Question on Regenerative braking

[whttiger25]

If you adjust the accelerator pedal such that the power line on the dashboard is 0, i.e. no orange or green line, essentially the electronics of the car not sending or accepting voltage/current to/from the motors. The rotors of the electric motors are essentially rotating without any electromagnetic resistance, so you are only losing energy from friction throughout the drivetrain. If you ease up on the accelerator to generate power and use regen braking, the electronics begin to accept current/voltage from the motors. What this does is generate mechanical resistance inside the motors as the rotors rotate past the fixed components of the motor, generating current and electromagnetic resistance. While you need a degree in physics to understand exactly how these motors work, basically as you rotate a coiled wire through a magnetic field, or a magnet past a coiled wire, voltage is created in the wire, and if that wire is connected to a complete circuit, current flows and resistance to the movement is also created. If the circuit is not complete, voltage is created but there is no current and thus no work/resistance. This is how mechanical work is converted to electric power. This current is sent to the battery. If that circuit is opened, or exposed to very high reistance, then only a small current is generated, and thus little or no resistance. The tesla increases resistance to infinity to disable e-braking (coast), minimizes resistance to maximize e-braking, and reverses the equation entirely and applies current and voltage to the motors to power the car.

The closest analogy to coasting in an ICE vehicle is to find this sweet spot of net zero power. The rotors of the electric motors already have their rotational kinetic energy and thus continue to spin without needing extra power to continue to do so (just like the earth continues to spin) and energy is only lost from friction. However, in the coasting scenario, an ICE vehicle still uses energy, as you need gasoline to idle, and a gasoline engine generates much more friction as you don't have a freely rotating rotor, but rather pistons that must be continually propelled to continue moving. If you remove gasoline from the system, the ICE freezes up immediately, while an electric motor at 0 V rotates like you have spun up a basketball on your finger, gradually spooling down from friction of air inside the motor and the axle only.

 

[MarcusMaximus]

If you adjust the accelerator pedal such that the power line on the dashboard is 0, i.e. no orange or green line, essentially the electronics of the car not sending or accepting voltage/current to/from the motors. The rotors of the electric motors are essentially rotating without any electromagnetic resistance, so you are only losing energy from friction throughout the drivetrain. If you ease up on the accelerator to generate power and use regen braking, the electronics begin to accept current/voltage from the motors. What this does is generate mechanical resistance inside the motors as the rotors rotate past the fixed components of the motor, generating current and electromagnetic resistance. While you need a degree in physics to understand exactly how these motors work, basically as you rotate a coiled wire through a magnetic field, voltage is created in the wire, and if that wire is connected to a complete circuit, current flows and resistance to the movement is also created. This is how mechanical work is converted to electric power. This current is sent to the battery.

The closest analogy to coasting in an ICE vehicle is to find this sweet spot of net zero power. The rotors of the electric motors already have their rotational kinetic energy and thus continue to spin without needing extra power to continue to do so (just like the earth continues to spin) and energy is only lost from friction. However, in the coasting scenario, an ICE vehicle still uses energy, as you need gasoline to idle, and a gasoline engine generates much more friction as you don't have a freely rotating rotor, but rather pistons that must be continually propelled to continue moving. If you remove gasoline from the system, the ICE freezes up immediately, while an electric motor at 0 V rotates like you have spun up a basketball on your finger, gradually spooling down from friction of air inside the motor and the axle only.

For true coasting in a Model S, you may actually need a bit of orange, particularly on a hot day with the AC on. The meter seems to show overall system usage, not just from the motors.

 

[whttiger25]

For true coasting in a Model S, you may actually need a bit of orange, particularly on a hot day with the AC on. The meter seems to show overall system usage, not just from the motors.

Or a cold day

I agree that's part of the meter but ideally you should use the accelerator to achieve the speed you want and use a gentle touch if you want to maximize efficiency. This turned into a more theoretical discussion.

Regardless, my point really is that you don't actually need to be in neutral to maximize efficiency of coasting liek you do in an ICE vehicle, as an electric motor rotating without current being applied requires very little energy to maintain, unlike and ICE, where you experience braking when you aren't applying the throttle and the drivetrain is still connected to the engine, because of the many internal frictional forces inside an ICE and the fact that it doesn't preserve rotational kinetic energy like an electric motor rotor.

 

[MarcusMaximus]

Or a cold day

I agree that's part of the meter but ideally you should use the accelerator to achieve the speed you want and use a gentle touch if you want to maximize efficiency. This turned into a more theoretical discussion.

