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.