Regen Efficiency

Regen is about 85% efficient. Producing power at the wheels has some inefficiency as well. Never going faster than you need to is 100% efficient, comparitively.

Consider two cases:
A) starting at 50mph, coast to zero (no green or orange arc).
B) starting at 50mph, regen to zero at max regen.

In the first case, you will have gone much farther. In the second case, your battery will have more charge. Regen loses about 15% energy (it's about 85% efficient). If you figure it all out, you'll find that case A is more efficient.

Taking that a step further, it's more efficient to accelerate from 0 to 60 mph and then coast to zero than to accelerate from 0 to some higher speed and regen down to zero to end up in the same place.

So technically, reaching lower max speeds and coasting to a stop is more efficient than regen.

Practically though, unless you're ok with driving pretty slow and really annoying other drivers, you can only use the coasting method to a limited amount.

So technically, coasting is better than regen, but practically speaking, higher regen will be better than lower regen because in the real world there are unexpected stops and misjudgements in which coasting to a stop is not realistic, and wasteful braking would otherwise be needed.

As for the energy app, the reason you don't see it go green is because it's integrating power consumption/production over a period of time. In other words, you need to travel something like a tenth of a mile (if showing last 5 miles of driving) and have overall net gain in energy for the graph to go green. So if you consume 20 kW continuously for the first 0.05 mi and then regen at 20 kW for the next 0.05 mi, the graph will show zero net power use for the last tenth of a mile. To be in the green, you actually have to have a net production of energy over the last tenth of a mile, which typically requires either a downhill stretch or a regen from a moderate to high speed.

I have been monitoring this over the last two weeks, and setting Regen to low definitely increases range for me. On average, there is about 50-70 Wh/mile *INCREASE* when I have regen set to standard. When I set it to low, I immediately begin to see improvements on Wh/mile (drops by 50-70). It's significant enough that I think I am just always going to have it set to low. I don't like saying this but I think Regen is just one of those "cool things that you like to explain to people" but in reality it actually isn't practical and reduces range. I don't drive slow either fyi. Not really sure what Todd's trying to explain but for me the reality is pretty clear - regen reduces range.

Todd's got a great explanation. I think a shorter way of saying it is:

Regen is not 100% efficient, so it's a bad thing to do if you can avoid it because it wastes energy.
However, regen wastes a lot less energy than braking.

So the simple rule is: if you have to slow down, use regen as much as you can instead of braking. Aside from that, try not to use regen at all. You don't have to turn it off - in fact you'd like it on high for when you need to use it to slow down - but you want to get in the habit of keeping the accelerator in the right position so it is not slowing you down when you don't want to slow down.

(That doesn't cover everything, but that's the vast majority of it. There are a few threads on this in the Roadster forum if you want to look up previous conversations. Including one where somebody insisted regen didn't help and all the manufacturers were doing it wrong. Some people here - me included - took measurements to show it did work. It turned out he was using his measurement tool incorrectly).

So in theory would it be better to have regen on in city driving (where there would be a lot if braking without it) and off in Highway driving (where coasting or taking your foot off the accelerator (but not braking) is more likely)?

carrerascott said:

So in theory would it be better to have regen on in city driving (where there would be a lot if braking without it) and off in Highway driving (where coasting or taking your foot off the accelerator (but not braking) is more likely)?

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I was unable to get better (lower) Wh/mile in either scenario with regen set to standard. After switching to low on both city and highway, my Wh/mile went down.

Here is a regen vs non-regen question.

Suppose you have a valley that is very deep with a steep slope. The slope is steep enough to exactly max out the cars regen and maintain constant speed. The valley is deep enough to accelerate you from 40mph to 80mph by the bottom.
At the top of one side you are going 40mph when you hit the downslope.
You can either let regen keep you at 40mph all the way down and then use the energy on the way up the other side -
or you can coast down with no regen and let the slope accelerate you to a high speed, and use that extra speed to help you up the other side.

