[electric_eel]

Regenerative braking (regen for short) is a great feature of EVs that saves the kinetic energy in a moving car into the battery during braking. If not for regenerative braking this energy would have been lost permanently as heat in the brake pads. So regen has a special place in the hearts of range anxiety stricken EV owners. However, beware of the ill effects of regen, particularly for range on highways. In this post I, give a simplified analysis of the regen that explains why one needs to be careful when using higher levels of regen in sparse traffic conditions. In fact I have found that in NExon EV, anything beyond regen 1 is useless except for certain really steep descents.

Since this is for building an intuition I will neglect all the other energy loses that occurs when driving the car (predominantly wind resistance). These losses that I have neglected only makes things worse.

TL;DR:

• For maximum range on highways the most important principle is to maintain speed. Aggressive regen can inadvertently cause a slow-down speed-up cycle and results in inefficiency.
  
• Using cruise controls can make the drive inefficient.

The above two recommendation contradict each other so read on if you want to know why it still makes sense.

Acceleration-deceleration cycle is the killer of range.

Imagine that you are cruising at a constant speed of say u. If we neglect the effect of other forces you can continue to cruise at this speed for ever without incurring any cost in terms of energy (Newtons first law). On the other hand consider an acceleration-deceleration cycle which involves increasing the speed to v and then coming back to u (via regen). The cost in energy is as follows

• You need to increase the kinetic energy of the car by K = 1/2 m(v² - u²) for which you will have to supply ηK energy from the battery, where η is the efficiency of the drive train. So the energy lost from the battery is 1/η K.
  
• While slowing down (fully by regen) you reduce the kinetic energy of the car by the same amount K. However not all of it is put back. If γ is the efficiency of regeneration then the energy gained through regen is γ K.

This means the net loss is (1/η - γ)K. The EV drive train is pretty efficient and hence we can assume optimistically that η = 0.9. The regen efficiency would be lower as it involves the generator (an efficiency of 0.9) and a charging loss (optimistically another 0.9). So γ = 0.81 would be reasonable to assume. So the net loss in about 0.3 K which is a nontrivial loss. Assuming the best of the drive train and regen efficiency, almost 1/3 the energy is lost in the regen process. The deceleration followed by acceleration is also similar.

For Hybrids (ICE + regen) things are staggeringly worse. The drive train efficiency η much lesser \< 0.5 and the calculation above shows we loose about 1.2 K.

Cruise control drains battery

Naturally, if we are to maintain speed then it would be best to just use the cruise control. This is indeed correct if we were driving on a flat high way. On the other hand if the high way goes up and down, which is often the case due to fly overs, Cruise control behaves like an acceleration-deceleration cycle: The car does not gain any kinetic energy through out the drive but at the top of the incline it has gained potential energy (P = mgh). We can repeat the above calculation with P instead of K and would arrive at similar conclusion

Recommendation for high way driving

• Accelerating followed by deceleration or vice versa is a big consumer of battery power and hence maintaining speed (on flat surfaces) is the best strategy.
  
  
• Aggressive regen leads to inadvertent deceleration followed by acceleration due to fluctuations in throttle pressure and hence is bad for efficiency on highways.
  
• Cruise control on undulating terrain behaves like acceleration-deceleration cycle and is to be avoided.
  

The moving car is a "battery" of mechanical energy; Drive such that you never supply/demand any energy from it. Needless to say be reasonable in applying this in strategy: safety trumps efficiency always.

The above principle is true for ICE+BEV hybrids as well and the effects are much more dramatic due to the lower efficiency of the ICE. Other considerations like the operational RPM also plays a significant role in this case.

[wocanak]

All good points, but don't forget these:

• When going downhill predominantly, you want enough regen to descent without hard braking, mild braking in most EVs will anyway use regen.
• Traffic in highways is not always light, some sections of highways have notoriously bad city like traffic, you again want regen there.

Most annoying is when the traffic conditions drastically vary in the highway, then you have to constantly adjust the regen level.

[ecenandu]

Originally Posted by electric_eel

• For maximum range on highways the most important principle is to maintain speed. Aggressive regen can inadvertently cause a slow-down speed-up cycle and results in inefficiency.
  
• Using cruise controls can make the drive inefficient.

In your analysis it make sense to compare acceleration - deceleration cycle with and without regeneration?

So, the real range killer is the acceleration - deceleration cycle rather than the regen. I think you are trying to conclude that higher regeneration cause more acceleration - deceleration cycles in turn reducing the range.

Your conclusion is applicable to all cars, ICE, BEV, hybrids etc, i.e. drive smoothly to increase range.

[roy_libran]

Excellent point and I agree. On my modest Tiago EV, I have found that setting Regen to Off on expressways where I can maintain constant speeds between 80-100 kmph leads to lower AEC than with Regen 1/2/3.