Regardless, my point really is that you don't actually need to be in neutral to maximize efficiency of coasting liek you do in an ICE vehicle, as an electric motor rotating without current being applied requires very little energy to maintain, unlike and ICE, where you experience braking when you aren't applying the throttle and the drivetrain is still connected to the engine, because of the many internal frictional forces inside an ICE and the fact that it doesn't preserve rotational kinetic energy like an electric motor rotor.

True! As a (leftist coastal elite) Californian, this notion of "cold" is foreign to me.

 

[TIppy]

An ice does preserve rotational kinetic energy in the flywheel, the rotating gears in the transmission and other rotating parts of the drivetrain. The flywheel energy is what causes a chirp in the wheels when the clutch is dumped. Engine braking occurs when the throttle is closed and the engine becomes a vacuum pump which extracts kinetic energy from the car.

The inverter uses about half a kilowatt to keep the magnetic fields turning at the same speed as the rotor to produce zero torque, so it's better to coast in neutral. After about 2 seconds in neutral, the inverter shuts down and consumes no energy.

 

[Brian-MS90D]

A few short statements/thoughts that helped me when I first received my MS and was stuck in ICE thinking mode:

• To minimize energy consumption on the highway, make your target zero kW (or slightly above, in orange). On a downhill, allow the car to gain some speed. On an uphill, allow the car to lose some speed. Slow over a long period to a stop instead of stopping quickly. Regen braking helps, but as you can see by the kW numbers on the display, it is not recovering everything.
• My VW diesel Touareg has massive engine braking when I let go of the gas; no coasting in that car.
• At more than 100 mpg equivalent efficiency, the MS is 2 to 5 times more efficient than ICE vehicle (depending on vehicle comparing to). This is enough of a difference for me to not worry at all about achieving perfect efficiency in my MS. I drive it and enjoy it.

 

[MarcusMaximus]

A few short statements/thoughts that helped me when I first received my MS and was stuck in ICE thinking mode:

• To minimize energy consumption on the highway, make your target zero kW (or slightly above, in orange). On a downhill, allow the car to gain some speed. On an uphill, allow the car to lose some speed. Slow over a long period to a stop instead of stopping quickly. Regen braking helps, but as you can see by the kW numbers on the display, it is not recovering everything.
• My VW diesel Touareg has massive engine braking when I let go of the gas; no coasting in that car.
• At more than 100 mpg equivalent efficiency, the MS is 2 to 5 times more efficient than ICE vehicle (depending on vehicle comparing to). This is enough of a difference for me to not worry at all about achieving perfect efficiency in my MS. I drive it and enjoy it.

Yep, when I'm really trying to conserve power, I go for your first point. I'll usually turn on autosteering so I can focus on speed adjustments and gradually reduce speed going uphill(toggling speed down by 1mph just as the orange bar starts getting high until I reach what I consider a minimum safe speed) and increase speed going downhill(basically reverse that procedure, increasing speed by 1mph as the green bar gets high until I reach a maximum safe speed). It's remarkable how much you can increase range doing that.

 

[TIppy]

I would think that as long as you don't regen on the back side of the hill, it shouldn't make any difference. If the cruise control maintains the speed as you gain altitude, the extra energy that comes out of the battery is stored as potential energy of the car. As you go down the back side of the hill, the motors will have to provide less power to maintain speed as you recover the potential energy as you lose altitude.

Now if you let the speed drop as you climb the hill, but never let it go above your original speed on the way down, you will use less energy because your average speed as you traverse the hill will be less. It will of course take you longer to get past the hill. You can always save energy if you slow down, as long as you stay above 22 mph.

If you let your speed on the down side climb so that your average speed is what your initial speed was, you will use more energy. This is because power increases with the speed cubed, so you are dissipating more energy on the down side than you saved on the up side.

 

[TIppy]

Not to hijack the OP thread, but been always wanting to pose this question and this seems like a fairly appropriate place to bring it up.

With regenerative braking in my Model S, I've always wondered when coming to a stop from say 30 mph if it's more efficient (for the regen system) to time it so I let off the accelerator completely for max regen to slow me to 5mph (where regen cuts out), or feather it to have an almost coasting effect/longer time in the green energy graph?

Basically, I'm asking: is there any difference to having a short duration of max regen or a longer duration of say half regen, all other things being equal.

The energy to cover a given distance increases as the speed squared. So if you double your speed, the amount of energy required is 4 times as much. This is true if wind drag is the prevailing force. As you slow down, it is drivetrain forces that prevail. The energy require to cover a fixed distance is then proportional to speed. If you double the speed it requires twice as much energy. In either case it is best to get your speed down as fast as possible. Regen quickly and then cover the distance at a slower speed. Since there is also a constant power draw, the most efficient speed for the car, as far as range goes, is 22 mph. Going slower than that will actually cost you range.

