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Impact of regenerative braking on MVSS-135 Compliance testing, a Q&A between Paul Gritt and Tom Hall

Updated: May 22, 2023

Question by Paul Gritt:

When running the failed booster part of the MVSS-135 on a BEV is the regen braking system active or not? I have read through 135 and it is not clear. In vehicles like the Tesla (one pedal driving) the regen system activates automatically as you lift your foot off the accelerator. That makes the regen an integral part of the service brake system. There are some EV's that have no ability to turn off regen. In the case of these vehicles if you disconnected all the electrical power to the electro-hydraulic booster/mc the regen would still work normally. to turn off the regen you would have to disconnect other things in the vehicle. 135 does say that the failed booster stops should be run in neutral so if that defeats the regen braking that is pretty well defined. If putting the vehicle in "neutral" does not stop the regen that it should stay active. This is very important because the extra brake force from the regen would make meeting the 168m failed booster stopping distance much easier. Response by Tom Hall:

This is my interpretation. I am not aware of any formal interpretation from NHTSA that would agree or disagree. I have used this argument in many discussions with EV Manufacturers,

To my knowledge none of them have relied on RBS to pass Failed Boost

If your Regenerative Braking System (RBS) is part of the service brake, then it can be active during any of the tests except burnish (so active in Failed Boost)

S6.3.13 Electric vehicles.

S6.3.13.1 (a) For an EV equipped with an RBS that is part of the service brake system, the RBS is operational during the burnish and all tests, except for the test of a failed RBS.

If RBS is not part of the Service brake system, it is disabled during all tests except burnish. The last paragraph is conflicting in my mind. It says that the operator (presumed to be the driver) can’t apply any emf to the motors. So, no driver controlled regen.

(b) For an EV equipped with an RBS that is not part of the service brake system, the RBS is operational and set to produce the maximum regenerative braking effect during the burnish, and is disabled during the test procedures. If the vehicle is equipped with a neutral gear that automatically disables the RBS, the test procedures which are designated to be conducted in gear may be conducted in neutral.

However, any non-operator controlled regen remains in effect.

S6.3.13.2 For tests conducted “in neutral”, the operator of an EV with no “neutral” position (or other means such as a clutch for disconnecting the drive train from the propulsion motor(s)) does not apply any electromotive force to the propulsion motor(s). Any electromotive force that is applied to the propulsion motor(s) automatically remains in effect unless otherwise specified by the test procedure.

So, in my mind to use regen during the failed boost.

  1. RBS must be part of the Service Brake System (more on this in a minute)


  1. Any Regen Braking Force must be automatically controlled by the vehicle system without input from the driver, a couple of typical examples.

    1. Regen to simulate a typical coast down drag that is always there can be active, typically in the range of 0.03 -0.05g

    2. Additional increment of regen that is pre-programmed to occur if the RBS has been notified of a failed booster, could be quite a bit depending on the choice of the system designer.

For the RBS to be part of the Service Brake System, 135 is clear.

S5.1.3 Regenerative braking system.

(a) For an EV equipped with RBS, the RBS is considered to be part of the service brake system if it is automatically activated by an application of the service brake control, if there is no means provided for the driver to disconnect or otherwise deactivate it, and if it is activated in all transmission positions, including neutral.

Two types of Regen control to consider.

Category B) RBS algorithm is in the ABS/Brake Controller. RBS only actuates when the Service brake is applied. This is considered a blended system. If you have this, then RBS is part of the Service Brake System, but vehicle can offer one pedal driving and is dependent on the ABS Manufacturer for all Regen control (this dependency can be its own long discussion). This is the less common system in the market at the moment.

Category A) RBS algorithm is not in the ABS System and is actuated by the accelerator pedal (progressive as you come out of the pedal). At first glance it is often argued that this means the RBS is not part of the Service Brake System and then by extension can’t be active during the Failed Boost Test. HOWEVER: Any ABS Algorithm has the function of always evaluating slip. Only IF it determines there is NO SLIP does it send a command to the RBS algorithm that RBS Braking is allowed. Therefore In my Opinion ABS is in control of the RBS by providing the overall decision whether it is allowed to function or not.

This leaves the threshold for Category A System to be part of the service brake system dependent on the portion stating that RBS must be automatically activated by the application of the service brake control. I have tried to make a compelling argument on this point but honestly none are very convincing unless the system adds a significant amount of regen after the service brake is applied (probably in an open-loop control). Possible but I haven’t seen any systems that have done this. Best argument I have is that if you use the detection of failed boost to then generate an increment of regen upon actuation of the brake pedal, this subset of regen, could then be considered part of the Service Brake System.

Lastly how do you execute the test?