[electric_eel]

Originally Posted by ecenandu

In your analysis it make sense to compare acceleration - deceleration cycle with and without regeneration?

Acceleration-deceleration cycle becomes worse without regen (just substitute gamma = 0). That is bad. But the point is even with regen around you loose considerably (1/3 would be an educated guess).

So, the real range killer is the acceleration - deceleration cycle rather than the regen. I think you are trying to conclude that higher regeneration cause more acceleration - deceleration cycles in turn reducing the range.

Yes acceleration-deceleration is the real killer or equivalently in cruise controlled cars, ups and downs (those pesky fly overs) are the killers of range. And aggressive regen does lead to such a cycle.

I also have other concerns with aggressive regen like for example safety issue (tailgating cars are a real issue on Indian roads) and drive comfort (regen 3 has exceptionally large head nodding effect). I rarely set regen to 0 (feels like/is like coasting in neutral) but do not go above 1 either.

Your conclusion is applicable to all cars, ICE, BEV, hybrids etc, i.e. drive smoothly to increase range.

Yes analysis is indeed applicable to ICEs as well. But the difference is that you do not have a regen=0 mode in ICE vehicles due to engine braking effect.

One point I missed making:

Based on the calculations I am more or less also convinced that the fuel saving in ICE hybrid will be marginal at best because of the notorious inefficiency of the ICE drive train. Assuming a 0.5 efficiency for the engine (which is very generous), the loss would be 1.2 * change in mechanical energy. Therefore for an ICE vehicles better driving style (smooth driving, right gears) will have much more effect on efficiency than just attaching a hybrid drive train. This is speculation as I do not have experience driving one. People who have used hybrids can confirm whether this is true or not.

[electric_eel]

Originally Posted by wocanak

All good points, but don't forget these:

• When going downhill predominantly, you want enough regen to descent without hard braking, mild braking in most EVs will anyway use regen.
• Traffic in highways is not always light, some sections of highways have notoriously bad city like traffic, you again want regen there.

Yes my analysis is meant for mostly flat, low traffic density driving. "Highway" was an informal description of that. And yes as you pointed out regen-0 has safety implications similar to coasting in neutral. So especially in hilly terrain there is a point of having higher regen.

Most annoying is when the traffic conditions drastically vary in the highway, then you have to constantly adjust the regen level.

I used to think a paddle shifting regen level shifter would be good for Nexons (like MG ZSEV) but then you can mildly touch the brake for similar effect. I find that more intuitive from the driving perspective.

[srini1785]

Get over it. Any EV when going down hill is going to produce power. How you use it is your choice. You can get it back to your battery or boil water/milk on a kettle, run a 2000W amplifier, label it as clean energy and store it in a battery and sell it (and get yourself a Greta award). What you cannot do is tell the motor not to generate. Period.

[SKC-auto]

Originally Posted by srini1785

What you cannot do is tell the motor not to generate. Period.

Isn't regen level 0 is telling the motor not to generate?

[ecenandu]

Originally Posted by electric_eel

Based on the calculations I am more or less also convinced that the fuel saving in ICE hybrid will be marginal at best because of the notorious inefficiency of the ICE drive train. Assuming a 0.5 efficiency for the engine (which is very generous), the loss would be 1.2 * change in mechanical energy. Therefore for an ICE vehicles better driving style (smooth driving, right gears) will have much more effect on efficiency than just attaching a hybrid drive train. This is speculation as I do not have experience driving one. People who have used hybrids can confirm whether this is true or not.

I disagree.

Hybrid drive train includes a supplementary electric motor, which can add additional torque to the ICE, can be run independently etc.

The conventional Otto cycle (higher power density, more low-RPM torque, lower fuel efficiency) is often modified to an Atkinson cycle or Miller cycle (lower power density, less low-rpm torque, higher fuel efficiency; sometimes called an Atkinson-Miller cycle).

Though marginal, it does offer better efficiency from the above.

Take a real world example, let us take Innova Hycross hybrid Vs non hybrid.
In highway 16.1 kmpl(Hybrid) vs 12.4 kmpl(ICE)
In city 13.1 kmpl(Hybrid) vs 6.9 kmpl(ICE)

Source

[SKC-auto]

Originally Posted by electric_eel

Based on the calculations I am more or less also convinced that the fuel saving in ICE hybrid will be marginal at best because of the notorious inefficiency of the ICE drive train. Assuming a 0.5 efficiency for the engine (which is very generous), the loss would be 1.2 * change in mechanical energy. Therefore for an ICE vehicles better driving style (smooth driving, right gears) will have much more effect on efficiency than just attaching a hybrid drive train. This is speculation as I do not have experience driving one. People who have used hybrids can confirm whether this is true or not.

