Mitigating Limitations of Regenerative Braking?

[Phrogz]

I live a ~mile up a steep mountain road (average grade about 13%, some sections around 20%). With an ICE car it is imperative to use lower gears to save brake pads over time.

When I leave home in the morning, with regen braking set to "High", I get about 2,000' along and 300' descent down the road before the regen braking on the R1T starts telling me that I'm force-feeding it too fast. For those that haven't seen it, there's a useful visualization of small blocks eating in from the far end of the regen showing the limit. Within a ~few seconds the blocks eat the entire regen region, the truck is coasting as though there is no regen braking present, and I have to use the brake pedal. After braking for a bit the blocks start to back off--even more quickly if I use the accelerator--and then I get regen braking back for another 50 or 100 feet before the regen capacity is gone and I have to start using the brakes again.

Once I get off the steep mile I can travel the remaining 4 miles at an average 3% grade with no problems. Regen braking works as you'd hope with no limitations.

I'm trying to understand what this "force feeding" is, and how to avoid it, or expand its capacity.

• Although I understand we can't use regen braking if the batteries are at 100%, this does not have to do with overall battery charge. I left home with 37% charge today and it was the same as when I'm at 70%.
• I don't think it is affected by ambient temperature. My truck is parked outside overnight and the behavior seems the same whether it's 75°F or 34°F in the morning.

Are there small ultra-capacitors drinking the braking juice, and they fill up? Please help me understand what causes this limit, and if there's any way to mitigate it.

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[electruck]

no ultra-capacitors, juice goes straight back into the battery pack.

Which drive mode are you using and what speed are you trying to maintain during the steepest part of the descent?

 

[thrill]

The power coming from the motors acting as generators is AC, and the rectifier circuit that converts to DC is, I suspect, heat limited. Level 2 (AC) chargers can provide up to 20 kw max (I think), so without going through the math of how much power 8000 lb rolling down a 13% grade can generate, I'm guessing it's hitting that 20kw limit that the truck needs to be able to accept.

 

[JoelD]

The power coming from the motors acting as generators is AC, and the rectifier circuit that converts to DC is, I suspect, heat limited. Level 2 (AC) chargers can provide up to 20 kw max (I think), so without going through the math of how much power 8000 lb rolling down a 13% grade can generate, I'm guessing it's hitting that 20kw limit that the truck needs to be able to accept.

I’m a retired computer scientist, not an electrical engineer ? but I routinely see my Bolt with 7 kW AC charge limit regenning 40-50 kW coming up to a stop, and the only time regen is limited is a full battery. I’ve not noticed this issue on the Rivian after the initial wait for a full battery to drop a few %. But I live at the coast and it’s very flat.

 

[SeaGeo]

I've run into similar issues a few times as well. Someone on Reddit had an idea that it may be a software bug and to try switching your charging limit to 85% when starting to drive. I haven't had a chance to test that out yet though.

 

[Phrogz]

no ultra-capacitors, juice goes straight back into the battery pack.

Curious! Thanks.

Which drive mode are you using and what speed are you trying to maintain during the steepest part of the descent?

Usually All-Terrain mode, occasionally Sport. Speeds are probably 10-15 MPH in the corners and 25-35 MPH on the straightaways before the first incident usually arises. Not held with ACC, just manually working the accelerator.

 

[Phrogz]

I've run into similar issues a few times as well. Someone on Reddit had an idea that it may be a software bug and to try switching your charging limit to 85% when starting to drive. I haven't had a chance to test that out yet though.

So, not charging to 85% (which I've done, and which limits the regen due to "high battery charge", a different message than this one), but charging to 70% and then telling it that it can go higher? Worth a shot. Seems that this morning's 37% charge with limit at 70% lowers the likelihood that it'll work.

I filed something related* as a support bug, and the person who contacted me said silly things like ~"regenerative braking isn't really about braking or helping you down hills, it's just an artifact of how the truck happens to drive; you shouldn't rely upon it for slowing you down on hills". Smelled like corporate CYA liability language.

*[I filed a bug that when setting the charge limit to 70% I sometimes hit 71% or 72%, and on those occasions I get messages from the system that regen braking is limited due to "high battery charge", and I opined that if the truck stopped at 65% or exactly 70% maybe my brakes would last longer.]

 

[SeaGeo]

Seems that this morning's 37% charge with limit at 70% lowers the likelihood that it'll work.

