Published on July 25th, 2018 | by Jake Richardson0
How Does Regenerative Braking Work On The Chevy Bolt? (Video + Interview)
July 25th, 2018 by Jake Richardson
Eric Way recently uploaded a review video of the Chevy Bolt’s regenerative braking to YouTube. It contains many comments and insights about how the regenerative braking system works, so it seemed natural to reach out to him for more details. Thankfully, he obliged and we are excited to share what we learned.
1. Why did you choose the Chevy Bolt over some of the other EV options?
I also own a first-generation Chevy Volt, and I was impressed with GM’s EV engineering and technology. In addition, none of the other reasonably priced electric vehicles had the range to conveniently support my typical driving, and most did not have the performance that I was looking for out of my primary vehicle. While I enjoyed the Volt, it was a bit anemic, especially transitioning from a Mitsubishi EVO X. With Josh Tavel as a lead engineer and performance reviews clocking the Bolt EV’s 0-60 mph time in the low 6 second range, it was the obvious choice for me.
I had considered a Tesla Model S 60 instead; however, it would have been too big for my shared parking space. The Model S would have struggled with some of my activities, such as camping and hiking, where the Bolt EV’s ride height and short wheelbase excel (rough, uneven, unpaved dirt and gravel roads). I also did not want an induction motor EV, due to the decreased efficiency.
2. Regenerative braking is a new concept for some people. Why is it important?
Regenerative braking is, in my opinion, the greatest strength EVs have compared to internal combustion vehicles. By recapturing energy from the vehicle’s momentum as you slow down, you store that energy for future use, reduce wear on your brake pads and rotors (which saves money), and (best of all, in my opinion) reduce one of the worst forms of air pollution (particulate matter in the form of brake dust). The reduction in brake usage due to regenerative braking was so significant on the Volt that GM ended up installing rust-resistant brake rotors.
3. In your video, you mentioned using a regenerative paddle, where is it? How do you use it? How often do you use it?
The Bolt EV comes equipped with a Regen on Demand (RoD) paddle behind the left-side of the steering wheel. To use it, simply pull back against the RoD paddle, and it will apply the maximum amount of regenerative braking force available, without using the friction brakes. When pulled back, the RoD paddle will bring the vehicle to a complete stop in either driving mode (D or L). In Drive, the RoD paddle will provide up to 65 kW of regenerative braking power. In Low, it will provide up to 70 kW. Apparently, you can also hold the RoD paddle down in D while modulating the accelerator to mimic driving in L, but I have no experience using it that way.
In my regular driving, I use the RoD often. It essentially replaces the brake pedal in all but emergency braking. It is a helpful supplement to the Bolt EV’s one-pedal driving available in L because it further decreases the times the brake pedal needs to be pressed (i.e., urgent but not emergency braking).
4. On an average 100-mile trip, how much electricity can you send back to the battery on average by using regenerative braking?
That data is not easy to gather, and it will vary based on the type of driving. If you only accelerate one time and drive a steady speed for the entire 100 miles, you will hardly recoup any energy from regenerative braking. If you are in stop-and-go traffic, the amount of energy recaptured can be significant. Likewise, if your 100-mile trip has a net elevation decrease (e.g., you are starting at the top of a mountain), you could add a significant amount of energy back to the battery.
So the energy available to be recaptured is never quite the same. However, for the energy that is available to be recaptured, I’ve found the Bolt EV’s regenerative braking system to be somewhere between 70 and 80 percent efficient. In the case of the first example (a single acceleration and deceleration), you might recover just a couple tenths of a kWh. In stop-and-go traffic, it could amount to a kWh or more. With a net elevation decrease, you could recover several kWh of energy.
5. The official range for the Bolt is about 238 miles per charge. How much can you extend that using regenerative braking?
Again, it will depend on the driving conditions and circumstances; however, in general, electric vehicles with regenerative braking will see a 10 to 20 percent range improvement over the equivalent electric vehicle without regenerative braking.
6. What is the maximum range you have achieved so far?
I’ve driven just over 300 miles on a single battery charge. I feel it is important to emphasize the single battery charge, though, because I often drive over 500 miles in a day, and I have driven well over 600 miles in a day on many occasions. With my typical driving style and conditions, my range per battery charge can be as low as 200 miles or as high as 280 miles.
7. For a complete beginner who just bought a Bolt, is there a particular strategy she or he could use to come up to speed quickly on how to use regenerative braking effectively?
I would say, simply read the manual to see the functionality that is available, and then drive the way that feels the best. My brother recently got his driver’s license, and while he was learning to drive, I took him out in the Bolt EV. After getting used to the vehicle in D, which feels the most like an internal combustion car, I had him switch to one-pedal driving in L. He exclaimed, “Why don’t all cars drive like this? It just makes sense!”
Personally, I think that is the genius behind GM’s regenerative braking system. It is highly customizable to fit the driver’s preferences, yet it maintains most of its effectiveness regardless of which mode is selected. As was demonstrated in my video, the blended brakes means that even those drivers who prefer to drive in D without using the RoD paddle will see a decent amount of regenerative braking.
