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oilerlord
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Re: miles due to regeneration

Thu Sep 14, 2017 8:39 pm

To add to Michael's point...preserving momentum is the key to efficient driving for any vehicle. Braking (regenerative or otherwise) kills momentum. Please post your source about regenerative braking recovering 80% of kinetic energy back into the battery. I don't know what the exact number is, but I'd be surprised if even 20% is recovered.
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michael
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Re: miles due to regeneration

Thu Sep 14, 2017 9:01 pm

oilerlord wrote:To add to Michael's point...preserving momentum is the key to efficient driving for any vehicle. Braking (regenerative or otherwise) kills momentum. Please post your source about regenerative braking recovering 80% of kinetic energy back into the battery. I don't know what the exact number is, but I'd be surprised if even 20% is recovered.


Yes, and to help convince yourself do another test:

Drive a mile at 60 MPH, note the energy consumed

Drive a mile with the car in L (for maximum regeneration) and go on-and-off the throttle repeatedly, fast enough that the car's average speed stays very close to 60 MPH. You will find that much more energy is consumed than in the steady throttle test.
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sparkyps
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Re: miles due to regeneration

Thu Sep 14, 2017 11:11 pm

oilerlord wrote:To add to Michael's point...preserving momentum is the key to efficient driving for any vehicle. Braking (regenerative or otherwise) kills momentum. Please post your source about regenerative braking recovering 80% of kinetic energy back into the battery. I don't know what the exact number is, but I'd be surprised if even 20% is recovered.


Regenerative braking efficiency is the product of the electric motor efficiency and the charging efficiency.

The motor is > 90% efficient and so is the charging.

While I'm not aware of any specific specs for the Bolt there are plenty for the Telsa that specify charging efficiency at 92% max and electric motors are well known to be very efficient, see https://www.energy.gov/sites/prod/files ... 097517.pdf for some graphs.

sparkyps
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Re: miles due to regeneration

Thu Sep 14, 2017 11:21 pm

oilerlord wrote:To add to Michael's point...preserving momentum is the key to efficient driving for any vehicle. Braking (regenerative or otherwise) kills momentum. Please post your source about regenerative braking recovering 80% of kinetic energy back into the battery. I don't know what the exact number is, but I'd be surprised if even 20% is recovered.


Momentum isn't preserved with coasting. Momentum is usually preserved with a positive power inputs.

With an EV there is a continuous range of power inputs to the motor, ranging from the max positive power input through the maximum negative power input (max regenerative braking). Zero power input (coasting) is not a magic number, it's just another power setting.

Rarely is no power input to the motor (when I say power input I am allowing for positive and negative numbers) the correct power setting.

If traffic is slowing ahead, coasting is unlikely to be the correct setting to decrease speed just enough to match the speed of traffic ahead without either crashing into a car or matching traffic speed but being too far behind traffic. Slowing by reducing power, but not to zero or negative, is often the correct power setting or some degree of negative power (regenerative braking).

sparkyps
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Re: miles due to regeneration

Thu Sep 14, 2017 11:31 pm

michael wrote:
oilerlord wrote:To add to Michael's point...preserving momentum is the key to efficient driving for any vehicle. Braking (regenerative or otherwise) kills momentum. Please post your source about regenerative braking recovering 80% of kinetic energy back into the battery. I don't know what the exact number is, but I'd be surprised if even 20% is recovered.


Yes, and to help convince yourself do another test:

Drive a mile at 60 MPH, note the energy consumed

Drive a mile with the car in L (for maximum regeneration) and go on-and-off the throttle repeatedly, fast enough that the car's average speed stays very close to 60 MPH. You will find that much more energy is consumed than in the steady throttle test.


Yes, braking for no good reason wastes energy

I've done tests of regenerative braking, it is very efficient. I drive up an approx 20 mile hill, about a 4000' elevation increase and then drive back down, pretty steady speed both directions. My Bolt regenerative brakes pretty much the entire way down. The round trip mi/kWh is pretty close to driving 40 miles at the same speed on flat ground. What is pretty close? Better than 90% for the R/T of the flat trip mi/kWh.

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oilerlord
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Re: miles due to regeneration

Fri Sep 15, 2017 2:04 am

sparkyps wrote:

Regenerative braking efficiency is the product of the electric motor efficiency and the charging efficiency.

The motor is > 90% efficient and so is the charging.



Not exactly. The motor is >90% efficient, however the battery-to-wheel conversion efficiency is typically only up to 80% efficient. The energy makes a full circle back into the battery and it gets converted twice for a net efficiency of at most 80% * 80% = 64%. Keep in mind those numbers are largely academic, because in practice; YMMV also applies to regenerative braking. In my experience regen doesn't recover anything close to 64% - much less the 92% as you suggest.