Agreed.

 

[VSG]

I would expect that the limitations to regenerative braking align with the charging curve, because after all that's what you're doing - charging the battery. When you are at a high state of charge, you can't put as much into the battery pack, either from a charger or from regen. But at 37% you should be able to put in the maximum allowed amount. I don't know how much the regen is actually generating, and whether the regen has its own rectifier to change A/C into D/C or whether it just pipes the current through the Level 2 charging port and into the battery. The latter would make some sense because sharing the circuitry means fewer components, more reliable, and easier to notice a problem with the regen (because that would also cause a problem with the Level 2 charging), but the downside is that it would limit the regen to just the 11.5 kW supported by the onboard Level 2 charger. Having a dedicated rectifier for the regen makes it more expensive, but could potentially raise that limit to around 200 kW.

 

[MountainBikeDude]

So, not charging to 85% (which I've done, and which limits the regen due to "high battery charge", a different message than this one), but charging to 70% and then telling it that it can go higher? Worth a shot. Seems that this morning's 37% charge with limit at 70% lowers the likelihood that it'll work.

I filed something related* as a support bug, and the person who contacted me said silly things like ~"regenerative braking isn't really about braking or helping you down hills, it's just an artifact of how the truck happens to drive; you shouldn't rely upon it for slowing you down on hills". Smelled like corporate CYA liability language.

*[I filed a bug that when setting the charge limit to 70% I sometimes hit 71% or 72%, and on those occasions I get messages from the system that regen braking is limited due to "high battery charge", and I opined that if the truck stopped at 65% or exactly 70% maybe my brakes would last longer.]

Out of curiosity....

If you set the charge limit to 70%, and start down a hill, will the truck limit it's regen ability to stay at that 70% threshold without going past? If so, that's definitely a bug.

I assume the 4 motors = 4 generators on that steep of a grade, begins to overwhelm the batteries ability to accept the overflow of power. With nowhere for that extra power to go, the regen starts to dial back for the batteries etc to absorb the high, rapid infux of power.

PS... I'm dumb.

 

[Dark-Fx]

Temperature makes a huge difference for what the battery is able to accept, but it's also possible you're heating up the pack enough that it starts to pull back because of it being too warm.

I haven't driven my R1T down a slope longer than a couple hundred feet and haven't seen Regen being reduced at lower states of charge.

The Hummer could put over 300kW back into the battery under hard Regen. Comparing how fast the Rivian brakes in comparison, and the weight, the Rivian is probably limited around 175-200kW.

 

[SeaGeo]

Temperature makes a huge difference for what the battery is able to accept, but it's also possible you're heating up the pack enough that it starts to pull back because of it being too warm.

I haven't driven my R1T down a slope longer than a couple hundred feet and haven't seen Regen being reduced at lower states of charge.

The Hummer could put over 300kW back into the battery under hard Regen. Comparing how fast the Rivian brakes in comparison, and the weight, the Rivian is probably limited around 175-200kW.

The thing that's confused me about this when I have had it happen is I swear I've seen screenshots showing a temperature icon showing the reason (cold or hot) for the limited regen. Temps make sense, but if the "cold" icon isn't flashing up and it should (I could be misunderstanding), then it isn't clear to the owner. Obviously.

Note: I've had this happen within 10 minutes of starting to drive and going down a hill when it's cold, but I've also had it happen going down a long-ish grade after the battery has been warm, but it's only say 50 degrees out.

 

[zipzag]

Much more likely too cold than too hot. Try charging for a half hour before leaving but not to more than 50% SOC

 

[BCan]

I’ve seen this routinely on local steep descents. My guess is that it’s a heat issue. I’m highly confident it has nothing to do with the battery’s charge level as I’ve had it at 30 to 70%.

 

[Prime]

I have a tesla model Y also live about a mile up a steep mountain / hill. I have had a similar experience with the Tesla. Regardless of battery percentage regen is limited on the descent... unless I had been driving it earlier around town that day, came home and went back down the hill. That tells me the batteries need to warm up or get to a state of "readiness" to receive the regen after driving for some time.

I used to get a warning message that "regenerative braking is limited" and to use regular brakes. Tesla just recently brought an update where the breaks will kick in to "feel" like regen when the regen is limited so there is no disconnect in using one pedal driving, there is a indicator on the main screen that shows if brakes are being applied or the regen is working. Hopefully Rivian brings something like this with a future software update.

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