8. Can you explain what one-pedal driving is and why it can be advantageous?
One-pedal driving is the ability to control a vehicle’s acceleration and deceleration using a single pedal (the accelerator). Pressing the accelerator down will propel the vehicle forward (as it does normally), but reducing pressure on the accelerator will cause the vehicle to decelerate more quickly than it would normally. To me, the primary advantage is that it assures that the friction brakes are not activated unless the absolutely must be. Even on EVs and hybrids with blended braking, pressing the brake pedal will apply some pressure on the brake pads and rotors. That wear on the brakes (and all the nasty stuff that comes with it) is abated with true one-pedal configurations.
An added benefit is that it simplifies the driving process. Only having to worry about a single pedal is simultaneously engaging and relaxing. Stop-and-go traffic becomes a game, and I’ve found myself less stressed and more relaxed after a long drive than I have any right being.
Also, for those hypermilers, one-pedal driving makes it easier and safer to maintain a neutral-energy driving state (i.e., coasting for as long as possible). By matching accelerator pressure to the energy display, you can constantly correct the vehicle’s energy consumption to be 0 kW. It becomes a bit of a game to match the speed of the vehicle, the pressure on the accelerator, and the slope that you are coasting down.
9. Do you combine regenerative braking with hyper-miling techniques like slow, gradual acceleration, not using the AC and driving a little slower than speed limits and coasting up to intersections to maximize your driving range?
I do not. While I am very familiar with hypermiling techniques, I practice very few of them. The Bolt EV’s range is more than sufficient for my regular driving, and when I am traveling long distances, it is faster and more convenient to drive with the flow of traffic (70 to 75 mph), accept the 3.6 to 3.8 mi/kWh of efficiency, and fast charge as necessary.
Also, it is important to call out that regenerative braking itself is something that hypermilers would want to avoid. Because the processes of converting kinetic energy to electrical energy, storing that energy in the battery, and drawing on that energy later all suffer from losses, regenerative braking is less efficient than simply maintaining a steady speed. As Homer Simpson said, “In this house, we obey the laws of thermodynamics.”
10. What do you think GM could do to improve the regenerative braking in the Bolt, based on your observations so far?
I think GM could do a few things:
First, I think they should consider a third option after D and L. Many drivers prefer no regenerative braking at all while driving on the freeway, so having some sort of a coasting mode (maybe C, D, L) would provide those drivers with that option.
Second, I think that the RoD paddle should be fixed to the steering column, not the steering wheel. Despite the fact that the Bolt EV’s regenerative braking is still effective while cornering, the RoD paddle becomes unavailable after a quarter turn of the steering wheel either direction. Having the RoD paddle in a fixed position on the steering column would make it more accessible.
11. In your video, you mentioned the Bolt has something called a hill reserve. Could you explain what that is and what it is for?
The Bolt EV comes with a setting called Hilltop Reserve Mode. When it is activated, the battery will leave the top 10% of capacity unused. The primary purpose, as I understand it, was to provide Bolt EV owners who live at the top of hills or mountains with enough unused battery capacity to allow for full regenerative braking. Because regenerative braking produces electrical energy, that energy needs to go somewhere. If the battery is full, it does not have sufficient room to store the energy, and the regenerative braking force is reduced.
Hilltop Reserve Mode also has a side benefit. Charging lithium batteries to their maximum capacity can increase wear and battery degradation over time, so activating Hilltop Reserve Mode when you don’t need the Bolt EV’s full battery capacity can increase the life of the battery.
This is a function I feel that GM could improve, though. While having the 10% reserve is nice, a better implementation would be to have an adjustable range (probably from 70% to 100%) so that the driver can choose the setting that best suits their needs. For instance, in my case, I would keep it at 80% for my regular 150 mile commute. Other drivers might be fine with 70%, while still others might want 95% (or simply leave it 100%).
12. Does the Bolt vehicle manual adequately explain what regenerative braking is, or did you learn about it online? Are there any info resources you could recommend to new Bolt drivers?
I think the Bolt EV Owners Manual is a great resource, and I think that it should be a new Bolt EV owner’s first stop. Especially for learning about the Bolt EV’s regenerative braking system in particular.
I would use caution when it comes to online resources. For general information about regenerative braking, sure. I think any regular, trustworthy source is fine. For information specifically about the Bolt EV, I would suggest any of the Bolt EV owner’s forums.
13. Does weather impact regenerative braking?
It can. Lithium batteries are sensitive to temperature, and similarly to a battery that is too full, a battery that is too cold can only accept a limited amount of energy. I have not personally been in temperatures cold enough to trigger that behavior, but I’m sure a Bolt EV owner in a colder climate will report exactly how it behaves at some point.
That is actually tied to another suggestion that I had for GM: They should include a driver-controlled battery conditioning function. The Bolt EV’s battery has an active thermal management system that cools and heats the battery as needed. Allowing access to those controls would let drivers warm or cool the battery in anticipation of a DC fast charging session, or in the case of regenerative braking, warm the battery so that it can accept full regenerative braking.
Hot weather doesn’t seem to matter much, and ironically, it might actually help. On several occasions when my Bolt EV’s battery was full, I should not have had significant regenerative braking available. However, because the climate control was drawing several kW of power, it actually increased the amount of regenerative braking that was available. Talk about a win-win.
Image Credit: CC BY-SA 4.0