Is accelerating hard in an electric (or any, for that matter) vehicle any more wasteful than accelerating slowly?

My flimsy grasp of physics suggests that it shouldn't be. The energy required to accelerate the mass of the car to 30mph is the same regardless of how long it takes; it's just that the use of energy is spread out over different time scales.

Assuming that the motor is as efficient at converting electrical energy into kinetic energy in the drivetrain at all power outputs, are there any other factors that mean it's more energy-efficient to accelerate slowly? For instance is wind resistance a factor?

Clearly one doesn't want to accelerate to a greater speed than is required, and one wants to decelerate as slowly as possible in order to engage regenerative braking, and in the hope that obstructions have cleared by the time you get there.

I guess my question comes down to: can I make driving more fun by always flooring it off the line?

  • $\begingroup$ " ... or any other vehicle..." ICE-based cars are grossly inefficient when accelerating hard. This is due to a bunch of things - poor torque at non-optimal RPM, sub-optimal ignition timing, and so on. Wind resistance is a function of body shape and speed, not acceleration. $\endgroup$ Apr 26 '16 at 13:38

The EV efficiency goes down very quickly with power output as $I^2R$ losses are mounting in the motor, the wiring, the power electronics and the batteries. Since an acceleration phase can only be so long (10-20s) and the car will not be used with constant velocity changes (unless it is designed for racing), the entire design will be optimized in such a way that the heat capacity of the materials will be able to absorb one or two consecutive acceleration cycles at max. power output. Beyond that the controller electronics will go into a thermal limiter mode to prevent damage to the motor, batteries and electronics until the cooling system can remove the excess heat. This also means that at the highest power output the system can be fairly inefficient (I would gestimate less than 70% or even 60%). In normal mode the EV will need less than one third of the peak current, which means that losses are an order of magnitude less. So even if, at peak, 40% of the energy is lost, during normal operation (constant speed) the system has roughly 4% losses. In essence, fast acceleration in an EV can waste quite a bit of energy.

  • $\begingroup$ This is only true if there's significant impedance (Z) inside the motor. A quick scan of a very nice research document, cpp.edu/~wakitch/arts/Wu_et_al_(2015).pdf seems to suggest that the efficiency loss at high current is not a major problem. If someone interprets the data differently I'll happily change my position. $\endgroup$ Apr 26 '16 at 13:32
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    $\begingroup$ @CarlWitthoft: It's not just the motor, although I give it to you that one could make the motor and most of the wiring superconducting, which leaves mostly the impedance of the batteries. That, too, can be minimized by adding capacitors in parallel, so there are (expensive) ways of improving the efficiency for short spurts. I think the better data to use for this case would be Tesla's "ludicrous speed" which seems to require an upgrade to the motor control electronics to minimize the losses in the switching elements. $\endgroup$
    – CuriousOne
    Apr 26 '16 at 18:14

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