Make guesses at the regen efficiency, drag vs speed and other constants and provide an answer...

richkae said:

Here is a regen vs non-regen question.

Suppose you have a valley that is very deep with a steep slope. The slope is steep enough to exactly max out the cars regen and maintain constant speed. The valley is deep enough to accelerate you from 40mph to 80mph by the bottom.
At the top of one side you are going 40mph when you hit the downslope.
You can either let regen keep you at 40mph all the way down and then use the energy on the way up the other side -
or you can coast down with no regen and let the slope accelerate you to a high speed, and use that extra speed to help you up the other side.

Make guesses at the regen efficiency, drag vs speed and other constants and provide an answer...

Click to expand...

Set your Regen setting to Low (Off if it was available) and try to maintain minimal consumption (at most a little orange) while maintaining zero regen (no green).

I think I would prefer to have Regen tied into the brakes, not the accelerator. I *really* hope they make that an option in a future software update. I believe I would get much better results.

lolachampcar said:

I found an interesting note in one of the electric motorcycle forums a while back. The thrust of it was that the bike in question (a Zero) got better range using the Sport mode which, in part, uses much less regen than the Eco mode.

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We are way over thinking this question. Regen is just another way to slow the car down. Regen is just a more efficient type of braking. You can use regen or hydraulic friction brakes to stop. Hydraulic brakes are very inefficient. Regen is less inefficient. Coasting is the most efficient.

Bottom line, yes, coasting is the most efficient method. But what would we expect?

Coasting isn't always more efficient. Consider a long and steep downhill. If you coast, you will increase speed. Increased speed will vastly increase losses due to air resistance. A speed increase from 55mph to 75mph will increase losses far more than the modest losses from holding 55mph by using regen. If you coast you will have 20mph more speed at the bottom of the hill, those 20mph will soon bleed off. The power from a few minutes of regen will get you a lot farther than those extra 20mph at the bottom. And by coasting you'll also risk getting a fine from driving 20mph over the limit

Also setting regen to low is not needed to coast. Just watch the power meter and feather the pedal so the meter stays at zero. I do this all the time in the Leaf, it has become second nature (and I always drive the Leaf in eco mode).

If you approach a red light, coasting to a stop is always more efficient. But you'll also induce road rage in those behind you...

yobigd20 said:

I have been monitoring this over the last two weeks, and setting Regen to low definitely increases range for me. On average, there is about 50-70 Wh/mile *INCREASE* when I have regen set to standard. When I set it to low, I immediately begin to see improvements on Wh/mile (drops by 50-70). It's significant enough that I think I am just always going to have it set to low. I don't like saying this but I think Regen is just one of those "cool things that you like to explain to people" but in reality it actually isn't practical and reduces range. I don't drive slow either fyi. Not really sure what Todd's trying to explain but for me the reality is pretty clear - regen reduces range.

Click to expand...

It sounds to me like you just aren't driving efficiently. Having high regen doesn't force you to use it, it just makes it available. The proper energy saving driving technique in the S is keeping the energy meter near zero and accelerating and slowing as little as possible. Low regen is just like a setting that limits acceleration (which some people have asked for). It just enforces something you can do by yourself.

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lolachampcar said:

I think one of the points earlier was that you could achieve this result by lifting off the throttle only to the neutral or zero power consumption point which is, by definition, no regen.... It's not comfortable or easy but I think it can be done.

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Actually I find it quite comfortable and easy. After a little bit of practice it's almost second nature.

yobigd20 said:

I have been monitoring this over the last two weeks, and setting Regen to low definitely increases range for me. On average, there is about 50-70 Wh/mile *INCREASE* when I have regen set to standard. When I set it to low, I immediately begin to see improvements on Wh/mile (drops by 50-70). It's significant enough that I think I am just always going to have it set to low. I don't like saying this but I think Regen is just one of those "cool things that you like to explain to people" but in reality it actually isn't practical and reduces range. I don't drive slow either fyi. Not really sure what Todd's trying to explain but for me the reality is pretty clear - regen reduces range.

