Will regenerative braking go over 'charging limit'?

[Oldrocky]

Hi,
We live on a steep hill. So I unplug the car and leave the house. The car has charged to my
personal choice of 80%. I only have to leave the garage, coast down and out of the driveway,
and I'm on a steep down (15%) for about 1/2 mile.
Does regenerative braking pay attention to the 'charge limit' - or will it charge above that
limit?

The reason I'm asking is that - it seems to me - that the full on braking of the regenerative
braking doesn't work (as well) unless the car has been driven enough to get it below that
limit set in the charging. For example, if I leave the house and come back for something
and then leave again without charging - the MX will keep the speed lower than it did the
first time down the hill. Or is this just my perception?
- Jim in PNW

 

[LoudMusic]

I'm fairly sure regen does not adhere to the charge limit.

What you might be experiencing is cold battery versus warm battery. Regen is diminished significantly when the battery is cold. Driving warms up the battery.

 

[mxnym]

In either case, unless you want to go slower, it is more efficient to regen less anyway, because there will be efficiency losses in regen plus efficiency losses in drawing that power back out of the battery. Neither of those efficiency losses exist while going down the hill with gravity instead (until you want to go slower and use the brake, wasting 100% of the energy removed by braking).

 

[mswlogo]

Of course not. Why do you think you completely lose regen when charged at 100%. There is no place to put it.

Even with a warm battery it will start tapering off regen starting at around 90%.

In a 1/2 mile at 15% you’ll probably only add 1-2%. Assuming the battery is not too cold. Not worth even thinking about.

 

[Harvey Danger]

Read the OP. He's wondering why the car limits regen after he charges to 80%. There's plenty of ability to accept charge at this level, and indeed if the hill were big enough there's no question the BMS would happily allow the pack charge level to exceed the 80% "charge limit" previously chosen for home charging: the only regen limit in the scenario described is a rate limit due to temperature.

 

[GtiMart]

I second that. It is most probably the battery temperature that is a bit low. Since a year or so, we still get good regen around 8-10C but it might be topped at 60kW instead of the maximum 85kW. It might not be full until the battery gets closer to 15-20C. I don't remember the exact values, and they changes in software updates sometimes, but this is a ball park. At 0C battery temperature or under, there is NO regen at all.
Obviously a battery over a certain high SOC would also not accept 85kW of "charging" by regen.
NOTE: My comments are based on a model 3, not an X, but the principles must be similar.

EDIT: I also second the statement that regen will NOT be limited by a set charging limit.

 

[Oldrocky]

Thanks all - it is unlikely that the behavior I'm seeing is seriously affected by the
temps since I live in the PNW where the changes in temp are fairly small. It is
also unlikely that I've left the house where the outside temps were less than 50F.
Finally, the car is kept in the garage and the typical scenario is that I take the
car out of the garage when the temps in the garage are at least 55F.
But since temp is the only good explanation for the difference in 'slowing
power due to regen braking' that has to be it.

Thanks also for the confirmation that the charging limit only applies to when
it is on a charger and not to regen. At the risk of being 'snarky' ... I hope that
those statements were not based upon educated guesses rather than actual
knowledge - it wasn't clear (to me) in the posts and I get it that the person
posting is "trying to help" and not mislead.
- Jim in the PNW

 

[Harvey Danger]

I have actual knowledge not educated guesses, regen is rate limited when the internal cell temperatures are anywhere under about 65°.

There is an app called scan my tesla that allows you to monitor both cell temp and a value reported by the BMS as 'Max Regen Power'. When the pack is at a normal operating temperature this value is reported as 60 (kW), at least in my X.

When the BMS limits regen to any level below 60 you will see a yellow dashed line in the energy meter on the IC. (I realize the legend in that graph only shows a label at 50, but the actual rate that can be accepted can go well beyond 50. I have observed levels near 100 after running the car insane mode.)

I recently spent some time monitoring the cell temps during a drive immediately after a cold soak to see what is the minimum temperature the cells must reach before Max Regen Power gets back to 60. I couldn't monitor constantly but I did observe that for any cell temp in the 50's Max Regen Power was well below 60, starting in the teens with a cold soaked pack and rising into the thirties after a bit of freeway driving.

As soon as I noticed the yellow line in the dashboard go away I checked the app to see the current cell temps and by that point had gotten up to 68° . Sure enough Max Regen Power had jumped from the most recent previous value of 34 all the way to the normal value of 60. Presumably the the limit had gone away a bit before I noticed it, so I'm guessing the actual temperature level the BMS shooting is somewhere lower, say around 65.

The proper way to do this is hit "record" button and inspect the resulting data dump later to see exactly what the relevant numbers were when Max Regen Power gets back to 60. Not sure if it's looking at average cell temp or if it waits until *each* cells has finally reached a minimum value (I just looked at average) but I think that is more detail than is really needed here.

