Regenerative Braking and Compliance Testing

The Brake Academy recently published a question-and-answer session between Paul Gritt and Tom Hall on the impact of regenerative braking on federal MVSS-135 compliance testing. Both men have an engineering background and extensive experience in the automotive-braking industry.

Here is the Q&A session:

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

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. (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.

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


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

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

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.

  • 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.

The entire Q&A, as well as short bios of Gritt and Hall, can be viewed by clicking HERE.

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