Regenerate Vs Build Speed While Going Downhill?

On slower speeds, no doubt it's best to build speed downhill then shed it when on the level, but I was wondering about higher speeds. The drop off in efficiency above 60 or 70 makes me think it would be better to hold the speed while on the downhill and regenerate the batteries. ... Any thoughts on what approximate speed this might be around?

Regeneration is, at most, about 40% efficient at recovering energy and returning it to the battery. Coasting is about 95+% efficient at moving the car. So you want to coast whenever you can, and use regen mainly when you can't coast, or can't use the additional speed safely. There is no magic speed number at which regen becomes better than coasting - it's all situational.

LeftieBiker said:

Regeneration is, at most, about 40% efficient at recovering energy and returning it to the battery. Coasting is about 95+% efficient at moving the car. So you want to coast whenever you can, and use regen mainly when you can't coast, or can't use the additional speed safely. There is no magic speed number at which regen becomes better than coasting - it's all situational.

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I'll disagree here. Aerodynamic drag increases proportional to the square of airspeed. This is why we have such a thing as terminal velocity. At some point even Earth's gravity itself is powerless to accelerate an object falling through the atmosphere.

There must be some cutover point where gravity is better employed to charge the battery than to waste it on drag for a marginal increase in kinetic energy, but I'm not sure I have the math to derive it. For me, it's more of a practical matter -- not willing to get a speeding ticket in the name of efficiency.

Do you have any source you could cite for those efficiency numbers? Where did you come up with those values? Isn't efficiency related to the ratio of output to input, either power or energy?

The way i understand it, Regen converts kinetic energy of motion back into electrical energy thru the magnetic fields in the motor. It seems that regen would have the same efficiency as that of the motor. The cells have different maximum rates for charge and discharge, but that is related to the chemical conversion of energy more than efficiency.

Coasting down a steep hill converts the potential energy at the top into kinetic energy with a higher speed at the bottom, so it might be considered to have over 100% efficiency since the input energy is nearly zero compared to the output. Coasting on a flat surface will slow the car due to the friction and drag forces that are always present and acting to retard the motion of the car, but it still has over 100% efficiency since the input is nearly zero. Like dividing by zero or a very small number, the ratio >1.

Terminal velocity while coasting without regen on my steep interstate hill is around 84 mph. i can use regen to adjust the speed down to the legal and will have more Range miles showing at the bottom than at the top.

Other than hills a better way to get more range is to travel at lower speeds, look ahead to anticipate stops and try to use regen instead of the friction brakes.

The 40% figure - actually 39% - was the result of some lab testing that was done years ago. I'm afraid I no longer remember the specific source. You are failing to take into account the efficiency losses from heat and from mechanical friction. It may well be that the motor's part of regeneration is ~90%, but the system as a whole, with battery charging losses (mainly in the form of heat), is much less.

This may be the study, although if it is then the result may have been misreported here - or there is a different number actually reported within the conclusions. I've also seen 16% - 90% quoted, so 39% might be for averaged conditions.

https://www.sae.org/publications/technical-papers/content/2013-01-2872/

i think the low regeneration "efficiencies" quoted in some old threads misinterpreted some of the parameters (by including rolling resistance, mechanical friction, and aerodynamic drag). The efficiency from the battery through the power electronics to the motor under load and efficiency from the motor through the power electronics to the battery under regeneration are nearly the same and the internal resistance of the battery is very low. Also, the efficiency of the motor itself does not matter which way the current is flowing. The mechanical losses in the gear reducer probably reduce the regeneration efficiency a little and account for the 70% figure noted in the summary of the SAE paper. The rolling resistance of the tires, the aerodynamic drag, and mechanical friction in bearings is present whether the car is drawing power from the battery or returning power to the battery.