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Discussion Starter · #1 ·
Is one of these trips better than the other in terms of amount regenerated? I realize that basic physics says NO, but there could be some subtlety about the system that makes it not so obvious.

Both trips start and finish at rest, with the end point 800 feet lower than the start.
Both trips never use any battery for motion. The regen. number is green throughout both trips.
Both trips start with room in the battery for storage.
Both trips are in L.

Trip A: Descent is at a speed that hits 35 often, using the paddle or L for the necessary slowdowns. In short, this trip gets down to the bottom quickly.

Trip B: Regen in L is used aggressively and the trip is taken much more slowly, perhaps twice as long.

I think we can ignore the increased drag at the higher speed, on the assumption that the effect is small, tho nonzero. My real question is: Does the 800 ft. drop lead to the same regen regardless of the time the trip takes (and perhaps even if one takes an alternate road that doubles the distance, but is still all downhill).

I do this route daily, and it appears that I get 0.6 or 0.7 kWh from the descent, but the numbers are not accuratee enough for me to even guess at the answer to my question.
 

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Is one of these trips better than the other in terms of amount regenerated? I realize that basic physics says NO, but there could be some subtlety about the system that makes it not so obvious.

Both trips start and finish at rest, with the end point 800 feet lower than the start.
Both trips never use any battery for motion. The regen. number is green throughout both trips.
Both trips start with room in the battery for storage.
Both trips are in L.

Trip A: Descent is at a speed that hits 35 often, using the paddle or L for the necessary slowdowns. In short, this trip gets down to the bottom quickly.

Trip B: Regen in L is used aggressively and the trip is taken much more slowly, perhaps twice as long.

I think we can ignore the increased drag at the higher speed, on the assumption that the effect is small, tho nonzero. My real question is: Does the 800 ft. drop lead to the same regen regardless of the time the trip takes (and perhaps even if one takes an alternate road that doubles the distance, but is still all downhill).

I do this route daily, and it appears that I get 0.6 or 0.7 kWh from the descent, but the numbers are not accuratee enough for me to even guess at the answer to my question.
For option B, why would you want to drive at only 15 mph? If you’re on public roads, driving that slowly may be an issue for anyone driving behind you. What’s the total distance travelled?
 

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Discussion Starter · #3 ·
It is a nonissue because traffic is very light. Further our neighborhood voted (I voted against) to put in speed bumps so we have 4 stupid bumps on the way down. In any case, of course if someone is behind me I do what is appropriate. Now, I think physics would say that distance (also speed) does not matter. But not sure that all the rules apply. The distance from my house to the stop sign at the bottom of the hill is 1.8 miles. I never need to call on any energy to get there.
 

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The total potential energy difference is m*g*h. Using 800' elevation, and 4100 lbs for loaded weight, converting to SI and kWh, I get:

1.227 kWh worth of energy.

Regen is usually considered to be about 50% efficiency from wheel to battery....so would predict 0.6 or 0.7 kWh.

This matches your result...physics works I guess.
 

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There should be a fairly easy way to model this, given enough data. There is a quantity of potential energy available, a regeneration electrical efficiency constant (or not constant) and losses to friction, both mechanical and aerodynamic. As speed increases, the largest and most speed-dependent loss - aerodynamic losses - compound non-linearly; energy that can in turn never be recaptured or utilized. Theoretically given a simplified model with a constant electrical efficiency and constant mechanical friction, speed should be kept under the point where the significant exponential inflection point of aerodynamic losses occur, while still progressing at an acceptable or useful rate.
 

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Discussion Starter · #6 ·
So if I was able to get a little more by going slowly (say 0.8 kWh instead of the .6 or .7 that I get) it sounds like it would be reasonable to attribute that to the less drag. So, so far, the answer seems to be that physics laws apply (was opposed to anything quirky about the regen system).

A related issue concerns acceleration. It has often been said that speedy acceleration is bad and should be avoided. But accelerating from 0 to 65 uses (theoretically) the same energy regardless of how long one takes to do it. Of course, the more time one spends at 65 the more energy one is using. But it appears that the pure acceleration to 65 uses the same kWh regardless of how quickly one does it. Therefore why not do it quickly? Habit.
 

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A related issue concerns acceleration. It has often been said that speedy acceleration is bad and should be avoided. But accelerating from 0 to 65 uses (theoretically) the same energy regardless of how long one takes to do it.
Power is a function of work and time. Would have to see a motor efficiency curve to see at what power level is the motor most efficient. Also, drawing more power would heat the battery and wires which changes the resistances. So there's curves for battery and motor with respect to temperatures. You need an auto acceleration button(s). One for most efficient. Others like warp speed would be nice.
 

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So if I was able to get a little more by going slowly (say 0.8 kWh instead of the .6 or .7 that I get) it sounds like it would be reasonable to attribute that to the less drag. So, so far, the answer seems to be that physics laws apply (was opposed to anything quirky about the regen system).

