Rabbit starts in EVs


#1

Most of us know that for fossil fuel-propelled vehicles, rabbit starts and stops are hard on fuel economy as well as on the engine, transmission, brakes, etc. It seems like none of these would apply, or barely, to electric drive-train/regenerative braking systems. True, or are there also significant negative impacts on EVs from such driving behavior?
Thanks for the best car column ever!


#2

Same is true for electric vehicles. Rabbit starts use more energy no matter where it comes from gasoline or battery. Regen has limits, too, so hard stops affect EVs as well.


#3

As this graph shows electric motor efficiency is low at low rpms.
Virtually all electric cars use a single “speed” transmission.
So applying a lot of power from start wastes more energy.

Another from Nissan:


#4

Also, mechanical shock doesn’t care where the energy came from and the parts that transmit the power to the wheels takes a beating same as if it was gas v electric. In fact, electric probably transmits more shock as it has less lag and losses…


#5

Another way to think about it, to rapidly accelerate a car using an electric motor requires more current in the motor windings (compared to slower acceleration), and the energy lost to heat in the motor windings goes as the square of the current. So double the current means 4 times the energy is lot to heat rather than moving the car forward. Another factor to consider, an electric motor generates electrical power as it turns. In an ideal world an ideal electric motor would generate as much energy as it used and not consume any energy at all when it was just moving the car along at a constant speed. But during acceleration even an ideal electric motor isn’t spinning very fast at first so isn’t generating much electric power compared to how much it is using. That’s just another way of saying it takes energy to accelerate an object, and the amount of energy it takes is proportional to the final velocity squared.

Oh, one more thing. When the motor is rapidly accelerating a car, there are forces on its bearing materials that would be less if the acceleration were less. Higher forces on the bearings will wear them out sooner.