Click to expand...

I'll try the experiment next week- after I get a good baseline with strong regen. I think the strong regen is working well for my commute since I live at the top of a decent sized hill (I usually put 0.2 kWh back in the battery by the time I get to the bottom) and I'm usually going 75 mph when I take my freeway exit both going to work and coming home (the Wh/mile for my 25 mile commute went down by about 10 after I exiting the freeway this morning). I also really like the whole one-foot driving thing- but I completely agree they should include a setting that only invokes regen when the brake is depressed and let people figure out what works best for them.

carrerascott said:

So in theory would it be better to have regen on in city driving (where there would be a lot if braking without it) and off in Highway driving (where coasting or taking your foot off the accelerator (but not braking) is more likely)?

Click to expand...

The way that we coast now in the S is by keeping your foot on the pedal and finding that sweet spot where the power meter is on zero. A bit harder to do than just taking your foot off the accelerator, but the benefit is that we now drive almost exclusively using one pedal. Out on the farm I'm quite used to this with my hydrostatic drive tractors, which behave in a similar fashion (although they have some limited hill-holding ability when the pedal is in the neutral position.)

The ways think about it is its no an either or question. Which is more efficient for you depends on your drive style. If you were able to perfectly execute with regen on hi, then you could be every bit as efficient as coasting, because you can coast even with regen hi if you feather the accelerator. But that's difficult to do perfectly.

So, you might assume then using low regen would be better. But it's not in practice always because the real world driving scenarios don't let you always coast to a stop. You need to slow down more quickly than a coast would often and probably most of the time. So with regen on low, the only option would be to brake, which is very inefficient.

So, depending on how you drive and the circumstances of your drive, either may be better in the real world.

jomo25 said:

The ways think about it is its no an either or question. Which is more efficient for you depends on your drive style. If you were able to perfectly execute with regen on hi, then you could be every bit as efficient as coasting, because you can coast even with regen hi if you feather the accelerator. But that's difficult to do perfectly.

So, you might assume then using low regen would be better. But it's not in practice always because the real world driving scenarios don't let you always coast to a stop. You need to slow down more quickly than a coast would often and probably most of the time. So with regen on low, the only option would be to brake, which is very inefficient.

So, depending on how you drive and the circumstances of your drive, either may be better in the real world.

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But you don't need to get it perfectly. The only thing you need to remember is slow acceleration and slow deceleration. I still maintain it's not that difficult to train your foot to ease down and up to control your speed. Taking your foot all the way off the accelerator is just an old habit that you need to get rid of (unless you're braking suddenly, in which case you take it off and hit the brake--there's no time in driving when I lift my foot from the accelerator unless I'm going for the brake). You can do one pedal driving with a gentle touch and get really good efficiency.

Ultimately this is similar to the creep discussion. You either want to drive the way you always have and have the Model S behave the same, or you are willing to embrace a different way of driving. I maintain in both cases the new way of driving is superior.

One note about coasting: the power meter is logarithmic. A given angle of orange or green arc near zero is a much smaller difference than that same angle of arc at higher levels of regen or power consumption.

You can still coast pretty effectively as long as your orange/green arc is fairly small. Just get it close to zero, and you'll find that's good enough.

lolachampcar said:

I think one of the points earlier was that you could achieve this result by lifting off the throttle only to the neutral or zero power consumption point which is, by definition, no regen.... It's not comfortable or easy but I think it can be done.

Click to expand...

It's not really uncomfortable to do (if you've adjusted your seat correctly) but it does take some practice (particularly if you are use to the release and press down a bit method that works best in the Prius). I can't say I've mastered it yet. My thought on the low regen setting is that if you need to brake quickly, you'll be using the friction brakes if you are in low regen rather than getting back at least some of the kinetic energy. Zero is still zero regardless of the regeneration setting.

Of course the biggest energy saver is not causing the Tesla grin when you have passengers--but who wants to do that?