  1. Disable the Boost Source (unplug power to the electric booster or pull the vacuum hose)

  2. If the vehicle can disable regen then also do this

  3. If the vehicle does not provide a means to disable regen then, by default the test will run with whatever level the vehicle generates, regardless of what your opinion on the rest of all of this is


-Difficult to argue that a Category A System is part of the service brake system for purposes of passing FMVSS 135 Failed Boost

-Some vehicles do use a lot of regen in real world failed boost situations that add significant benefit to the driver but don’t count to pass the test

-Category B Systems can utilize regen as part of the service brake system during a failed boost stop. Not aware of any that rely on it to comply though.

Follow up by Paul Gritt:

I agree with your positions Tom. The vehicle I am currently envisioning is set up for one pedal driving with up to 0.3g regen decel without applying the brake pedal As you say if you are running the failed boost test on this type of vehicle you would have to do some major modifications to stop the regen. On a related topic. What about the fact that the stop lights may not be activated if only regen is being used to slow down the vehicle. I see this as a possible rear end collision issue.

Additional comments by Tom Hall:

I have always had some concerns about the Brake Light Activity with Regen. For some reason, that I have never understood, a lot of EV Manufacturers don’t like the idea of brake Lights with regen even at high decels.

I can accept this up to maybe 0.05 g or so but anything above that, I would want to light the lights, why not?

Somewhere long ago I had the phrase. Lights on for any Driver induced deceleration.

Now with significant regen and autonomous commands of higher decel braking, I think both should be lighting brake lights to alert other drivers above coast down levels. You can add some kind of stealth mode and turn them off for racing or law enforcement maybe.

ECE 13 had some requirements at one point but I haven’t seen anything from FMVSS.

Additional comment by Paul Gritt about Vehicle’s Stop Lights.

I have an association who owns a Chevy Volt.

He told me that the stop lights activate when he uses the steering wheel mounted paddles to increase regeneration.

Mr. Gritt holds a Bachelor of Mechanical Engineering from Carnegie Institute of Technology (now Carnegie Mellon University), and a Master of Mechanical Engineering from Ohio State University. He joined the Chrysler Corporation brake engineering department in 1969 after serving two years in the U.S. Army core of Engineers.

For the past 40 years he has been involved in all aspects of brake engineering with concentrations in the areas of warranty reduction, quality, brake actuation, dynamometer testing, and system simulation and modeling.

Paul has been active in several SAE technical brake committees and has been a member of the Brake Activities Committee for over 24 years. Paul has co-organized and chaired many brakes technical sessions at the SAE International Congress in Detroit. He is the liaison between the SAE Chassis Activities Committee and the Executive Committee of the SAE Brake Colloquium. Most recently he was a member of the task force that created SAE J-2784 the recommended practice for single ended dynamometer simulation of MVSS-105 and 135.Mr. Gritt retired from Chrysler in November 2008 and is the principal of Paul Gritt Consulting LLC

Thomas Hall has been active in the Brake and Chassis Control Industry for more than three decades. Blending Education and Experience in areas of Engineering, Finance and Law, Tom strives to bring a holistic and practical focus to Industry Activities. Tom’s professional experience spans long tenures at Continental and Robert Bosch with experience leading Project Teams for the Application and Industrialization of complete Brake Systems to many well-known vehicle platforms. Tom is an early pioneer in developing Brake Systems for Pure EV’s for the Tesla Model S as well as experience developing Brake and Chassis Systems for other Startup Manufacturers including Rivian, Faraday, Aptera and several others. Tom also has experience in support of Product Liability Litigation serving in the role of Expert Witness and Advisor to Legal Teams representing both Plaintiff and Defendant Arguments. Active in Professional Development, Tom has developed and lead several SAE Brake Courses. Mr. Hall is a member of and one of the instructor on Brake Academy.

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May 27, 2023

A reader from the USA wrote:

Just finished the Regen Braking v 105/135. Very good and for me very informative. A lot of things I had thought/wondered about that got answered. I did send an email to Tom as I had not had contact with him for some time. A heck of good guy as well as very knowledgeable.


May 27, 2023

A reader from the UK wrote:

I read the latest blog and felt as though I had entered a room and

joined 2 guys part way through a conversation. I had no idea what they

were talking about. It needed some form of introduction to give some

background to the issues. At the very least it needed to explain the

existing system. What is the electro-hydraulic booster/mc? I presume

there is a foundation brake in addition to regenerative braking but I

fail to understand the link between a failed booster and the

regenerative braking system, other than a test exists.

Do you think the 2 guys could follow-up with a blog to outline the

systems and the current problems -…

Replying to

Sorry for no intro.

here is the background. in MVSS-135 as you know there is a requirement to stop a vehicle with the power brake assist system disabled. for many years this was a chanlenge because of low pedal ratios and big vaccuum booster for good pedal feel.

in many BEV's PHEV's and Hybrids the vacuum booster is replaced with an electro hydrualic unit. With modern vacuum systems the ESP pump is used to provide additional pressure when the booster is disabled. this makes meeting the 168m stopping distance very easy. The the electric vehicles with electro hydrualic actuation in some cases there is no seperate ESP pump but it is integral withe booster. The result is that th…

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