Optimal speeds for recapturing energy during descend and lowering consumption on ascend

[V__2]

Good day everyone!

Living in a hilly area (many >25% grades) in a city with 50km/h - 60km/h (30mi/h - 40mi/h) speed limit. In most areas there is no traffic to keep up with, so have flexibility to vary my speed between 50% and 100% of the speed limit. The car is mostly lightly loaded, if not just the driver and a passager.

From looking at charge speed efficiency in the past, I remember at very low speeds the charging efficiency is not great due to the constant losses. With faster charging speeds, the losses are smaller percentage even if as absolute value they are similar. On the other hand, in general, the car is most efficient (kWh/mi) at lower speeds.

While there may be little practical value, still interested in what is the best speed on descends to recapture the most energy and similarly, wonder what the optimal speed for ascend is. For example, not sure if driving longer time at lower speed on descend is better than shorter time at higher speed. Similarly should I go as slow as practical on ascends?

 

[ucmndd]

At the speeds you’re talking about there will be essentially no discernible difference in consumption.

Remember that the most efficient regen is no regen at all - letting the car coast is always going to be the most efficient way of driving. With that in mind you’d do better to carry more speed downhill (regen less) if it means you can carry that momentum back up the next hill.

 

[V__2]

So I may have used the incorrect adjective. I live in the mountains and it is not much about keeping momentum but rather you descend going one way of your trip and the return is uphill.

Being that my current Tesla is a 2023 model, control of regeneration level is only via the accelerator. So even during steep descend I need to apply pressure on the pedal and pick what my speed is. Even at the steepest of roads here (and mind you, often my ears need to pop after driving just 5 min downhill on some of the streets), the car would still decelerate to a stop if I don't press a bit on the accelerator.

Any suggestions on the speed when going uphill? Does it take meaningfully more energy to move uphill at say 50mph vs. 30mph?

 

[ucmndd]

Any suggestions on the speed when going uphill? Does it take meaningfully more energy to move uphill at say 50mph vs. 30mph?

At the speeds you’re talking about there will be essentially no discernible difference in consumption.

 

[QtownTaxiGreg]

Let you know in a few weeks when my 2024 MY shows up. Same deal here. I have yet to figure out the finer points of one foot driving that you appear to be locked into.

As I understand it you control the amount of regen within available ranges you can preselect, with the accelerator acting in reverse. Absence of pressure gives max regen in that range to bring you to a halt.

The other choice should be brake = brake, and accelerator coasts no power applied no regen applied until you brake.

I expect there should be very little difference between coasting down at speed limit under regen vs 50% of limit.

Likewise between one pedal and two pedal modes.

Afaik brakes usually only get applied in hard stops. If you have enough regen selected it will just use regen even if you are in two pedal mode?

 

[Rocky_H]

From looking at charge speed efficiency in the past, I remember at very low speeds the charging efficiency is not great due to the constant losses. With faster charging speeds, the losses are smaller percentage even if as absolute value they are similar.

You're trying to apply a principle that does exist to a situation where the parameters are so far from that, that it's not relevant. That thing about charging efficiency, is in the realm of like 1 or 2 kW power levels, where a constant draw of 300 W is a sizeable portion of it. So charging at a somewhat higher power level, like 6+ kW would be more efficient. Regen while driving is usually going to be tens of kW, so it's not related to that really low power charging for many hours.

 

[boulder.dude]

When downhilling, the air resistance will be the dominant component?

 

[QtownTaxiGreg]

When downhilling, the air resistance will be the dominant component?

I don't think so. Without a hill you coast with clutch in to a stop from 30mph but with a hill your speed runs away like crazy. That says to me the gravity induced change in elevation energy is far in excess of the combined rolling resistance and aerodynamic resistance dissipation at speed limit.

I don't know how efficent regen is at taking the power back from the rolling wheels and putting it back in the battery. Maybe 50% of what you used to elevate the car, as opposed to driving it 'along' the hill, is returned?

The OP question is an interesting one for sure. How does regen efficiency change with speed? It's really two questions and an assumption.

We know the car bleeds more energy to friction running faster downhill because eventually it will stop accelerating!

Instead of freewheeling we introduce regen to soak up the excess.

My guess is 10mph under the speed limit will be quite a bit less aerodynamically lossy than the speed limit and also a bit less lossy for the tyres on the road.

In theory all of that difference is available to the regen system. How good the regen is at making more stored energy from a little less speed but a little more time is the real question.

If you have a regular really long hill it will be easy to make an experiment. I have many so I will let everyone know once I get going.

