# Could an EV reverse to brake?

One of the benefits of electric cars is that many parts can be eliminated, simplifying the overall system. Could EVs eliminate brakes as well by slowing the car down using reverse torque from the motor?

I know this is a horrible idea for a number of reasons, but I'm wondering if there are any fundamental reasons why it couldn't be done. E.g. perhaps axles would fail from the torque required for extreme braking.

EDIT: I'm specifically asking about the case where the car provides a retarding torque using the motor that exceeds the amount that can be effectively used for regen braking. Or is regenerative braking torque limited by something that can't be solved with more power?

• Google "regenerative braking". Sep 9, 2019 at 21:50
• That's what regenerative braking does. Regenerative power falls with decreasing speed so at some point you have to blend in conventional brakes. Sep 9, 2019 at 21:51
• You could look up KE flywheels as well as retarders used on buses, coaches and trucks Tecalemit is a name that comes to mind... Sep 10, 2019 at 14:56

As pointed out above in the comments, this is exactly how "regenerative braking" works: the motor is switched electrically so it works as a generator, and then it soaks up the kinetic energy of the moving car and pumps the resulting electric current into the battery, which gets charged up. while not 100% efficient, this process is good enough to significantly increase the useful range of an electric vehicle that does a lot of stopping and starting, and thus is an essential feature in all electric propulsion schemes currently out there.

• Yes, I'm aware of regen braking and should have discussed it in the question. What limits regen braking torque? I believe at some point providing a retarding torque from the motor will result in net energy loss from the battery, which is the situation I'm describing.
– Doug
Sep 10, 2019 at 3:27
• @Doug if you want to break faster then just apply DC voltage to the coils. This causes the system to break as fast as the motor is energized to stand still. More DC more breaking. Allmost all manufacturing machines break this way on emergency stop. Its very fast if you have enough energy. Sep 10, 2019 at 5:41

There are two ways that the "reverse" torque can be used. Technically speaking, of course.

The first way is the literal understanding of the phrase reverse. Instead of delivering the power to move the car forward, the motor now makes the car moving backward. This idea can be used for marine shipping with extremely heavy container ships, but I haven't heard any idea on it being used on cars.

The second idea has been mentioned at the comments and the first answers: regenerative breaking. The motor in this case, instead of "discharge mode" (or normal working), works as a generator now. The efficiency was about 60~70%. Still, it was enough to improve the deceleration and increase the range of the EV. Some sources suggests that energy consumption can be decreased by 20% in certain scenario.

Hope that help.

Lets consider Tesla model S with approximate weight of 2240kg, say the weight is distributed evenly between the front and rear axels.

Assuming new tires with friction coefficient of $$\mu=0.8 \$$the max friction force F, imparted on each wheel while braking to a full stop will be $$F=\mu mg =(2240/4kg)*9.8m* 0.8/s^2 = 4390N$$

which if divided by the radius of the wheel will be the torque needed.

$$T_{braking}=\frac{4390}{18*0.0254/2} =19512N.m$$ The max torque provided by the engine is $$\ 330N.m \$$ per Tesla specs.

As we see the engine has to be nearly a 100 times more powerful to provide that much rejuvenating resistance force, even if they could come up with something near that power, disseminating the huge current created in just a few seconds will be a task of fire protection and other logistics.

• That can't be correct. If the engine only provided 330 N.m at the wheels, the acceleration from a standing start would be glacial. In fact the Model S has a gearbox of 9.73:1 between the engine and the wheels, and according to Wikipedia the maximum torque from the front and rear motors combined is 1250 N.m. So the maximum torque at the wheels is about 12000N.m, not 330. Sep 10, 2019 at 0:15