It goes without saying we are talking about internal cell temperatures inside the battery pack, not outside air temperature.

 

[Harvey Danger]

Ok now you got me curious.

I just repeated the experiment today and the upshot was basically the same: When the temp average reached 65.8 the yellow dashed line had just gone away.

But the details were different this time around so clearly I have some more work to do to be able to characterize how this all works.

Now I think the 50 kW regen level marked on the energy display is in fact significant, and my supposition that 60 might be the real threshold was wrong.

Today Max regen power climbed steadily as the car warmed up, any seemingly big jumps the last time I looked into this were probably just due to me not checking very often. The highest I saw prior to the yellow lines going away was 48. Shortly after the yellow lines went away the value was 50.

With more driving, it continued to rise into the 60's .

But I discovered there is nothing special about 60 kW max Regen Power, at least on my car. This isn't necessarily a target value. My car just happened to spend a lot of time there the last time I looked into this, so I figured the car likes to be there unless you are actively heating the pack.

But today it didn't seem to hold back: it went on past 60 and didn't seem to converge on any particular round number. I guess the longer you drive the warmer the pack gets, and the warmer the pack, the higher the max Regen Power is allowed. Simple as that.

The label for this value in the app is actually "Rr Regen Power Max". Rr means rear. On my 2016 Dual Motor model X anyway, the front motors do not supply Regen Power or at least none that is reported here.

 

[TIppy]

In either case, unless you want to go slower, it is more efficient to regen less anyway, because there will be efficiency losses in regen plus efficiency losses in drawing that power back out of the battery. Neither of those efficiency losses exist while going down the hill with gravity instead (until you want to go slower and use the brake, wasting 100% of the energy removed by braking).

But the faster you go down the hill the more your original potential energy is dissipated as heat. Tesla says their current drive train efficiency is about 96 percent, so round trip efficiency is about 92 percent. If you use regen to slow your decent so that the car is dissipating at least 8 % less energy, it's better to regen.

If a model 3 lr with 18" wheels would coast down a hill a 70 mph with no regen or brakes, slowing down to 66 mph with regen would use 8% less energy and would be the break even speed.

 

[peety3]

Thanks all - it is unlikely that the behavior I'm seeing is seriously affected by the
temps since I live in the PNW where the changes in temp are fairly small. It is
also unlikely that I've left the house where the outside temps were less than 50F.
Finally, the car is kept in the garage and the typical scenario is that I take the
car out of the garage when the temps in the garage are at least 55F.
But since temp is the only good explanation for the difference in 'slowing
power due to regen braking' that has to be it.

Thanks also for the confirmation that the charging limit only applies to when
it is on a charger and not to regen. At the risk of being 'snarky' ... I hope that
those statements were not based upon educated guesses rather than actual
knowledge - it wasn't clear (to me) in the posts and I get it that the person
posting is "trying to help" and not mislead.
- Jim in the PNW

I too am in the PNW and I'm seeing the same thing. Car set to charge to 90%, and of course if I just unplugged it, it's likely morning and cold(ish). Going down the hill to Main Street Duvall sometimes results in a need for that brake pedal more than what I'm used to (having bought the car used in April, when it was warmer).

 

[Harvey Danger]

Well, you raise a topic that has actually been discussed here pretty thoroughly over the years. To wit: some say it is good to choose a time to charge the car so that charging completes just prior to departure which leaves the battery pack in a warm state and is thereby capable of accepting lots of regen immediately after departure.

Of course some people have taken this mentality a step too far and figured that just turning on the interior cabin heater while connected to 'shore power', which also preheats the pack without running down the battery, would helpfully accomplish the same thing without worrying about timing a charging session just right.

Generally, if you live on a hill, preheating this way could be a meaningful strategy.

But for the rest of us, at least from a physics standpoint, you're probably using a higher (net) amount of electricity from the grid just to make the yellow line go away sooner than it would have done anyway if you had just started cold and driven the car like normal. Most scenarios wouldn't result in so much usage of the friction brakes that it would have been better to preheat.

So preheating with the heater is usually not ideal; timing your charging session is more of a win-win since you were going to charge your pack at some point anyway. Note either method will involve turning on the pack thermal management system and running the battery coolant pumps and so on.

Final caveat, starting to charge when the pack is cold is slightly harder on the pack. To avoid increased risk of degradation due to lithium plating, it's better to charge right after you arrive somewhere while the pack is till warm.

Me personally I never worry about it. Every few trips if I notice the battery getting low I plug it in when I get home. Then the next time I go to drive it, if it's cold, so be it. Occasionally I might feel like preheating the cabin with shore power but I'm doing it for comfort not so the yellow regen line goes away.

 

[GtiMart]

I'm unsure about your last caveat @Harvey Danger . The BMS reduces the charge rate (or prevents it completely) until the battery gets to the right temperature. It does so with home charging as well as supercharging. I thus think the car will never charge the battery at a "too low" temperature. What remains is to decide whether the Tesla BMS has a lower temperature limit that's colder than what the research literature says.