Battery reserve should be same for each run.

Load should be the same up and down. Not gonna happen with me as a taxi too often but if I record enough empty down trips it should be obvious what is the real story.

I will note conditions of battery at bottom and top and back at bottom at various states of charge and loadings and road conditions.

 

[Rocky_H]

I don't know how efficent regen is at taking the power back from the rolling wheels and putting it back in the battery. Maybe 50% of what you used to elevate the car, as opposed to driving it 'along' the hill, is returned?

Oh, that has been tested and measured some over time, and it's over 50%. I seem to recall around 60 to 70%? This search seems to show some consensus in that area.

how much energy recovered from regen braking hills - Google Search

 

[QtownTaxiGreg]

There's a bunch of stuff but I couldn't find anything specific to the question of how efficient the Y
Tesla cab be at different speeds.

It would appear at least that we can't exceed the charging capacity with regen since supercharging charges faster than you can burn power climbing.

 

[V__2]

Appreciate the input so far.

My comparison to charging efficiency was more in terms of both processes having some fixed and some variable losses. It is unlikely regen efficiency is the same at different speeds or different grades of slope. And it is likely there is a speed that provides peak regen efficiency. Intuitively, it also seems there are also different efficiency depending on the slope.

It is also important that we do not associate the kWh number seen in a situation where the accelerator is no longer pressed and the car is coming to a complete stop via regen, with what the car can harness downhill while maintaining speed. The on-board electronics may be consuming around 400w but this is likely a magnitude less than the "losses" the car has while moving. From the arguments above it seems, while maintaining speed downhill, the car will have a speed where it will have the least amount of energy spent for moving and the most energy left to be converted by regen.

Seems peak efficiency on a flat road is at ~35mph for most Teslas and a bit less for other EVs.

 

[QtownTaxiGreg]

do not associate the kWh number seen in a situation where the accelerator is no longer pressed and the car is coming to a complete stop via regen, with what the car can harness downhill

I am not properly sure what you are getting at there but I think you have to be wrong!

All else equal for two trips up and then down hill X at down speeds A and B surely one trip will have more left in the battery than the other. The difference has to be the rate of regen applied. Reveals the relative regen efficiency v speed.

Very interesting driven speed v efficiency graph there. Looks like from Tesla? I had no idea lower constant speed would rob energy. Intuitively I expected a cliff like climb up from 0 to maybe 15mph and flattish line across 30mph with a steady ranking after that due to aero and heat loss everywhere. Not at all what we see there! 35mph better than 30mph.

The best data I can get will come from the ski hills in the winter. Problem is the load up, temp, SOC at bottom, road condition will be all over the grid day to day. A pattern will surely show itself over a few trips and I will be using best practice from there.

Sometimes we are more in a hurry than other times to get up and down so there will be variation.

There was a strange graph I saw of EV motor regen torque vs angular velocity. It was not making a lot of sense to me so I left it. I think it was saying torque and speed were in a straight line over much of the range. There was a whole set of curves for another condition which may have been different motors but didn't really say. The original.paper is behind a paywall. Anyone IEEE?

At some stage we get traffic lights on the route to the airport. This will preface frequent crawling along the last 2 or 3 km or sometimes 4km with stop go every 800m the whole 7km all day every day. Did someone say we live in interesting times?

 

[ewoodrick]

At the speeds you’re talking about there will be essentially no discernible difference in consumption.

Remember that the most efficient regen is no regen at all - letting the car coast is always going to be the most efficient way of driving. With that in mind you’d do better to carry more speed downhill (regen less) if it means you can carry that momentum back up the next hill.

Nope, not always true.

If downhill causes an increase in speed, maximum speed is often limited, as it seems to be in this case. Therefore braking would be needed. Braking is bad.

 

[ewoodrick]

Good day everyone!

Living in a hilly area (many >25% grades) in a city with 50km/h - 60km/h (30mi/h - 40mi/h) speed limit. In most areas there is no traffic to keep up with, so have flexibility to vary my speed between 50% and 100% of the speed limit. The car is mostly lightly loaded, if not just the driver and a passager.

From looking at charge speed efficiency in the past, I remember at very low speeds the charging efficiency is not great due to the constant losses. With faster charging speeds, the losses are smaller percentage even if as absolute value they are similar. On the other hand, in general, the car is most efficient (kWh/mi) at lower speeds.

