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Regen limited after descent

Plagued

Member
Apr 9, 2019
240
191
Uk
I'm aware that you can get the regen limited if it overheats from too much regen (although that's not a stated reason for it limiting it in the "handbook") Although I am a little surprised at how often I seem to hit this with my SR+.
For example on my drive home, there's a fairly steep hill that climbs for about half a mile (40 zone) that then drops off the other side over about a mile at 30/40 mph.
Without fail by the time I get to the bottom of the hill I've got the warning and almost no regen for the last mile home.
It's hardly hot outside, and it's a fairly steady decline down the road, so I'm surprised to hit an overheating issue quite so easily, especially after reading about people regenning several % coming down mountain roads.
Apologies if I've missed an existing thread, they all seemed to be about SOC and Battery Temperature other than one American thread with a Model Y.
 

Jason71

Active Member
May 8, 2019
3,038
3,046
Shropshire
I have not heard of this. I have never had this issue and I used to drive up and down the Brecon Beacons a lot. I have an LR though so presumably the bigger battery would increase the regen capacity?
What sort of SoC are you at typically and does that have an impact? What does the warning say?
 

chemirocha

Member
May 8, 2021
10
3
North Wales
Could it be related to how quickly your battery can charge, akin to how fast you can charge sometimes when cold etc when plugged in? I mean, that is effectively what regen is doing - if the battery canna take any more, cap'n, for whatever reason, then the regen will stop.
 
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Plagued

Member
Apr 9, 2019
240
191
Uk
so I'm generally at quite a high state of charge as I often charge at work, the last time I went down this road I was at 79% and prior to the hill I had full regen, About 25 minutes into the drive home with an ambient temp of 15 degrees. So available charge speed will have been a bit limited as I was quite full, but I dropped to 79% going up the hill then regained back to 80% on the way down. the warning flashes up and I get almost no regen for the remainder of my journey.
There's no reason why I should have less regen than before I went up the hill, other than something overheating.
 

simonh

Member
Sep 21, 2020
61
37
Leeds, UK
I've had the same on my LR with a high state of charge of around 80% and a journey starting mainly downhills. After a few miles back on the flat with some hard acceleration, regen was restored to normal. I'm presuming it's a combination of temperature and SOC. Maybe have a go one day with 50% or 20% SOC and see if it still does the same?
 

Jason71

Active Member
May 8, 2019
3,038
3,046
Shropshire
I don't know how steep your hill is but as an example using my long forgotten GCSE physics and maths skill if your hill was at an angle of 20 degrees and is 1 mile long and you travel it at 40mph then that means you are dropping a height of 800 meters in 90 seconds according to Pythagoras (1600sine(20)
if the car weights 2000Kg and falls 800m then it gains 4KWh of energy in 90 seconds. (U=mgh)
I have ignored friction and air resistance in true GCSE form and regen is not 100% efficient but I also made up the angle of the road so bear with me anyway
so lets knock a random 1kwh hour off for all that so now we have 3kwh gained in 90 seconds which is the equivalent of charging at a rate of 120kwh while at 80% state of charge.
Not suggesting this is exactly what is happening to you but you can see that in principal traveling down a steep hill for a sustained period of time could well represent a regen charge rate that, at a high SoC the car is not comfortable with.
I would guess this is what is happening to you. Why it sulks and refuses regen for the rest of your journey I don't know.
please feel free to take issue with my attempt at maths.
 

Jez_GB

Supporting Member
Supporting Member
Dec 28, 2019
684
629
Nottinghamshire, UK
I don't know how steep your hill is but as an example using my long forgotten GCSE physics and maths skill if your hill was at an angle of 20 degrees and is 1 mile long and you travel it at 40mph then that means you are dropping a height of 800 meters in 90 seconds according to Pythagoras (1600sine(20)
if the car weights 2000Kg and falls 800m then it gains 4KWh of energy in 90 seconds. (U=mgh)
I have ignored friction and air resistance in true GCSE form and regen is not 100% efficient but I also made up the angle of the road so bear with me anyway
so lets knock a random 1kwh hour off for all that so now we have 3kwh gained in 90 seconds which is the equivalent of charging at a rate of 120kwh while at 80% state of charge.
Not suggesting this is exactly what is happening to you but you can see that in principal traveling down a steep hill for a sustained period of time could well represent a regen charge rate that, at a high SoC the car is not comfortable with.
I would guess this is what is happening to you. Why it sulks and refuses regen for the rest of your journey I don't know.
please feel free to take issue with my attempt at maths.
It might not be right (🤷‍♂️) - but it sounds frickin genius..... I'm with the maths fan!
 
