Regen B vs Coasting

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Conan

Active member
Joined
Aug 3, 2013
Messages
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New owner here, I am trying to figure out the advantage of having the B regen option (whatever you want to call it) which gives you better regeneration when not stepping on the gas pedal (is it called gas pedal, or electric pedal? maybe power pedal?)

If I am going down a large hill followed by a long flat road. Do I:
Switch to B and get as much regen as possible down the hill and then drive normally on the flat road (since I regenerated energy, I can go further down the flat road)
or
Do I just let it coast picking up more speed than if it was in B regen which would help with momentum and get me further down the flat road.

Basically, to narrow it down without getting too much into physics. Does the regeneration using the B option ever generate more power than the momentum/kinetic energy from letting it coast down a hill? my guess is no, and is always best to coast and avoid regen at all except when breaking.
 
It depends how long the hill is.

Also remember there is still some regen in D so use N if you really want to save every last Watt second.
 
Q:
Conan said:
Does the regeneration using the B option ever generate more power than the momentum/kinetic energy from letting it coast down a hill?
Click to expand...

A: Yes, you will recapture more energy if you use B on the descent than if you coast down in D and then coast longer.

Why? If you coast down in D instead of B, then you will go faster. You will therefore get more braking from aerodynamic drag (wind resistance). How much more? The drag force is proportional to the square of your velocity. So at any given time, you are losing 60% more power to aerodynamic drag at 70 mph than at 55 mph. You would have recaptured most of that energy via regen if you coasted downhill in B instead.
 
Conan said:
New owner here, I am trying to figure out the advantage of having the B regen option (whatever you want to call it) which gives you better regeneration when not stepping on the gas pedal (is it called gas pedal, or electric pedal? maybe power pedal?)
Click to expand...
How about accelerator pedal?
Conan said:
If I am going down a large hill followed by a long flat road. Do I:
Switch to B and get as much regen as possible down the hill and then drive normally on the flat road (since I regenerated energy, I can go further down the flat road)
or
Do I just let it coast picking up more speed than if it was in B regen which would help with momentum and get me further down the flat road.

Basically, to narrow it down without getting too much into physics. Does the regeneration using the B option ever generate more power than the momentum/kinetic energy from letting it coast down a hill? my guess is no, and is always best to coast and avoid regen at all except when breaking.
Click to expand...
There are a number of considerations. What is your speed at the top of the hill, how steep is the hill, and how concerned are you about law enforcement? Regeneration will only put a percentage of the recovered kinetic energy into the battery. I don't know what the percentage is, but I'll guess 75% to 85%. So, if you can regen less and use the momentum to continue down the road longer before you use more battery energy, the better. However, there are certain high speed cases where coasting in neutral and getting up to 75mph would actually be worse than setting the cruise control for 60 and letting the car take the regen as it goes down the hill. The reason is that you're burning up the potential energy with wind resistance at high speed. The wind resistance is a function of velocity squared, so it goes up really fast over 45mph.
 
I use B mode to control speed down a hill so as to not waste energy to aero drag. It may also require VERY light application of brakes (that hopefully will be all regen and no friction brakes). If you need to stop at the bottom, of course, all regen.

For stopping on level terrain, for highest efficiency, coast to a stop with no regen or brakes.
 
I use "B" when driving in the mountains all the time. My house up there is at 3300'. I can drive 17 miles while using only 3 miles of "range" dropping to an elevation of about 1400' using "B". If you don't want the full "B" effect, you can use the accellerator to reduce braking slightly if needed on a less-steep slope.

This seems to work really well. I generally am going 40-45mph going down hill (which is a pretty good speed for a compromise in aero drag and forward speed and is about the limit of what's safe on the mountain roads).

Once i get to the bottom of the hill i switch out of B into "D" and mostly use cruise control.

I had a 119 mile drive from my house in the mountains to the Bay Area at 3.7mi/kwh this weekend. I had 24 miles of range left when i arrived. (in fact i drove 18 miles to work the next morning and had 10 left when i put it on the charger at Google). That's about the best i've done so far.
 
n3ckf said:
I use "B" when driving in the mountains all the time. My house up there is at 3300'. I can drive 17 miles while using only 3 miles of "range" dropping to an elevation of about 1400' using "B". If you don't want the full "B" effect, you can use the accellerator to reduce braking slightly if needed on a less-steep slope.

This seems to work really well. I generally am going 40-45mph going down hill (which is a pretty good speed for a compromise in aero drag and forward speed and is about the limit of what's safe on the mountain roads).

Once i get to the bottom of the hill i switch out of B into "D" and mostly use cruise control.

I had a 119 mile drive from my house in the mountains to the Bay Area at 3.7mi/kwh this weekend. I had 24 miles of range left when i arrived. (in fact i drove 18 miles to work the next morning and had 10 left when i put it on the charger at Google). That's about the best i've done so far.
Click to expand...

wow. congrats. sounds like an epic drive...150 mile range!

I tried the N strategy and was surprised how much momentum the car had in neutral. I find myself shifting between D, N, and B as if I had a stick shift! I love it!
 
The thing to keep in mind is that once you have kinetic energy, it is better to keep it rather than to convert it to electricity and convert electricity back back to kinetic. If there is a stop at the bottom of the hill or you desire to slow down, then more regen is good. If you will need to speed up again, or even just maintain speed, keep the kinetic energy. Of course, the length of the "coasting" is important. On a long hill, factors like wind friction come into play. Safe driving speed is also crucial to consider. For me, I try to avoid friction braking as much as possible, and minimize regen when I know I'll be hitting the accelerator before hitting the brakes.

I really wish there was a breaking indicator to let us know when friction braking engages. Small presses on the brake pedal seem to be just be increasing regen, and obviously at some point friction is engaged.

Dave
 
Seriously, I learn SO much from you guys. I really do! I had presumed that coasting was better due to the momentum but gathering extra energy is the goal here... so I'll use my *B* more often on the 2 relatively good downhills I am faced with on the 73 on my way home....
 
I think SeaMonster got it right. You lose a good percentage of energy converting kinetic to electricity only to turn around and convert it back. Of course, the other factor is wind friction. I think it would be cool to figure out at what speed are you better off regenerating vs coasting. I used to ride a motorcycle with no wind shield at all. Going 70mph the wind resistance was tolerable almost enjoyable. At 80mph or 85mph it changes exponentially and you have to start leaning down otherwise your going to quickly fatigue your upper body. I would guess the 80 to 85mph down a hill is where you may want to switch to regen. Risk of a hefty 450 dollar ticket increases exponentially and so does wind resistance.
 
I drive exclusively in B. Only time I need the brake is when coming to a stop. In traffic, I can use just one pedal.
 

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