Limited regen going downhill

[PeterOT]

I've noticed with my Cybertruck, the regen seems to be limited more often than my other Tesla cars when the battery is warm and my SOC is not high. This seems to happen mainly when I'm in the mountains heading downhill for extended times. Yesterday heading back home, the regen was 100% with no limited icon. However after 15 or 20 minutes (I wasn't really paying attention to the time) I noticed regen going down the hill was not as strong as earlier and the regen limited icon had appeared. This happened several times during the trip. I'd be on fairly flat ground for miles and full regen, then followed by mild downhill for many miles (not steep) and eventually the limited regen light would come on.

On the trip home when I saw the limited regen, SOC was just over 50% and I was not towing and the truck only had 2 people and two backpacks, so very little weight. Temp was in the high 30's to low 40's. The regen light had been off for most of the trip until the sections of continued downhill where regen was used a lot. By the way, this also happened several times this past summer, so I don't think it's battery temperature related.

In 2019 I drove to Pikes Peak with my Model S. On the entire trip down the mountain I never had limited regen.

I'm just wondering if others have seen this? I saw a few YT videos where folks had this issue going down much steeper mountains towing a heavy load, but I've never seen anyone mentions what I just experienced.

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[HaulingAss]

I never have to use my brakes unless I have to stop faster than planned due to another motorist being inconsiderate or unaware. Even when I had the truck loaded with 2400 lbs. of payload and was driving down a 12% grade the regen checked my speed to 30 mph. If the battery is cold I will notice reduced regen but I just give myself a longer distance to stop so I don't have to move my foot to the actual brake pedal.

 

[yohst]

Related to this perhaps?

 

[PeterOT]

Related to this perhaps?

I had not seen that article, so thanks! I'll wait until that update and see if that fixes my regen issue.

 

[Gaximus]

I never have to use my brakes unless I have to stop faster than planned due to another motorist being inconsiderate or unaware. Even when I had the truck loaded with 2400 lbs. of payload and was driving down a 12% grade the regen checked my speed to 30 mph. If the battery is cold I will notice reduced regen but I just give myself a longer distance to stop so I don't have to move my foot to the actual brake pedal.

Just because you aren’t using your brakes doesn’t mean your brakes aren’t being used. The truck will apply brakes when needed to supplement the regen, there unfortunately isn’t any indication of when this is happening. There is a setting to turn this off, but it’s on by default.

 

[yohst]

Just because you aren’t using your brakes doesn’t mean your brakes aren’t being used. The truck will apply brakes when needed to supplement the regen, there unfortunately isn’t any indication of when this is happening. There is a setting to turn this off, but it’s on by default.

“Apply breaks when regenetive breaking is limited” is the setting i presume you are refering to. However, the OP isn’t concerned with limited breaking power but with the phenomenon regen isn’t available or limited at times he expected it to. The article above suggests this may be a bug crept in or exposed by the latest software. Something to be aware of when planning a route through a cold, hilly area.

 

[HaulingAss]

Just because you aren’t using your brakes doesn’t mean your brakes aren’t being used. The truck will apply brakes when needed to supplement the regen, there unfortunately isn’t any indication of when this is happening. There is a setting to turn this off, but it’s on by default.

I keep that setting off because I like knowing when the friction brakes are being used.

Even with that setting off, I think there are limited times when the truck will apply light braking without it being obvious to the driver. Like right before the vehicle comes to a complete stop. But that doesn't change my main point, even when the battery is cold, and my regen is limited, I still drive mostly with one pedal driving. It's very rare for me to have to push the brake pedal.

 

[PeterOT]

I never have to use my brakes unless I have to stop faster than planned due to another motorist being inconsiderate or unaware. Even when I had the truck loaded with 2400 lbs. of payload and was driving down a 12% grade the regen checked my speed to 30 mph. If the battery is cold I will notice reduced regen but I just give myself a longer distance to stop so I don't have to move my foot to the actual brake pedal.

I'm curious if when you had the truck loaded and going down the 12% grade, was that an extended length of time going downhill? I don't see the problem if I'm only going downhill for a mile or two. It's usually after I've been driving downhill for quite some time using the regen the entire time.

 

[yohst]

Sooo, I just experienced this issue myself. Its 59 outside, hardly can call that "cold" but on drive to Home Depot the "limited regen" light came on and stayed on for my 6mi to HD. On the way back it was off. Perhaps truck wanted the batts more cozy and light came on but in 6mi hardly anything has the time to warm up so limited regen should have been on going back also. Weird. Certainly something to keep an eye on when planning a long trip in the planner (which counts on regen being active) and not letting your SoC go down as much perhaps (and use the "nearby super charger" trick)

 

[brur]

I've noticed with my Cybertruck, the regen seems to be limited more often than my other Tesla cars when the battery is warm and my SOC is not high. This seems to happen mainly when I'm in the mountains heading downhill for extended times. Yesterday heading back home, the regen was 100% with no limited icon. However after 15 or 20 minutes (I wasn't really paying attention to the time) I noticed regen going down the hill was not as strong as earlier and the regen limited icon had appeared. This happened several times during the trip. I'd be on fairly flat ground for miles and full regen, then followed by mild downhill for many miles (not steep) and eventually the limited regen light would come on.

On the trip home when I saw the limited regen, SOC was just over 50% and I was not towing and the truck only had 2 people and two backpacks, so very little weight. Temp was in the high 30's to low 40's. The regen light had been off for most of the trip until the sections of continued downhill where regen was used a lot. By the way, this also happened several times this past summer, so I don't think it's battery temperature related.

In 2019 I drove to Pikes Peak with my Model S. On the entire trip down the mountain I never had limited regen.

I'm just wondering if others have seen this? I saw a few YT videos where folks had this issue going down much steeper mountains towing a heavy load, but I've never seen anyone mentions what I just experienced.

one thing I didn't realize, if the charging limit is set the ct will not regen above that limit.

 

[PeterOT]

one thing I didn't realize, if the charging limit is set the ct will not regen above that limit.

I've never tested that before. So you are saying that if I charge my vehicle to 80%, and then put the charge down to 50%, head off down a steep hill, I will not have any regen? I can't imagine the software is set like that, but I have never tried to test it.

All the times in the Cybertruck when I have experienced this (not expected) lack of regen, I've been at a SOC well below whatever my set charge limit is.

 

[yohst]

one thing I didn't realize, if the charging limit is set the ct will not regen above that limit.

Reference? Regen is limited to protect the battery from over charging as the manual states. The charge limit setting is independant from that. But this is easy to test and verify on a down slope. Going to Big Bear in a few days…

 

[HaulingAss]

I'm curious if when you had the truck loaded and going down the 12% grade, was that an extended length of time going downhill? I don't see the problem if I'm only going downhill for a mile or two. It's usually after I've been driving downhill for quite some time using the regen the entire time.

No, the steepest portion of the grade was only for about 90 seconds. But this is the steepest grade I know of in the area. In my F-150 on the same grade, with the same load, I was in 2nd gear (of 6) and still having to ride the brakes down to prevent over speeding/runaway. But here's the thing: the friction brakes in the Cybertruck have more thermal capacity than those in my F-150, and I wasn't even using the friction brakes a little bit. I didn't even need to use full regen, I was feathering the throttle pedal to prevent slowing down too much. Even if regen started to fade on on extended steep descent with a heavy load I would be far better off than in my F-150 because the friction brakes are BETTER, not worse (and they start cooler due to regen).

I don't know why anyone is concerned about this, given the size (thermal capacity) of Cybertruck's brakes. Could I overheat them on purpose? Maybe, but I would need some really exceptional grades, very long and steep grades, and I would need to be over-loaded and travelling at dumb speeds, and on/off the throttle, and have regen fade to zero. I've had regen fade slightly to moderately in my other Tesla on exceptional grades with a cold battery (while sport-driving) but I've never had it go completely away.

Pro tip: I've driven over-loaded ICE trucks in many environments. If you ever lose regen completely while hauling a heavy load, however unlikely that would be, and you think there might be an issue, the solution is dead simple:

Slow down more as soon as your regen fades too much. Going slower spreads the braking over a longer time. This extends the amount of time heat can radiate from the brake discs. It also maximizes whatever regen you might have (for the same reason, more time for thermal management). Slow solves everything in the unlikely event you are threatening to use the equipment past it's engineering limits.

I can't imagine this ever being an issue in the Cybertruck but, in the unusual event it was, never hesitate to slow down even more, even if it requires you to come to a complete stop. The Cybertruck has a HUGE advantage here because ICE drivers are accustomed to having to ride their brakes down steep grades so things can creep up on an inexperienced driver. Cybertruck drivers are not accustomed to using the friction brakes. Thus, they have more warning signs before they even touch the friction brakes for the first time on a steep descent.

Ironically, the solution to avoiding a runaway truck is counter-intuitive. If you notice you are using the brakes more than one thinks they ought to, use them more (to slow down more). And please, no fake concerns about getting rear-ended. Big trucks routinely descend long, steep grades at slow speeds.

The Cybertruck has such a huge head start here, it's not even comparable to the challenges faced by ICE trucks in the same class, with the same load and grade. I think concerns about the brakes are either ignorance about how good the friction brakes actually are or they are simply trying to throw shade on the Cybertruck wherever the anti-Tesla cretins think they can. It's simply not credible that this is a valid and informed concern.

 

[yohst]

one thing I didn't realize, if the charging limit is set the ct will not regen above that limit.

I tested this yesterday on slopes down hill from Big Bear. Starting at 68% soc, i set charge limit at 60%. First part down hill has actually quite a bit of up hill so soc reached 63% but all the way back down (some 25mi later) i had 70% in thr tank. So the notion that regen wont go past the charge limit setting seems incorrect.

Temps were around 45F when i started down hill and after 20m of driving regen limited light came on (outside was then 40F). Added a super charger destination to the nav to have batts conditioned/warmed up so regen would be restored. I Heard the conditioning starting and had that going on for 10m or so. Regen lim light stayed on and regen indeed felt soft. So the idea that overcoming the regen limit on with temporarily adding a charging desitination seems debunked also. Regen limited on changed on its own much later on the flats with env temp around 57.

Even with regen limited on, the truck was recharging the batteries though i can’t tell if that was at a limited level or not.

 

[AlmostHuman]

one thing I didn't realize, if the charging limit is set the ct will not regen above that limit.

That is not true. Do that all the time. We live at the top of a hill, at least two miles downhill. I gain at least 1% sometimes 2% on my way down and it goes above the SoC limit I have set if it just finished charging.

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