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Mrklaw

Member
Mar 5, 2020
444
232
Berkshire
I don’t care what the car does but I would like the pedals to be consistent - it’s something you don’t want to suddenly change on you

so if regen is limited, blend in friction brakes to provide the same level of deceleration based on pedal position.
 

Jason71

Active Member
May 8, 2019
3,038
3,046
Shropshire
I don’t care what the car does but I would like the pedals to be consistent - it’s something you don’t want to suddenly change on you

so if regen is limited, blend in friction brakes to provide the same level of deceleration based on pedal position.
some makes choose to do their re-gen on the brake pedal rather than lift off the accelerator. I have heard complaints that it makes the feel of the brake pedal inconsistent and the transition from regen to mechanical brakes can be less than silky smooth.
Based on that what you ask for would indeed be nice but I suspect it may be a more difficult engineering challenge than you think. One day maybe
 

VanillaAir_UK

Well-Known Member
Jun 17, 2019
8,396
5,919
Surrey, UK
I don't find a blend of accelerator feathering and brake pedal feathering to be be significantly different to that of a vehicle with no regen braking such as my ICE. It's all done by feel and running out of regen and compensating with the brake is as natural as lifting the accelerator on an ICE and applying the correct amount of brake. On low regen setting (I had to use low regen for a month or so to allow Tesla to investigate a brake problem) the feel to my ICE is even more similar.

I would much prefer to be in control of the regen/brake balance than letting the car decide. Its not like regen remains a static force, so you always have to drive according to what is available at the time and compensate with the brake on the odd occasion where you start to run out of road :eek:

I think Tesla regen is spot on (apart the the first couple of reverse down off our driveway after they introduced hold - muscle memory had already learned roll mode) and hope it doesn't change behaviour.
 
Last edited:

Adopado

Active Member
Aug 19, 2019
4,262
3,222
Scotland
I'm aware that you can get the regen limited if it overheats from too much regen (although that's not a stated reason for it limiting it in the "handbook") Although I am a little surprised at how often I seem to hit this with my SR+.
For example on my drive home, there's a fairly steep hill that climbs for about half a mile (40 zone) that then drops off the other side over about a mile at 30/40 mph.
Without fail by the time I get to the bottom of the hill I've got the warning and almost no regen for the last mile home.
It's hardly hot outside, and it's a fairly steady decline down the road, so I'm surprised to hit an overheating issue quite so easily, especially after reading about people regenning several % coming down mountain roads.
Apologies if I've missed an existing thread, they all seemed to be about SOC and Battery Temperature other than one American thread with a Model Y.

Well there's a coincidence ... I had the reduced regen message yesterday in similar circumstances. It's on a route not far from home so was still on a high level of charge (mid 80%s I think). There is a longish climb followed by a similar length downhill. Interestingly on the decent I mostly balance the accelerator and am rarely seeing the colour change on the power bar to indicate any significant regen. I have to say that despite the screen warning the reduced regen, if there was any reduction, was barely noticeable. It even crossed my mind that the reduction was due to a cold battery due to no power use during the downhill, because neither had I been doing strong regen.
 

hgmichna

Member
Jun 17, 2020
324
260
Germany
You can get more of the recuperation energy by driving downhill more slowly. There is a speed at which all the energy goes back into the battery, minus unavoidable technical losses.

Depending on circumstances like steepness and outside air temperature this speed may be very low though. You can still experiment with going more slowly and see how that helps.
 

