A question about regenerative breaking vs. coasting

[Kiros]

When I see videos explaining the Energi regeneration they just mention the breaking regen. As I understand it the car will also generate electricity when the vehicle is coasting (the electric drive motor turns into a generator). This bears out when I look at the center info display during coasting - it shows electricity flowing back to the batteries. If I'm slowing down to an upcoming stoplight which is more efficient? Thanks!

[47Minutes]

IMO the regen braking is more efficient.  I haven't seen anything official on it, but here is what I base my opinion on.  If you have the battery display configured to show either the down arrow when battery is outputting or up arrow when it is charging, watch the up arrow during each of the two behaviors.  When costing it looks just normal.  When regenerative braking is applied it sort of bolds or the outline glows more predominantly.  That leads me to believe it is a visual queue showing more energy being put into the battery.

[lokicat]

It's difficult to say for certain and not sure how one could calculate with so many variables. I don't believe there is a simple yes or no. Here are the pros and cons of each:

 

Coasting

 

This certainly doesn't power as braking, but it also only adds a very small resistance to the car's momentum.

 

Braking

 

Lots of power regenerated, but also consumes a lot more power to get back to the speed prior to braking.

 

Neither gives anything close to 100% efficiency since there is no such thing as a perpetual machine. All things being equal, my intuition tells me coasting my hold an edge because there is little to no loss of energy to heat. If you brake really hard, a lot of the energy gets converted to heat instead of electricity. But I am a chemical engineer and not a physicist.

[Energized]

To be clear, there's regen braking and regen coasting, and then just regular coasting (without regen, i.e. being in neutral).

 

The only real difference between regen braking and regen coasting is the amount of regen applied.  The method of regenerating electricity is the same.  With throttle off regen braking, the system provides a consistent and modest level of regen.  With regen braking, the driver controls the level of regen up to the point where the regular friction brakes are applied.

 

When slowing down for a light, the most efficient way is to judge your distance and use then use the appropriate amount of regen coasting and regen braking to arrive at the light with a 100% braking score, meaning you didn't have to use the friction brakes.  How much regen coasting vs regen braking isn't really a factor.  Turning your kinetic energy into electricity rather than heat (thru the friction brakes) is the goal.

[jsamp]

Well stated, Energized.  

 

The real key is to not press the brake pedal so hard as to engage the friction brakes.  Other than that, the 2 methods (regen brake or regen coast) are going to generate the same amount of energy per MPH of slow-down.

 

FWIW, batteries do better when charged at low current rates, which the coast regen and light braking offers.  Harder braking puts a higher current load to the batteries, which can reduce their life.  So the softer you brake and the more you use coast regen, the better for your batteries.  This is less critical with Li-ion batteries than it was with NiMH, but still holds true.

[Russael]

I would prefer a more aggressive 'coasting' regen.  I noticed that the car has a downhill mode which increases regen while coasting, and you can even drive in L which further increases regen when taking the foot off the gas.  I don't think anybody here has experimented with it so I don't know if it would be more efficient or not, but it's something I'll putz with when I get my car.

 

I'll never be able to not touch the brake at all since the car DOES creep while in gear (and you can't turn that on or off like the Tesla S), but I think I'd be able to get much higher regen that way.

[jsamp]

"downhill mode" and "L" are a more aggressive version of coasting regen.  They are not more efficient, just more aggressive.  It just means you slow down faster.  The total energy recouped would be the same going from 35 mph to a stop no matter which one you used, only the distance to stop would change.

I use "L" on my escape hybrid quite a bit when coasting up to a red light.  It's more even than trying to hold a gentle brake pedal and do a smooth regen stop.

[sturoberson]

During my 27 mile commute, I drive up and down about a 600 foot peak (Sunol Grade).  I am experimenting with the ideal way to use batttery power when going up and regen power when going down the hill.  Of course, with stop and go traffic, this goes out the window but occasionally I have no traffic.

 

As for going up the hill, I have just been using auto and seek your advice.

 

Going down, this is what I have learned:

1 letting the car coast without applying the brake can regen about a mile into the EV battery.  The car will exceed the speed limit if I do this as it still gains speed due to the grade.

2 pressing just hard enough on the brake to light up the animated regen icon puts about 2-3 mile into the EV battery.  I only do this to maintain 65 mph during the descent.

3 have not yet tried low gear or hill assist.  will try next week.  let me know if you have done similar tests.

 

For now, seems that using the brake pedal to maintain speed is the most efficient.

 

Question for all, when pressing the pedal just enough to generate a braking regen, do the brake lights illuminate?  Since I use this on a grade to maintain speed, I can imagine it could trouble drivers behind me if they thought I was really braking or was a two-footed driver.

Edited April 21, 2013 at 06:11 PM by sturoberson

[bibolton]

Yes, your brake lights do come on, but they won't if you use hill assist. You will find that the car will stay around the same speed it was going when you pressed "hill assist", even on a good down slope, and you get quite a bit of charge to the battery.

Edited April 23, 2013 at 01:38 PM by bibolton

[47Minutes]

You're braking my heart.

 

Yes, your break lights do come on, but they won't if you use hill assist. You will find that the car will stay around the same speed it was going when you pressed "hill assist", even on a good down slope, and you get quite a bit of charge to the battery.

[bibolton]

So sorry... I should have caught that. English is hard, and I was born here!!!

[BadRock]

So sorry... I should have caught that. English is hard, and I was born here!!!

 

 

Amen!

  :rockon: