Shift Selector??

[WetEV]

Yes, not changing speeds is the most energy efficient. That's just Newton's First Law and it applies to everything. Keeping the speed down is also more efficient. The laws of thermodynamics. Totally agree.

However, the question here (I thought) was whether it makes sense to "coast down" in neutral on an EV, or allow it to recapture energy through regeneration. I don't see the case for neutral. For discussion purposes don't add slopes. Assume flat terrain.

Ok, assume flat terrain.

We agree that friction brake use is worse.

Maximum regeneration rate slowdown is next worse. As the car will be charging the battery at the maximum rate allowed at that speed, the losses are Resistance *Imax*Imax. So rather than capturing all of the kinetic energy, you capture 1-R*I^2-other_losses, which could be near 50%.

Lower rate regeneration will be better. Losses are less. At half maximum regenerate rate, losses are Resistance*Imax/2*Imax/2, or 25% of the full regeneration case. Also the period of time used slowing down increases, so energy use to maintain speed stops earlier. You capture 1-R*I^2-other losses, which could be near 70%

Keep reducing down to zero regeneration, and you are near the minimum.

For the nitpicker, due to constant use of power by computers, lights, etc, a very low rate of regeneration is actually the minimum loss point. And at a very very slow speed, just stopping with friction brakes is actually best.

 

[wwhitney]

whether it makes sense to "coast down" in neutral on an EV, or allow it to recapture energy through regeneration. I don't see the case for neutral. For discussion purposes don't add slopes. Assume flat terrain.

I think it's unclear which method would be better, assuming constant speed followed by a deceleration phase: coasting to a stop or regen'ing to a stop. Regen captures some of the kinetic energy, but you also start decelerating later, so your constant speed portion is longer, meaning your power consumption to overcome drag forces, etc, lasts longer. The exact answer will depend on a few parameters.

If I did the calculation correctly, a 3700 pound vehicle plus contents traveling at 55 mph has a kinetic energy of 140 Wh. If that same vehicle gets 4.0 mi/kWh at 55 mph, then it is using 250 Wh/mi. So if max regen is 50% efficient and can put 70 Wh back into the battery, that equates to 0.28 miles. The question is reduced to the comparative stopping distances for coasting versus max regen. If coasting takes more that 0.28 miles longer to stop, then it is more efficient. If coasting doesn't get you that 0.28 miles further, then regen is more efficient, under the assumptions made.

It is entirely possible that the optimum will be some level of regen in between 0 and maximum.

Cheers, Wayne

 

[wwhitney]

Also the period of time used slowing down increases, so energy use to maintain speed stops earlier.

Correct, but you have ignored that during the deceleration phase, the same forces that cause a car to coast to a stop will still be acting on the car and reducing its kinetic energy and hence the energy available for capturing by regen.

Cheers, Wayne

 

[WetEV]

Also the period of time used slowing down increases, so energy use to maintain speed stops earlier.

Correct, but you have ignored that during the deceleration phase, the same forces that cause a car to coast to a stop will still be acting on the car and reducing its kinetic energy and hence the energy available for capturing by regen.

Cheers, Wayne

The car takes no drive power while coasting, and does take drive power for longer to maintain speed if stopping faster than coasting. WWhitney has it better explained. It's usually better to leave the energy in the battery than take it out and put it back in.

 

[roundpeg]

I believe the fly in that particular ointment is whether an EV has a true neutral. Can the motor be entirely disengaged from the wheels? I don't entirely know but I suspect not, since it lacks gearing and a clutch. So that to me says that any effort to coast the car is going to be slowed by resistance in the drivetrain for which you get no benefit in terms of regeneration, just loss to friction. In any case even if is arguably more efficient to time a coast to a freeway exit and expend the last drop of energy at the bottom at the bottom of the ramp, the practical issues of attempting to do so probably don't need to be explained.