 

[mach.89]

The energy to cover a given distance increases as the speed squared. So if you double your speed, the amount of energy required is 4 times as much. This is true if wind drag is the prevailing force. As you slow down, it is drivetrain forces that prevail. The energy require to cover a fixed distance is then proportional to speed. If you double the speed it requires twice as much energy. In either case it is best to get your speed down as fast as possible. Regen quickly and then cover the distance at a slower speed. Since there is also a constant power draw, the most efficient speed for the car, as far as range goes, is 22 mph. Going slower than that will actually cost you range.

Thank you- So if I'm understanding you correctly, to get every last electron out of the regen system (in my example of coming to a stop from highway speeds), you're saying to let off the accelerator pedal completely for maximum regen to 22mph and then feather the accelerator pedal to maximize distance while continuing to slow to a stop? Obviously this is not practical in traffic and requires some good judging and timing, but I am still curious to how to maximize it, so thanks for your response.

 

[TIppy]

Thank you- So if I'm understanding you correctly, to get every last electron out of the regen system (in my example of coming to a stop from highway speeds), you're saying to let off the accelerator pedal completely for maximum regen to 22mph and then feather the accelerator pedal to maximize distance while continuing to slow to a stop? Obviously this is not practical in traffic and requires some good judging and timing, but I am still curious to how to maximize it, so thanks for your response.

Keep it at 22mph until you have just enough distance to use full regen to stop.

 

[TIppy]

Using the power vs speed curve for the Tesla roadster, I calculated the energy use for traversing a particular hill. These calculation show that it's better to let cruise control maintain your speed rather than let it drift down on the up side

In non-mountainous terrain, the maximum grade on an interstate highway is 6 percent. If you approach a hill with this grade at 70 mph and keep your accelerator pedal fixed (constant power) your speed will begin to drop. If the hill has a height of 24m, your speed will be 55 mph, the minimum legal speed, when you reach the top. To make your average speed 70 mph as you cover the total distance of the hill by the time you get to the bottom, you will have to increase the power output of the motors The peek speed on the down side of the hill will have to reach 100 mph. The car will travel 400m on the up side in 14.49 secs and 400m on the down side in 11.2 secs. The motors on the up side will consume 293 KJ and 979 KJ on the down side, for a total of 1272 KJ. If you let the cruise control maintain your speed at 70 mph as you traverse the hill, it will require 535 KJ. This includes 28 KJ that are lost by round trip efficiency of the small amount of regen required to keep the speed at 70 mph on the relatively step down side of the hill. The regen loss would only be 14 KJ if the car was also drawing an additonal 5.5KW to support heating or cooling and all other electrical loads.

So if you maintain your average speed at 70 mph, it requires 2.5 times as much energy if you let the speed drift down on the up side of the hill.

 

[SageBrush]

First off, I am a big fan of regenerative braking, absolutely loving "one pedal driving."

I was wondering however if during periods of non-braking, does it actually create more electricity requirement? If I let go of the accelerator in an ICE vehicle, there only rolling resistance of the tire to the ground which allows one to coast and thus increase fuel economy. In the same setting, if the go pedal is not minimally pressed, there is a large resistance from the motors which are in regenerative braking mode (which I take to mean that there is a large resistance that requires some use of battery to overcome).

In a long distance highway trip with little braking I would think the benefits of regenerative braking would pale in comparison to the loss in ability to coast.

Curious if others have a better explanation.

You can coast just fine in a Tesla -- just find the foot pressure that does not engage regen.
By the way, coasting in a regular ICE is pretty lossy because the transmission and engine continue to rotate. Considerably more losses than coasting in an EV.

 

[SageBrush]

If you let your speed on the down side climb so that your average speed is what your initial speed was, you will use more energy. This is because power increases with the speed cubed, so you are dissipating more energy on the down side than you saved on the up side.

I'll have to calculate later, but this approach works well for me:
Level ground: say 110 kph
Before a hill: speed up to 120 kph;
Bleed off speed down to 100 kph by top of hill;
Reach 110 kph again at bottom of hill

Advantages:
Moderated load demand
No regen
Minimal additional aero losses
Reasonable speed going up the hill

The technique is ingrained in me from years of hybrid driving but it still has some benefit in EV driving. My other technique is to bleed off some speed before the top of the hill so that I do not travel too fast or use regen on the way down.

 

[TIppy]

I'll have to calculate later, but this approach works well for me:
Level ground: say 110 kph
Before a hill: speed up to 120 kph;
Bleed off speed down to 100 kph by top of hill;
Reach 110 kph again at bottom of hill

Advantages:
Moderated load demand
No regen
Minimal additional aero losses
Reasonable speed going up the hill

The technique is ingrained in me from years of hybrid driving but it still has some benefit in EV driving. My other technique is to bleed off some speed before the top of the hill so that I do not travel too fast or use regen on the way down.