The effeciency in hybrids is because during deceleration and slow acceleration the ICE engine switches off, even when charging the battery the ICE engine runs at its peak effeciency. Meanwhile, ICE engine in non hybrids run all the time.

The reason why there is lot of difference in city mileage between a Hybrid and non hybrid. The mileage will be closer if you are driving in 6th gear at constant speed of 100kmph.

[JLS]

Originally Posted by SKC-auto

Isn't regen level 0 is telling the motor not to generate?

How I understand is, that for zero level regeneration, motor will generate only while breaking and not while coasting ( and slowing if on plane surface ) or rolling down.
Thanks
JLS

[srini1785]

Originally Posted by SKC-auto

Isn't regen level 0 is telling the motor not to generate?

Originally Posted by JLS

How I understand is, that for zero level regeneration, motor will generate only while breaking and not while coasting ( and slowing if on plane surface ) or rolling down.
Thanks
JLS

As long as there is a current carrying conductor rotating inside a magnetic field there would be resistance to its movement and this resistance is called braking. There no telling the motor not to do it. What you can do is control the current thereby reducing the effect. EV makers have various ways of doing this. Motors that are used in EV's are not the normal coils and permanent magnets ones that we are used to in daily life like say a grinder or a water pump. They have complex thyrister driven circuits that can control which set of coils to turn ON or OFF depending on the situations. What companies do is use this tech to their advantage and introduce modes like Level 0/1/2 so as to reduce or maintain the regeneration effect. Me. personally, I don't mind braking while going down hill but i don't want the effect to kick in every time i take my foot off the A pedal. This is done using circuits which limit the current flowing through the coil wires. Remember that the regeneration is proportional to the coil current.

A lot of people confuse Regen Braking and the physical act of pressing the brake pedal. Regen does not depend on whether you have your foot on the brake or not but it is automatic and is intrinsic to motors that cannot be avoided.

[electric_eel]

Originally Posted by srini1785

As long as there is a current carrying conductor rotating inside a magnetic field there would be resistance to its movement and this resistance is called braking. There no telling the motor not to do it.

The point with regen 0 is that there is no current carrying coil inside the magnetic field:

When one is regenerating either by taking the foot off from the accelerator in regen mode 1,2,3) or by depressing the brake lightly (heavy pressing activates the actual brake pads) the motor is run as a generator. It is this mode of operation that needs to be understood.

A generator with no load connected has no current in the coil by definition. Hence the resistance is minimal (those in the bearings and due to some minimal eddy currents may be). When the load is connected --- for charging the battery in this case --- there is current in the coil (drawn by the charging system) and hence the back emf adds resistance to the rotation (which is what you were alluding to). This is what acts like braking.

Consider the situation when the regen is 0 and we take the foot of the accelerator. The motor is now like a generator (the rotor is rotating) but without a load. As a result there is minimal resistance in regen 0, and it feels (to the driver) like coasting in neutral.

(Note: Regen 0 has all the safety implication of coasting in neutral. During hill descents regen 0 leads to excessive use of the brakes and this causes heating up and failure. Hence regen should be activated on hill descent with the level set depending upon the steepness of the descent)

What you can do is control the current thereby reducing the effect. EV makers have various ways of doing this. Motors that are used in EV's are not the normal coils and permanent magnets ones that we are used to in daily life like say a grinder or a water pump.

"Normal" motors are induction motors and do not have permanent magnets at all and both the stator and rotor are essentially coils. With no permanent magnets, to run them as generators requires external current supplied to the stator.

See

https://en.wikipedia.org/wiki/Induction_motor
https://en.wikipedia.org/wiki/Induction_generator


The only motors in domestic use which uses permanent magnets are the ones in BLDC fans which however is a DC motor. The old cycle dynamos too had permanent magnet but they are now obsolete (well coming to think of it that is a good example for regen braking, if you have ridden one you will know). Most EVs on the other hand uses permanent magnet synchronous motors which can be run as generators by essentially drawing current out of it (while in rotation) instead of supplying current to it (to rotate it). Teslas dual motor setup I think and has both an induction motor as well as a synchronous motor.

A lot of people confuse Regen Braking and the physical act of pressing the brake pedal. Regen does not depend on whether you have your foot on the brake or not but it is automatic and is intrinsic to motors that cannot be avoided.

This is correct but as I said the braking effect depends on whether you have loaded it (are drawing current or not). So regen braking is a combination of generator + charging circuit current. As such if we set regen to 0, there is no change in the mechanical energy of the car (assuming other forces are zero) and hence this is the most efficient mode of operation when cruising. Remember energy is lost (not lost to the universe but from our system) during conversion and not otherwise.