While there may be little practical value, still interested in what is the best speed on descends to recapture the most energy and similarly, wonder what the optimal speed for ascend is. For example, not sure if driving longer time at lower speed on descend is better than shorter time at higher speed. Similarly should I go as slow as practical on ascends?

As long as you are going over around 30mph, you should be good, don't worry about it.
Once you start going below 30mph, indeed, the motor speed starts to be non-optimal for regeneration

 

[ucmndd]

Nope, not always true.

If downhill causes an increase in speed, maximum speed is often limited, as it seems to be in this case. Therefore braking would be needed. Braking is bad.

If I can sum up your point: Duh.

It’s not really related to the point I was making though - which is that regen is not perfect energy recapture and has a significant round trip loss. If you can avoid it via your driving style and conditions, that will always be more efficient.

If you can’t, because your downhill coasting speed exceeds what is safe, then yeah, use regen instead of friction brakes. Duh.

 

[QtownTaxiGreg]

If I can sum up your point: Duh.

It’s not really related to the point I was making though - which is that regen is not perfect energy recapture and has a significant round trip loss. If you can avoid it via your driving style and conditions, that will always be more efficient.

If you can’t, because your downhill coasting speed exceeds what is safe, then yeah, use regen instead of friction brakes. Duh.

No need for that.

If you read the OP he is looking at controlling his speed on a long hill so telling him he needs to brake less is less than helpful also.

 

[rrolsbe]

We have a mountain property that has a 10 mile stretch that is mostly downhill but not real steep. I have monitored the Wh/mile using standard regen and reduced regen and reduced uses slightly less energy; however, the reduced regen option has been removed in newer Tesla's. I think the regen can be reduced/turned off using the Sexy buttons?

Follows is a comment I made to a Youtube video four years ago.

On rolling hills an option to select a smarter cruise control mode could be a real winner. For example if you set your cruise control to 60MPH, allow the car maintain 55 miles an hour if it detects it is going uphill then on the downhill side allow it to coast (in neutral) up to 65 or even 70MPH (you could get a ticket) if downhill speed exceeds 65/70 speed engage drive mode and use regen. If downhill speed does not exceed 65/70 continue to coast until 55MPH is reached on the next hill then switch from neutral to drive mode to maintain 55 until the next downhill road section.. The parameters of 5 below going uphill and 10 above going downhill could/should be selectable parameters. Basically have the cruise be smart enough to switch automagically in/out of neutral/drive and use regen when necessary to keep the downhill speed safe. All of this could be done by the driver but would not be very relaxing way to drive. Having this cruise control option should extend range and not add much, if any, travel time to the trip.

 

[QtownTaxiGreg]

neutral

Really bad idea disengaging drive like that. Nice idea from milage point of view but freewheeling down the road not.

 

[rrolsbe]

Driving many electric cars when the traction batteries are very cold is basically equivalent to neutral when you let up on the go pedal. The cars computer would need to handle the engagement/disengagement of neutral, regen and possibly the automatic blending of the friction brakes in a safe manner. I do agree coasting in neutral can be dangerous.

 

[ewoodrick]

We have a mountain property that has a 10 mile stretch that is mostly downhill but not real steep. I have monitored the Wh/mile using standard regen and reduced regen and reduced uses slightly less energy; however, the reduced regen option has been removed in newer Tesla's. I think the regen can be reduced/turned off using the Sexy buttons?

Follows is a comment I made to a Youtube video four years ago.

On rolling hills an option to select a smarter cruise control mode could be a real winner. For example if you set your cruise control to 60MPH, allow the car maintain 55 miles an hour if it detects it is going uphill then on the downhill side allow it to coast (in neutral) up to 65 or even 70MPH (you could get a ticket) if downhill speed exceeds 65/70 speed engage drive mode and use regen. If downhill speed does not exceed 65/70 continue to coast until 55MPH is reached on the next hill then switch from neutral to drive mode to maintain 55 until the next downhill road section.. The parameters of 5 below going uphill and 10 above going downhill could/should be selectable parameters. Basically have the cruise be smart enough to switch automagically in/out of neutral/drive and use regen when necessary to keep the downhill speed safe. All of this could be done by the driver but would not be very relaxing way to drive. Having this cruise control option should extend range and not add much, if any, travel time to the trip.

But I believe that this ONLY applies to situations where you can coast down a hill and then just about up the next.

Around me, the hills are a hundred plus feet tall and there generally isn't a straight road, so you have to slow down to go around the curves and not far from here is a 5,000ft hill , for which there is no way to even think about coasting. Back in the 70's it was common to pass by a dozen cars with overheated brakes.