Anything less than about a 5 percent grade and you won't need regen on the back side at 70 mph. And even if you do need regen, it will be pretty low. It will cost you much more energy having to increase your speed to maintain an average speed of 70 if you drop below that speed.

 

[SageBrush]

Anything less than about a 5 percent grade and you won't need regen on the back side at 70 mph. And even if you do need regen, it will be pretty low. It will cost you much more energy having to increase your speed to maintain an average speed of 70 if you drop below that speed.

A 5% grade over one km is about 250 Wh of potential energy.
I'll guesstimate level driving at 110 kph at 200 Wh/km and regen at no more than 50% efficient, so your extra losses are a good 10%.

 

[SageBrush]

Keep it at 22mph until you have just enough distance to use full regen to stop.

There are more variables at play than just air resistance, e.g. regen efficiency.

In general, it is a poor idea to regen any more than absolutely necessary, meaning you have to slow down and road friction is not enough. Slowing down to 22 mph and then using power to stay at that speed instead of using kinetic energy until the stop is not a recipe for efficient driving.

 

[MarcusMaximus]

Using the power vs speed curve for the Tesla roadster, I calculated the energy use for traversing a particular hill. These calculation show that it's better to let cruise control maintain your speed rather than let it drift down on the up side

In non-mountainous terrain, the maximum grade on an interstate highway is 6 percent. If you approach a hill with this grade at 70 mph and keep your accelerator pedal fixed (constant power) your speed will begin to drop. If the hill has a height of 24m, your speed will be 55 mph, the minimum legal speed, when you reach the top. To make your average speed 70 mph as you cover the total distance of the hill by the time you get to the bottom, you will have to increase the power output of the motors The peek speed on the down side of the hill will have to reach 100 mph. The car will travel 400m on the up side in 14.49 secs and 400m on the down side in 11.2 secs. The motors on the up side will consume 293 KJ and 979 KJ on the down side, for a total of 1272 KJ. If you let the cruise control maintain your speed at 70 mph as you traverse the hill, it will require 535 KJ. This includes 28 KJ that are lost by round trip efficiency of the small amount of regen required to keep the speed at 70 mph on the relatively step down side of the hill. The regen loss would only be 14 KJ if the car was also drawing an additonal 5.5KW to support heating or cooling and all other electrical loads.

So if you maintain your average speed at 70 mph, it requires 2.5 times as much energy if you let the speed drift down on the up side of the hill.

Of course, that's only true if you're worried about maintaining an average speed. If not, bleeding off speed on the way up, and regaining the same speed lost on the way down will absolutely be more efficient. You're using less energy on the way up and just not regenning as much on the way down(regen being lossy, this is where the gain comes from)

 

[TIppy]

Of course, that's only true if you're worried about maintaining an average speed. If not, bleeding off speed on the way up, and regaining the same speed lost on the way down will absolutely be more efficient. You're using less energy on the way up and just not regenning as much on the way down(regen being lossy, this is where the gain comes from)

Of course you can save energy if you slow down. But if you want to compare energy use for traversing the hill in the same amount of time, it is better to maintain your average speed rather than slowing down and then speeding up.

 

[TIppy]

There are more variables at play than just air resistance, e.g. regen efficiency.

In general, it is a poor idea to regen any more than absolutely necessary, meaning you have to slow down and road friction is not enough. Slowing down to 22 mph and then using power to stay at that speed instead of using kinetic energy until the stop is not a recipe for efficient driving.

The power to maintain speed increases with your speed cubed. Twice the speed, eight times the power. So if you are coasting at 60 instead of 30, you're dissipating your stored kinetic energy eight times faster per second. But you're only covering distance twice as fast.

If you look back at post #20, I calculated the energy used for coasting from 70 mph to 5mph, 681.3KJ(189 WH). By integrating the power vs speed curve, I was able to determine how far the car coasted(0.983 mile) and the amount of time it took(106.6 secs). That gave me the average speed (33.2 mph) during the coast down. If you drive the 0.983 miles at that speed, it'll take exactly the same amount of time.

If you look at the plot, at 33.2 mph the car requires 5.2 KW to maintain speed. 5.2KW * 106.6 secs is 554 KJ (154 WH). The difference of 681KJ (coasting) and 554KJ (motoring at 33 mph) is 127 KJ (35 Wh). Where did that difference go? 80 percent of it (regen efficiency) is in the battery and 20 % was dissipated as heat from the drivetrain.

So coasting is not the best strategy. You are wasting valuable kinetic energy at high speeds.