A related issue concerns acceleration. It has often been said that speedy acceleration is bad and should be avoided. But accelerating from 0 to 65 uses (theoretically) the same energy regardless of how long one takes to do it. Of course, the more time one spends at 65 the more energy one is using. But it appears that the pure acceleration to 65 uses the same kWh regardless of how quickly one does it. Therefore why not do it quickly? Habit.
The thing you're missing here is losses in the battery. It has a (temperature dependent) internal resistance.

The power out is more or less proportional to the current you are pulling from the battery, while the power lost in the battery is quadratic I^2*R_internal

At low output powers, (and currents) this loss from internal resistance is low. At high powers, this internal loss can be substantial....when you are pulling 150 kW (max) from the battery, a significant additional power is being dissipated in the battery itself as heat....so you lose more kWh/mi.

The bigger factor of jack-rabbit driving is that regen in not 100% efficient (it too has to go through the internal resistance). So braking faster has lower recovery than braking slower (the LEAF has a dash 'tree game' to teach you to brake slower). Every time you accelerate and then regen back to the same speed you lose energy, and more if you do it at higher acc/deceleration.

This is one reason why EVs with smaller batteries (like the Gen 1 LEAF) are limited to lower HP motors, and have blended regen/physical brakes, and rely on the driver hitting a modest regen sweet spot. The Bolt has a lot more wiggle room to accommodate high HP and regen power. No 'trees' required.
 

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Regarding your road trip and going slower or faster: a factor that will make a difference is the wind drag, which increases by the square of the speed of the vehicle: double the speed means four times as much drag. It does make a difference, but not much, since the car has a pretty low drag coefficient. Just like the EPA numbers state, my Bolt gets slightly better mileage in town than on the freeway - just the opposite of a ICE car. The Bolt uses almost no energy when sitting at a stoplight, while the idling ICE car is burning gas. Both cars have about the same drag at a given speed, but the multi-speed transmission on the ICE car contributes to its efficiency on the freeway.

But I don't think it's going to make much difference what speed you travel at on your trip (within reason). The Bolt's motor and single speed transmission appear to be pretty efficient at all speeds, judging by the EPA numbers (128 MPGe city and 110 MPGe highway) and my own experience (3.2 mi/kWh in mixed driving, and 3.1 mi/kWh on the freeway). I doubt you will save more than 5% by going slower.
 

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The total potential energy difference is m*g*h. Using 800' elevation, and 4100 lbs for loaded weight, converting to SI and kWh, I get:

1.227 kWh worth of energy.

Regen is usually considered to be about 50% efficiency from wheel to battery....so would predict 0.6 or 0.7 kWh.

This matches your result...physics works I guess.
Is Regen really only 50% efficient in this mode? While not doing precise measurements, I live at the top of a hill and go up and down several times and starting with no miles, the average comes out about 4.5 mi/kWh, whereas if it were only 50% I would expect less since the best I get at moderately low speed on the flats is 5 mi/kWh. Maybe sometime I will go up and down 10 times and see how it goes.
 

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This should be a simple thing to test. Go down a steep hill at 50 km/h, then go down it again at 25 km/h (or pick a different pair of suitable speeds that differ by a factor of 2). Note the kW of regen in both cases. Subtract 1 kW from each to account for the non-propulsive load on the battery (assuming no heat or A/C). The faster descent should regenerate at twice the rate since you're covering twice as much distance per second - if not then you have your answer.
 

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It has often been said that speedy acceleration is bad and should be avoided. But accelerating from 0 to 65 uses (theoretically) the same energy regardless of how long one takes to do it. Of course, the more time one spends at 65 the more energy one is using. But it appears that the pure acceleration to 65 uses the same kWh regardless of how quickly one does it. Therefore why not do it quickly? Habit.
I can't speak to the efficiency of the Bolt regarding slow vs fast acceleration, but it's a myth in regular gasoline vehicles that slow acceleration is more efficient. Heavy acceleration is slightly more efficient in gasoline vehicles because the engine is much more efficient near full load and very inefficient at partial loads. The efficiency is lost though if the car is quickly accelerated, only to hit the brakes. Brakes are the enemy of efficiency.

Is Regen really only 50% efficient in this mode?
The many energy conversions are what work against efficiency. When the Bolt uses regen to slow down, and then uses that energy to accelerate, the energy conversion is:

kinetic > electrical
electrical > chemical
chemical > electrical
electrical > kinetic

So if the first 2 steps involved in regen are 70% efficient, and reversing the process in the last 2 steps to accelerate is also 70% efficient, that results in an overall 50% efficiency (0.7 * 0.7 = 49%).
 

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Discussion Starter · #13 ·
Totaled: Note that if you start with 0.0 kWh used and go down the hill storing X kWh, then that X is not included in the KWh count, since negative numbers do not exist for the Bolt. I believe this hold true for the trip odometer as well if you reset that.

