your reasoning.

Maybe another solution (which could be implemented in software) would be another selectable option for charging besides "standard" or "range". It could charge the car to 98% or something, which should leave plenty of room for regen.

Or better yet, Tesla could just give us a slide bar to select our target charge level, with markers on it for "standard" or "recommended" at the 90% level, and another marker at your max charge to retain full regen.


I have an idea: why not get the regen to apply the friction brakes to utilize the energy?

Oh wait . . . that's where we started.

I like the 3rd-row barbecue better!

Aw fughedaboutit! Just teach the driver to apply the friction brakes....


Ok, let's turn the frunk into a barbeque pit then. That would go well with Elon's suggestion about making cross-country trips too: "If you bring food and stay with relatives, you can leave your wallet at home." With the Frunk-e-que, you don't even have to get propane :)

Frunk-gating at footbal games!

That would be frunking cool! Minus all that grease inside the frunk after?

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