I use cruise control a lot when driving. I use it where you’d expect on the highways, but I also use cruise control when driving the back roads. I don’t use it so I can stop paying attention to the road but I do it to avoid speeding. It’s far too easy to speed in the Model S with the smooth instant acceleration, the lack of engine noise, the low center of gravity, etc.
I wrote about how cruise control works on the Model S back in the spring of 2014 but it turns out there’s more to the story. As we entered the winter months here in New England I wrote about how the colder temperatures affected range and the regeneration capabilities of the Model S but I didn’t realize that there’s a relationship between cruise control and limited regeneration.
Warning: When Model S regeneration is limited, your cruise control may not work as expected.
Model S Cruise Control Limitation
Cruise control is meant to maintain a constant speed once set. On level terrain its pretty reliable in most cars. When going uphill the accuracy depends on a number of factors like the rate of change of the slope, the throttle response of the car and how fast the cruise function responds. Generally most cars are pretty accurate to the set speed going uphill.
Going downhill is usually a different story. In most ICE cars cruise control will not apply the brakes but will leave the car in gear allowing the engine vacuum to slow the car down. This slowing varies based on the make and model of the car but often is enough to overcome the pull of gravity. Some cars will also downshift to increase the engine “drag” if needed. My wife’s Mercedes ML350 is particularly good at maintaining a constant speed on all terrains. At faster speeds you also have the benefit of the drag from the wind helping to slow you down when needed. Regardless of the approach, the behavior is consistent throughout the year.
The Model S has no resistance through an engine and a very low drag coefficient. The Model S relies on the effects of regeneration to slow the car down when cruise control needs to decelerate. This is especially true at lower speeds. When regeneration is limited, the deceleration from it is reduced and the cruise control system has limited ability to slow the car down. In the image above, I was cruising along at 40MPH with very limited regeneration and started heading down a hill. I noticed my speed increasing and when I looked I saw I was already 13MPH over the speed limit.
The Model S picks up speed quickly going downhill without regeneration.
As an owner you get to pick between standard and low regenerative braking options in the settings. But the Model S will subject to you very low to non-existent regeneration settings depending on the temperatures.
Your driving experience on the same roads in the same car will vary depending on the outside temperature.
Not a small concern
I spoke to an owner from California and they were surprised I was concerned about it. Their thought was that the Model S would warm up and work normally after a short period so its not a big deal. They suggested I accelerate hard a few times to get back to “normal.”
Thats not the way the limited regeneration works. You have to get those 7,000 batteries up to some target temperature before your regeneration comes back. That takes time. Now that we’re constantly below 40F here in New England it often takes 45 minutes of highway speed driving before my regeneration limit is above 30kW so we’re not talking about a short time that you’re living with the limitation. More often than not I’m driving with regeneration limited now thanks to the weather.
On that same hill I did some testing. I found that for regenerative braking to work well I need the regeneration limit to be 30kW or higher. Anything lower than that gains speed as I head downhill. How fast it gains speed depends on how limited the regeneration was.
If you live in colder climates and use cruise control on your Model S, beware the impact of limited regeneration or risk some hefty speeding tickets.
Tesla Owner said:
Living in California, and only driving rarely in the cold, I would probably never have discovered this problem. I also don’t much like cruise control as it takes away the fun of driving and also just gets into the way when there is much traffic.
I also had the thought — “Rich persons problem” not having cruise control at all times. Somewhat facetious somewhat not. I’ve been in a lot of unsafe by western standard vehicles in developing countries.
I also wonder how auto-pilot will handle this?
I drive 30K miles a year and cruise control is important to me. There are still many fun and enjoyable driving periods but a lot of mindless ones too. Unfortunately I don’t have autopilot hardware but it will be a problem for them there too unless they’ve engineered a solution.
ALL the problems on the Model S are “Rich persons problems” right now by definition.
Excellent blog post. I’m waiting patiently for a Model X so this is good information for next winter. I use cruise all the time in my 2011 Volt. It is clear to see the regen working (drive train display) when in cruise in moderately hilly terrain. As well when you -/down the cruise to slow down for traffic or a local town. One thing I am spoiled about about in the Volt is the regen works ALL the time. It was -1F today and it worked the same as the summer.
I think in some ways the Volt and Leaf have experienced and dealt with some of the issues we’re still facing on the Model S. All of them are bringing EVs forward which is great to see.
Very Interesting… so then, your Winter range is somewhat reduced by virtue of the fact that regeneration is limited to nonexistent in very cold weather if the pack is cold soaked. A question for you then, the range estimate in miles that the S gives you probably reflects some amount of regeneration in the calculation I would imagine, if that is the case… do you see less real range in the cold winter periods? Also is you kW/mile the same in cold weather or is it higher or lower?
Great question and a post coming soon. Practically regeneration doesnt add much for range. The cold has a huge impact on the range not due to limited regen but due to the need to get the (battery) pack to the right operating temperature and the efficiencies around that. Real world data and thoughts coming soon.
You have to wonder how the new 6.1 Tesla release that includes Traffic-Aware Cruise Control (TACC) will deal with this? Perhaps just not allow you to engage it?
Yup, not sure. And unfortunately i’ll be unable to test it :p
I would think that, at some point, Tesla will enhance the cruise-control software to do exactly what you must do when you see your velocity increasing on down-slopes in the absence of regen, which is to apply the brakes.
The question is if they can do it on pre-automated driving HW.
My understanding is that traction control (and stability control as well) are implemented by selectively applying the brake to a drive wheel that is slipping. (Actually, I think I learned that from your 8/24 post.) So the software could certainly activate the rear brakes to slow the car, right? (And both front and rear on dual-motor configurations.)
What I really don’t understand is why it takes so long to heat the battery in the first place. If the battery is discharging enough current to drive the car, there should be ample current to heat the liquid that controls battery temp. And the cabin heater is certainly able to heat cabin air quite quickly. So why in the world does it take up to 45 minutes of driving for the batteries to reach the temperature required to regain regen? (I can’t say that I’ve had it take that long, but I haven’t had mine very long nor do I drive anywhere near the miles you do.) I just don’t get it.
I think traction and stability control can do that, but they aren’t using it on the regenerative braking and I don’t know if the would add it or only invest in the “newer code” areas used for autopilot functions.
The batteries are large, heavy and dense. Think about 7000 AA (not quite the same) batteries sitting at 19 degrees F for 8 hours and then trying to warm them up to 60. Too much mass to do quickly. Its like heating a pool, etc. You can heat the outside/ambient environment quickly but to get the whole pack to a certain temperature requires a lot of “soak” time.
It sounds as if you might have your car in “range mode,” which doesn’t warm the battery as much while you are driving. Also, you can warm the battery while the car is plugged in; I find that I usually start out with 30 kW regen available if I warm the battery by charging right up to my departure time and pre-heating the car.
I tried it in both range mode and not in range mode for several days and didnt notice a difference. Timing the charging so it ended with the start of my AM drive helps the AM but not the PM. But then i’m having UMC issues now so timing the charging has become difficult.