If you start down the hill with 10 kWh used, then that number will indeed go down and you will learn how much you regen on descent. For me it is very consistent on the 750 foot hill: I always regen either 0.6 or 0.7 kWh. I have never hit 0.8.
 

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Totaled: Note that if you start with 0.0 kWh used and go down the hill storing X kWh, then that X is not included in the KWh count, since negative numbers do not exist for the Bolt. I believe this hold true for the trip odometer as well if you reset that.

If you start down the hill with 10 kWh used, then that number will indeed go down and you will learn how much you regen on descent. For me it is very consistent on the 750 foot hill: I always regen either 0.6 or 0.7 kWh. I have never hit 0.8.
I have seen that effect, when my Bolt was charged to 100%, rather than with hilltop mode. The data on the trip display and that on the energy used on the central panel no longer matched. On other occasions, when I start out with some kwHr used, that number certainly decreases as I regen down the hill. I just haven't done controlled measurements. :nerd:
 

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Totaled: Note that if you start with 0.0 kWh used and go down the hill storing X kWh, then that X is not included in the KWh count, since negative numbers do not exist for the Bolt. I believe this hold true for the trip odometer as well if you reset that.

If you start down the hill with 10 kWh used, then that number will indeed go down and you will learn how much you regen on descent.
Yes, I've seen the total kWh used go down on hills where I regen a significant amount of energy. But if I were to start down a hill with 0 kWh used I'm not sure if it would go below that.

You'd have to do that test in Hilltop Reserve Mode, since the only way to get 0 kWh displayed is to "fully charge" and if you don't do it in Hilltop Reserve Mode then you probably wouldn't be able to put any additional charge into the battery after fully charging it.
 

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I can't speak to the efficiency of the Bolt regarding slow vs fast acceleration, but it's a myth in regular gasoline vehicles that slow acceleration is more efficient. Heavy acceleration is slightly more efficient in gasoline vehicles because the engine is much more efficient near full load and very inefficient at partial loads. The efficiency is lost though if the car is quickly accelerated, only to hit the brakes. Brakes are the enemy of efficiency.



The many energy conversions are what work against efficiency. When the Bolt uses regen to slow down, and then uses that energy to accelerate, the energy conversion is:

kinetic > electrical
electrical > chemical
chemical > electrical
electrical > kinetic

So if the first 2 steps involved in regen are 70% efficient, and reversing the process in the last 2 steps to accelerate is also 70% efficient, that results in an overall 50% efficiency (0.7 * 0.7 = 49%).
Is the choice of .7 based on measurement or just an estimate?
Based on what the Bolt reports, we have kWhr stored in the battery. Is that electrical energy in your list or chemical energy, not sure how the Bolt software computes that. I assume if you had 40 kWHr stored and connected it to a resistive load and drained it, you would measure 40 kWhr worth of energy dissipated in the load.(E1) Now as a thought experiment, with a 40 kWhr charge, go down a vertical drop (delta H) with regen supplying all the braking. At the bottom, put the battery on the load and drain it. Then compare the power dissipated in the load (E2) This efficiency would be (E2-E1)/(mass x g x delta H). What do you think that efficiency is?

Anyway, the regen is a huge reason my Bolt is more efficient than my old Jetta, which wasted almost all of the the potential energy going down a hill through braking. Maybe it is just my wishful thinking that the efficiency is greater than 50%.:nerd:
 

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Yeah my 50% figure comes from experiments with other EVs on other fora, perhaps poorly remembered. For the OPs data, propulsion of the car certainly took some energy. So maybe a bit higher.

I think you are right that the second conversion to from battery to wheels should not be counted in, as the kWh are on a battery potential energy basis.

Maybe its 65% from wheels to battery (one-way) and the OP lost 10 and 20% in propulsion at his two speeds?

Please feel free to create a richer model. But I think you will need more data from the OP for it to be worthwhile.
 

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Discussion Starter · #18 ·
Sean N: When using Hilltop Reserve (which I do always) the DETAIL screen resets to 0.0 kwH used and 0.0 miles driven. So the right way to do these experiments is to note the amount of kWh used when it is not 0 at the start, and see what happens at the bottom. In short "fully charge" includes the case of a charge to the Hilltop Reserve value. the "score" screen also resets after such a 90% charge.
 

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Sean N: When using Hilltop Reserve (which I do always) the DETAIL screen resets to 0.0 kwH used and 0.0 miles driven. So the right way to do these experiments is to note the amount of kWh used when it is not 0 at the start, and see what happens at the bottom. In short "fully charge" includes the case of a charge to the Hilltop Reserve value. the "score" screen also resets after such a 90% charge.
My point was that you can't tell if the "kWh Used" would ever go below zero unless you start with the reading at zero before you go down the hill, which would require you to complete a charge (either "Hilltop" or "Full") before you go down it.

The value certainly goes down with regen, I see that happen on a regular basis when I go down hills. I just don't know if it would ever go below zero, which is what the other poster claimed couldn't happen.
 
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