[Jeroen]

Originally Posted by electric_eel

Cruise control drains battery

Naturally, if we are to maintain speed then it would be best to just use the cruise control. This is indeed correct if we were driving on a flat high way. On the other hand if the high way goes up and down, which is often the case due to fly overs, Cruise control behaves like an acceleration-deceleration cycle: The car does not gain any kinetic energy through out the drive but at the top of the incline it has gained potential energy (P = mgh). We can repeat the above calculation with P instead of K and would arrive at similar conclusion

.

I see these comments a lot on the internet. Similar for ICE cars where cruise control is claimed to be inefficient in hilly terrain. I am not convinced about that at all. It might be true, but only under specific circumstances, if at all.

What nobody seems to be taken into consideration is the fact that there is a hill on the road you are driving on. You need to get over it, no matter what. You have two options, you go across using cruise control, or not. So if you want to make any statements on the effectiveness of cruise control it should, my opinion, be a comparison between those two scenario’s. Hill up and down with cruise control on and hill up and down without cruise control.

Modern cruise controls are capable of controlling the cruising speed within a very narrow bandwidth. They have virtually no hystereses.

So when you drive up a hill, the speed is kept constant, which means you don’t loose any kinetic energy (no change in speed) and gain potential energy (height). You have used extra energy going up the hill compared to driving at the same constant speed on a flat road.

When you come down the hill, whether and how much regen takes place is, my opionion, dependent on how steep the decline is and how the regen logic on your car works.

So this is what needs to be looked at: How much extra energy is required going uphill at constant speed and how much of that extra energy can be captured going downhill. Next you need to compare that with a driver taking manual control.

My gut feeling is that in most case the cruise control scenario will be more efficient than the manual driving scenario. One of the reason is that almost anybody will slow down going from a flat road up on a hill. Which means you loose kinetic energy. And as the speed is squared (v²) in the formula of kinetic energy dropping a few kilometers/hour means a lot of energy is lost.

On ICE cars the question whether cruise control makes (economic) sense in hilly terrain is identical in the sense, you need to compare the two scenario’s, with and without cruise control. Obviously an ICE cars does not have regen, so in most cases will be worse off than the EV with regen.

On many ICE cars, the cruise control controls the engine RPM and thus the speed. When the speed picks up going downhill the cruise control starts to close down the throttle to the point where the engine is idling or driving by the wheels/transmission and thus effectively engine braking. Not all cruise controls have the capability to also control the brakes. So the most braking power you can get is engine braking. Which means you end up going faster than the set speed on the cruise control, thus building up additional kinetic energy, or wasting it and braking manually.

In the end the efficiency of cruise control in any scenario, is comparing it to a scenario without cruise control. All else is irrelevant. That hill is in front of you and you need to get over it (literally and figuratively )

And that is a very different way of looking things than most of what I see on the Internet.

Maintaining a steady constant speed does indeed mean no change in kinetic energy, but a reduction of speed be it a poor cruise control or your typical human twitchy right foot, means a tremendous loss of kinetic energy, much more than your typical gain in a bit of extra potential energy.

Look at it this way, if you drive an EV at constant speed up and down a hill, compared driving an EV up and down a hill on cruise control what is the difference? The only difference is precise your right foot is in maintaining the constant speed like the cruise control would. In these sort of case, man against machine, man almost always is less efficient! If your right foot is capable of mimicking the cruise control a hundred percent I don't think there would be any difference in regen either?

Jeroen

[electric_eel]

Originally Posted by Jeroen

I see these comments a lot on the internet. Similar for ICE cars where cruise control is claimed to be inefficient in hilly terrain. I am not convinced about that at all. It might be true, but only under specific circumstances, if at all.

I have tried cruise control on highways (Palakkad-Thrissur) which have a lot of fly overs and I have not got anything better than 125 Wh/Km ever. On the other hand with manual control I routinely get 110 Wh/Km for high way drive for round trips (so as to negate the effect of terrain). My PKD-TVM trip both had an efficency of 109 Wh/Km (both ways). Just recently Palakkad-Coimbatore-Palakkad trip I got something like 111 Wh/km with PKD-COM on the higher side (130s) understandably due to the increase in elevation. It seems that for long enough highway drive with sedate driving at 70-90 Nexon EV consistently gives 110 Wh/km. Cruise control on Nexon EV at least has never
matched this efficiency.

Do I think cruise control can be designed better ? Absolutely. By making it less aggressive in maintaining the speed I think it can be better.

Also your assumption that drivers keep the speed constant as we climb is wrong. I mostly drive with an aim of maximising passenger comfort and then keeping the speed constant as we climb/descent is as bad as acceleration --- steeper the climb worse it is. So I modulate the throttle (of course with limits). In that sense, it is different from cruise
control. In addition to safety issues of using cruise control without ADAS, I find cruise control on undulating terrain seriously compromising passenger comfort and hence use
it sparingly.