 

[mxnym]

But the faster you go down the hill the more your original potential energy is dissipated as heat. Tesla says their current drive train efficiency is about 96 percent, so round trip efficiency is about 92 percent. If you use regen to slow your decent so that the car is dissipating at least 8 % less energy, it's better to regen.

If a model 3 lr with 18" wheels would coast down a hill a 70 mph with no regen or brakes, slowing down to 66 mph with regen would use 8% less energy and would be the break even speed.

Interesting thought experiment, and one I've wanted to see for some time. Presumably the speed would be even lower with a Model X, which has more drag, unless the regen is also less efficient. In any case, I should imagine my "unless you want to slow down" would pre-empt the potential efficiency gains speed in most driveways.

Final caveat, starting to charge when the pack is cold is slightly harder on the pack. To avoid increased risk of degradation due to lithium plating, it's better to charge right after you arrive somewhere while the pack is till warm.

Outside of the question from @GtiMart, I wonder if you are implying that charging a cold battery is more likely to cause lithium plating than discharging a cold battery. It seems to me that charging at just the right time to cause the battery to be warm when leaving could negate this potential caveat. It also occurs to me that scheduled charging on weekends/off days would not provide this benefit, so assuming charging the cold pack isn't inherently worse than departing with a cold pack (especially considering the substantially higher current involved in departure as compared to charging), I wonder if departing with a pack just warmed from charging 80% of the time and charging a cold pack that won't be used for departure 20% of the time is better or worse than charging the pack warm.

 

[Harvey Danger]

Good questions. I mean it's a fair point, this is a minor caveat in the first place. Tesla wouldn't put a system into the world without making sure that they're not damaging batteries in normal usage scenarios. Still, i just figure, 'all other things being equal', I figure its better to charge when it's already warm.

I'll see if I can locate some original references, but it was my understanding that Li plating is more of a problem during charging than discharging. .

 

[TIppy]

Presumably the speed would be even lower with a Model X,

It turns out 66 mph is also the break even speed for the Model X Long Range with 20 inch wheels.

 

[Oldrocky]

Thanks to all who replied/provided answers/speculation (educated guesses). I now know that
the behavior is 'normal' and can just relax.
We're having a lot of flooding here ... but that hill we live on means that we're not likely to be
affected other than -perhaps- not being able to go somewhere due to road closures. So far
we haven't even seen a blip in our power grid. The rain is lessening, the wind is dropping, the
sun even came out late in the afternoon and we got to see the moon just after sunset. So life
is good ... except I'm having trouble coming up with even a sketchily plausible reason for
going for a drive. Oh, Wait! Wait! I don't know how the Snow Geese and Trumpeter Swans
are dealing with the flooding - tomorrow MUST be a good day to go birding. ;-)

- Jim in the wet PNW

 

[Harvey Danger]

Wow I see Tesla continues to tweak the regen vs pack temp relationship. Guess I won't keep staring at ScanMyTesla trying to understand it. This guy is way ahead of me:

https://www.reddit.com/r/teslamotors/comments/r2iuqb

 

[Oldrocky]

I -have- noticed that some times the regen braking is 'reduced' compared to other times.
The above doesn't really help me - to be honest I couldn't follow it well enough to see the
implications it has on regen. The times I have had reduced regen were -not- cold. I live in
the PNW (West side of the Cascades, near sea level, about an hour North of Seattle) and
although we have some cold days the typical daytime lows are usually in the high 40s and
low 50s. None of the times I experienced what I am calling "lower regen" were at night.
My times when I noticed were "in the same place" on the route away from my house.
And it was noticeable because I had to brake when a couple of days earlier I did not in
order to control the speed down the same hill.
- Jim in PNW

P.S. Took car to San Francisco today. Made use of "Valet Mode". Car handled very well
and passengers who were new to Tesla found the car 'amazing'. Oh yes, the Dim Sum
at Osmanthus was VERY good.

 

[PagodaY]

Just found this thread and wanted to chime in.

Got a 2020 MYLR.

Live at the top of a mtn/hill about 800' above the main road, partially paved.
I normally charge to 75%, immediately when I arrive home (after climbing the mtn)

During the warmer months, I just get in and drive the car without conditioning the battery. At the bottom of the hill I have 77-78%. At the bottom, I've driven about 3 miles with -200kw usage.

In the winter I precondition the car for 20 min before disconnecting and driving down. Regen works just fine, but I have to do the preconditioning.

All the extra free juice is almost used up coming back up the mtn when returning home so it's kinda a wash electrically.

But for me the real benefit is the lack of wear on the brakes going down. When I had the car in for the annual inspection my mechanic commented on the almost pristine condition of the brakes, something he wasn't used to seeing on my car!