Which Regen Mode is Most Efficient?

[Tony Montana]

After the implementation of a LOW regen mode, I grew very curious on how much additional efficiency we get with regen.

My instant reaction would be that HIGH regen would yield more efficiency as it's more powerful(?, not sure if that's the best way to put it).

But another part of me feels that LOW regen might yield more efficiency as it's regen-ing for a longer period of time.

Anyone have any concrete data or knowledge on which would be most efficient? Or is it so negligible that it's best to follow my preferences (standard).

Appreciate any input.

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[racekarl]

In theory there should be almost no difference between them. The energy recaptured is relative to the change in velocity, not the rate at which velocity changes.

In practice, whichever mode allows you to avoid using the friction brakes is the one that will be most efficient, but this depends almost entirely on your driving style.

There may be some very minor differences (e.g. perhaps high regen wastes some energy heating up the tires, which have to exert more force against the road) but I expect those are small enough to ignore.

 

[zefram47]

Entirely depends on how you drive it. Regardless of mode, if you find yourself getting into regen and immediately back to throttle a lot, then you aren't going to get much of a benefit. If you're able to modulate the throttle/regen such that you basically never use the friction brakes when coming to a stop and aren't constantly on/off the power, then you'll probably see a sizable benefit. There's no option to blend regen with the brake pedal as other EVs sometimes have, so it's kind of an all or nothing deal and Rivian hasn't added a mode to disable regen to allow coasting as a result. My commute into work has rolling hills with several long descents with a net loss of about 300 ft. On the way in to the office I'll frequently get 2.5-3.1 mi/kWh depending on how judicious I am with regen and squirting away from lights with authority. On the way home I'll see more like 2-2.4 mi/kWh as it's a net uphill. As long as you don't hit thermal limits on regen and have to use the friction brakes to make up the difference, my experience is that it can be a big deal for overall efficiency. But you can't drive it like an ice vehicle and expect it to be magically efficient.

 

[Tony Montana]

I almost never use the friction brakes. However, I find it hard to believe that there would be no difference between the efficiency of regen modes.

Maybe it's cause I don't have a complete understanding of the system but that doesn't compute for me.

 

[Tony Montana]

Interesting take. Where would I find the kWh history from previous trips? Is that in the infotainment?

 

[cjbot3000]

I almost never use the friction brakes. However, I find it hard to believe that there would be no difference between the efficiency of regen modes.

Maybe it's cause I don't have a complete understanding of the system but that doesn't compute for me.

Yeah I would think low regen > lower resistance from the motors > less energy captured

I always drive high regen but it quickly limits on long downhill stretches

If low regen generates energy at a lower rate, I presume it would do so for longer without regen limiting itself

So do we capture more energy by going high and fast before hitting a thermal limit or do we capture more at a lower rate for longer without hitting a limit

 

[Tony Montana]

Yeah I would think low regen > lower resistance from the motors > less energy captured

I always drive high regen but it quickly limits on long downhill stretches

If low regen generates energy at a lower rate, I presume it would do so for longer without regen limiting itself

So do we capture more energy by going high and fast before hitting a thermal limit or do we capture more at a lower rate for longer without hitting a limit

I guess that's the real question

 

[racekarl]

I almost never use the friction brakes. However, I find it hard to believe that there would be no difference between the efficiency of regen modes.

Maybe it's cause I don't have a complete understanding of the system but that doesn't compute for me.

The two big assumptions are 1) that both high and low regen are able to recover the same percent of energy from stopping back into the battery and 2) neither mode exceeds the maximum rate at which the system can recover energy. I think that these are generally both safe assumptions to make, and if you do, then there would be no efficiency difference between them.

Recall that E(k) = 1/2 mv^2 where E(k) = kinetic energy, m = mass and v = velocity.

For example: if your truck weighs 3200 kilograms (~7100 lbs) units and is going 30 meters per second (~67 mph). It will have 1,440,000 joules of kinetic energy. Let's assume regeneration is 90% efficient, meaning that 90% of the kinetic energy can be recovered into the battery as potential energy. So 1,296,000 joules goes back into the battery when you stop (meaning v, and thus E(k) are now both 0).

Because there is no term for time or distance in the energy equation, it shouldn't matter how quickly you stop. That's the theory as I understand it, at any rate...

 

[Chewy734]

Is it obvious when the truck is using the brakes instead of just regen?

The reason I ask is although I rarely ever hit my brakes in high regen, I’ve noticed a lot of brake dust still accumulates on the calipers over time.

 

[Southern R1S]

Is it obvious when the truck is using the brakes instead of just regen?

The reason I ask is although I rarely ever hit my brakes in high regen, I’ve noticed a lot of brake dust still accumulates on the calipers over time.

Do you have the Regen Brake Assist feature turned off? If not, then that feature will occasionally use your physical (friction) brakes rather than regen even though you aren't physically touching the brake pedal.

Likewise, Highway Assist will also use the friction brakes while active if/when it determines that to be necessary in order to safely slow down. Some have speculated Highway Assist is always using friction brakes (rather than regen) since the brake pedal seems to modulate, but haven't seen anything definitive on that one way or another so YMMV.

Automatic emergency braking (with our without Highway Assist enabled) will always use your friction brakes.

Otherwise, the friction brakes should only operate when you're touching the pedal, so brake dust should be pretty minimal outside of those scenarios.

 

[krockett]

I don’t have any scientific data but when casually testing on my own, I had better efficiency in normal vs high re-gen.

 

[Lrak1973]

Entirely depends on how you drive it. Regardless of mode, if you find yourself getting into regen and immediately back to throttle a lot, then you aren't going to get much of a benefit. If you're able to modulate the throttle/regen such that you basically never use the friction brakes when coming to a stop and aren't constantly on/off the power, then you'll probably see a sizable benefit. There's no option to blend regen with the brake pedal as other EVs sometimes have, so it's kind of an all or nothing deal and Rivian hasn't added a mode to disable regen to allow coasting as a result. My commute into work has rolling hills with several long descents with a net loss of about 300 ft. On the way in to the office I'll frequently get 2.5-3.1 mi/kWh depending on how judicious I am with regen and squirting away from lights with authority. On the way home I'll see more like 2-2.4 mi/kWh as it's a net uphill. As long as you don't hit thermal limits on regen and have to use the friction brakes to make up the difference, my experience is that it can be a big deal for overall efficiency. But you can't drive it like an ice vehicle and expect it to be magically efficient.

North Wadsworth? that's my commute, and I get roughly the same numbers as you do.

 

[Singletracker]

I aways assumed that the friction brakes activated in the last few feet prior to actually stopping, blending into a smooth stop. Up until that point, it would be all regen. I could be wrong. It seems that most have the understanding that the friction brakes are not used, at all, unless one steps on the brake pedal. This assumes that Bended Braking is not activated. So, which is it, really?

 

[UnsungZero_OldTimeAdMan]

Interesting take. Where would I find the kWh history from previous trips? Is that in the infotainment?

Gauge view screen. You have A and B trip computer options. Use whichever you fancy, just remember to reset at the start of each trip. At the end of each trip, you'd see what the average mi/kWh number is.

 

[zefram47]

North Wadsworth? that's my commute, and I get roughly the same numbers as you do.

CO93 to down around Colorado Mills.

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