Plagued

Member
Apr 9, 2019
240
191
Uk
...snip...
if the car weights 2000Kg ....snip...
please feel free to take issue with my attempt at maths.
Are you suggesting I weigh 355KG ... how dare you, I'm highly offended! 😦
The car only gains I guess about 0.8KW (going from the fact it goes up 1% after just dropping. so it's regenning about 0.5KWh over 2 minutes.
The max a SR+ can charge at from a super charger at about 80% SOC is 35KW odd or about half a KWh a minute... so I'm regenning about half the amount of power the main charging system is capable of, but I have no idea what the difference is between the Regen charging circuit.
I tried the back roads home again last night and it did the exact same thing. I'm not going to charge at work today and see if I get the same issue with a 60% SOC
 
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Adopado

Active Member
Aug 19, 2019
4,262
3,222
Scotland
You can get more of the recuperation energy by driving downhill more slowly. There is a speed at which all the energy goes back into the battery, minus unavoidable technical losses.

Depending on circumstances like steepness and outside air temperature this speed may be very low though. You can still experiment with going more slowly and see how that helps.

There would be no point trying to put as much energy as possible into the battery if instead you can coast down a hill and not use regen at all. Using regen when you would otherwise require to use brakes is always a benefit but it does not return anywhere close to 100% of the potential energy loss of braking .. but it's worth having. The most efficient use of gradients is to coast without any regen at all! If you are suggesting slowing beyond what you would normally drive just for the sake of getting some regen then you are effectively "shooting yourself in the foot"... wasting energy (i.e. you would travel further coasting without using battery than you could subsequently drive with the regenerated battery power).
 
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jamherber

Member
Sep 7, 2020
63
46
Bridport, UK
I get this regularly with our SR+. My commute back from work has a very steep hill (drops around 500m in about 3/4s of a mile) and I'm usually at a sub 40% SoC. The road is a 50mph limit. After lots of trial and error, I've found that at 35mph and below, I get full regen all the way to the bottom (with a corresponding 1% increase in indicated charge). At any speed above 35mph, regen restricts half way down and carries on reducing until almost zero at the bottom.
 
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Jason71

Active Member
May 8, 2019
3,038
3,046
Shropshire
There would be no point trying to put as much energy as possible into the battery if instead you can coast down a hill and not use regen at all. Using regen when you would otherwise require to use brakes is always a benefit but it does not return anywhere close to 100% of the potential energy loss of braking .. but it's worth having. The most efficient use of gradients is to coast without any regen at all! If you are suggesting slowing beyond what you would normally drive just for the sake of getting some regen then you are effectively "shooting yourself in the foot"... wasting energy (i.e. you would travel further coasting without using battery than you could subsequently drive with the regenerated battery power).
yes except for the fact that higher speed = greater air resistance. energy consumption vs speed is non linear.
Go too slowly and you run out of regen part way down, go to fast and the energy consumption goes up exponentially.
There is no doubt a sweet spot in terms of the most efficient speed to travel down any given gradient but I would not pretend to know what it is.
The higher speed option will definitely get you to your destination sooner that one is a constant.
 

Alset2go

Member
Mar 21, 2019
123
101
Oxfordshire , UK
Not sure if correct but I remember watching an old video of Bjorn’s where he reckoned flicking into Neutral when descending a long slope earned him more distance than regen.
I must admit I sometimes do that.

best wishes and good health to all.
 

hgmichna

Member
Jun 17, 2020
324
260
Germany
an old video of Bjorn’s where he reckoned flicking into Neutral when descending a long slope earned him more distance than regen.
So much is obvious, but it works only if you do not have to brake because of excessive speed. And, as has been mentioned already, at high speeds you lose more energy to air resistance because of the nonlinear characteristic.
 

Adopado

Active Member
Aug 19, 2019
4,262
3,222
Scotland
So much is obvious, but it works only if you do not have to brake because of excessive speed. And, as has been mentioned already, at high speeds you lose more energy to air resistance because of the nonlinear characteristic.

... but don't forget you are lucky to get 70% of energy back through the regen process so even some aero losses wouldn't really change things in its favour. Anyway, the best option is to balance the throttle, then you can bring in a shade of regen if you really are building speed beyond what you would normally drive down the gradient in question.
 
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