"...a body in motion will remain in motion unless it is acted upon by an external force"

Braking is braking. It's the external force that wastes the energy spent to get the car in motion. Regenerative braking only wastes somewhat less of it.
Last edited by oilerlord on Fri Sep 15, 2017 2:30 am, edited 3 times in total.
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Re: miles due to regeneration

Fri Sep 15, 2017 2:24 am

I know pretty well the expressway timing of lights around my house (45-55 mph, with occasional traffic lights, maybe every mile). When I am on an expressway I find I get *much* better mileage if I drive the correct speed for the timing of the lights so that I don't have to stop, or even slow much, at the lights. That *includes* the case of driving about 10 mph faster in one direction (because I know the timing) than in the other (because the timing sucks in that direction, depending on the expected traffic programmed into the system). It also includes flooring it instead of stopping if the light turns yellow when I am close enough so that I can *safely* make it through the intersection instead of stopping. (I obviously don't do it if I don't think I can beat the light, or there is an unsafe condition.)

Driving 10 mph faster in one direction and hardly ever stopping gets me between 1.2-1.5m/kWh better mileage than when I drive 10 mph slower and have to stop at every other light. And it's even a larger difference when I am in slow commute traffic at 15-25 mph but stopping twice at every light (because all the stopped cars can't make it through in one cycle of the light). I try to avoid driving at the height of rush hour, but sometimes I don't have a choice.

I don't believe that any EV gets anywhere close to 60% back from braking. Otherwise why would I get better mileage at 55 mph and not stopping than at 25 mph and stopping twice every mile?

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Re: miles due to regeneration

Fri Sep 15, 2017 3:03 am

sparkyps wrote:Coasting is 0% efficient

Try this test, speed up to 30mph, coast to a stop. Measure energy recovered by coasting. Answer is 0

Now speed up to 30mph and regen brake to a stop. Measure energy recovered by Regen braking. Answer > 0

The big, big thing you're missing here is: how far did you go? It's pretty useless to recover 80% of your energy (which I suspect is pretty optimistic) if you've only gone, say, 200 feet when just coasting may have taken you 1500 feet.

gpsman
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Re: miles due to regeneration

Fri Sep 15, 2017 8:10 am

Coasting is always most energy effiecient.
Ok, let me be precise, coasting is most energy efficient 99% of the time.

CHANGING speed requires energy to be lost, both accelerating, and decelerating. "A body in motion stays in motion unless acted upon by some outside force." And as others said, we want the fewest number of conversions from one energy type to another as possible.

There are a few, absurd, specific cases where coasting may not be the best.

A) Say you are using climate control on the maximum setting. Prolonged coasting may lengthen the time of your trip a significant amount, causing the climate control to eat up all your gains from coasting and then some.

B) as very high speeds (near or at terminal velocity due to air drag), it may be best to regen brake to a lower speed so less energy is lost to air drag overall.

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oilerlord
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Re: miles due to regeneration

Fri Sep 15, 2017 3:29 pm

sparkyps wrote:Coasting is 0% efficient

Try this test, speed up to 30mph, coast to a stop. Measure energy recovered by coasting. Answer is 0

Now speed up to 30mph and regen brake to a stop. Measure energy recovered by Regen braking. Answer > 0


SeanNelson wrote:The big, big thing you're missing here is: how far did you go? It's pretty useless to recover 80% of your energy (which I suspect is pretty optimistic) if you've only gone, say, 200 feet when just coasting may have taken you 1500 feet.


Sean's example is correct. Let's say the car that coasted to a stop went 1500 feet. The car that braked with regen goes 200 feet. It's all fine & good that some energy was recovered, but you'd then be spending all of it + more to go the extra 1300 feet to reach the car that coasted.

This is a debate that comes up once in a while, probably because there's so much misinformation on the Internet suggesting that it's possible to bend the laws of physics. I watched a guy on YouTube who was illustrating / teaching the "best" way to hypermile a Volt. He maintains that driving in "L" (maximum regen) at all times is the key to efficient driving. He uses the same logic that coasting recovers 0 energy / Regen braking recovers > 0 energy - but blissfully ignores how much farther he could have traveled slowing down / coasting in N.

The end goal in hypermiling is about driving the farthest distance on the least amount of energy. Regardless of the cars we drive, efficient driving always comes back to Newton's Law that a body will remain at rest or in motion unless it is acted upon by an external force. For that reason, the amount of energy your brakes recover is irrelevant because it is NEVER as much energy that was spent in getting the vehicle in motion.
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