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In July - regen limited at 90% charge?

Discussion in 'Model 3' started by Zidarich, Jul 13, 2019.

  1. Zidarich

    Zidarich Member

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    Have my new, second model 3, and noticed something I never did with my first in the summer. After a night of charging in July (lows in the 70s), I have a small amount of limited regen.

    Is this something new with a 90% charge to preserve the battery better?
     
  2. DJVoorhees

    DJVoorhees Member

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    You can set the % full charge, from the factory it's set at 90%. My SR+ shows 214 at 90%.
     
  3. Zidarich

    Zidarich Member

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    What I mean is that a charge to 90% limits my regen the next morning.
     
  4. Kognos

    Kognos Member

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    This is completely normal. At 100% you have even less. The BMS won't let you overcharge your battery, so you'll have limited regenerative charge as you get closer to the top.

    This otherwise affects nothing at all, except when you might rely on regenerative braking to slow down, hence the warning.
     
  5. puckpurnell

    puckpurnell Supporting Member

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    When fully charged or 90%, there’s no or limited room for regen to charge.
     
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  6. jdcollins5

    jdcollins5 Member

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    I routinely charge to 90% and do not see limited regen. So this would not be normal to me.
     
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  7. AlanSubie4Life

    AlanSubie4Life Active Member

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    #7 AlanSubie4Life, Jul 14, 2019 at 8:27 AM
    Last edited: Jul 14, 2019 at 9:01 AM
    #Actually ;) , we need to be a little more careful about that statement. If the OP got the regen limited message right away with the car cold soaked to 70F at 90% SOC, that would be unusual. But if the OP were just seeing regen dots at 90% (without the message), that would be totally normal.

    In addition, regen limiting has some interesting behavior. You’ll find that if you start with a couple regen dots even with a relatively low state of charge, say 70% (because the battery is chilly...say 60 degrees)...and then you just drive downhill, relatively soon into the downhill, your regen will become even MORE limited - and you’ll even get the regen-limited message.

    The battery is very reluctant to regen too large an amount when it is cold, even at SoC well below 90% (say 70%). So it can start with nearly full regen and then as you regen and drive more it reduces the amount you can do. It’s an interesting phenomenon and I don’t exactly understand why - since I figure regen should be a warming event.

    Practically, this only happens on long downhills with a battery that is not up to temperature.

    I saw this a week ago in an AWD coming down from Vivian Creek Trailhead below Mt San Gorgonio. We had 213 miles, battery was cold soaked probably to about 55-60 degrees, had been warming up in the sun for about 4 hours, ambient temps were in the high 70s, and as we descended the regen kept getting reduced to the point the brakes had to be used. The more we drove and regenerated, the more regen we lost. We started out with nearly full regen, and once we had added about 4 miles to get to 217 miles, after driving about 5 miles, it REALLY became limited. (So the reduced regen was obviously not due to the state of charge change.). Eventually after a bit more actual driving (Black bars) we managed to get some dots back - but it took quite a while - I think it was more than 20 miles.

    It does appear that Tesla is very cautious about any possibility of lithium plating. I wish I understood why a certain amount of regen is ok with a cold battery, but then when the battery is warmer even LESS regen is allowed, though. It’s very counterintuitive with what little I know about lithium plating.

    So there are at least three inputs to the formula:

    1) State of charge - higher means less regen
    2) Battery temperature - lower means less regen.
    3) Recent regen history - the more you have done, the less regen you are allowed. (Note this is unrelated to the state of charge getting higher due to regen, to first order.) But I believe this input becomes a non-issue once the battery is fully warmed (someone would have to experiment - but I imagine we would have heard from people previously if this were an issue with warm batteries).
     
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  8. antoinearnau

    antoinearnau Member

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    I noticed it too. It came back at 88%.
     
  9. earthwormjim

    earthwormjim Member

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    Seems normal to me, the battery can accept only so many watts of input at a time, and the amount it can handle at any given moment drastically goes down, the closer you get to 100% charge. Just look at the charging rates when supercharging, they nose dive after about 60%. Regenerative braking is no different, it's just another form of charging.
     
  10. AlanSubie4Life

    AlanSubie4Life Active Member

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    V3 Supercharging Profiles for Model 3

    Based on @Zoomit's plots above, looks like @90% (with a fully warmed battery) you'd be limited to 35kW.

    Power = m * a * v

    For the situation I mentioned above, we were dealing with 1400 feet descent over 5 miles. This is about a 5% grade.

    Google Maps

    To maintain speed (at say 45mph), you need:

    Power = m * sin (alpha) * g * v = 19.7kW

    Where alpha is the angle of the slope (not the grade), alpha = arctan(0.05) = 0.0499
    FWIW, (sin(tan-1(grade)) is approximately equal to the grade for "typical" grades...

    So with a warm battery it would be no problem to go slowly enough on such a grade.

    Wolfram|Alpha: Making the world’s knowledge computable

    For actually slowing down, forgetting about the slope, you'd be limited to 35kW with a warm battery, which works out to be (at 45mph):

    a = 35kW/(m*v) = 0.089g.

    Wolfram|Alpha: Making the world’s knowledge computable

    This isn't a lot of slowing (certainly not enough for an abrupt stop), but 0.09g is actually a fairly noticeable stopping force. It's enough to keep you going quite slowly on fairly steep grade...per the above, it's basically enough to keep your speed at 45mph if you're going down about a 9% grade (which is quite steep!).

    But again, in the OP's situation, and the situation I described, the battery is not up to temperature, so you won't get the full 35kW.

    Going back to our situation I described above, we were at 69% SoC. In @Zoomit's plot that corresponds to 95kW maximum charge rate for a warm battery. Based on us needing to use the brakes on a 5% grade with a cold battery (this requires ~20kW to maintain speed at 45mph), empirically, it looks like the regen with a cold battery is limited to less than one quarter of the maximum possible value at that 69% SoC.

    I'm not taking into account conversion efficiency losses and drivetrain losses in any of the above. In general taking those into account would mean the regen energy allowed into the battery is even more limited with a cold battery than stated above.
     
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  11. AlanSubie4Life

    AlanSubie4Life Active Member

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    #11 AlanSubie4Life, Jul 14, 2019 at 2:17 PM
    Last edited: Jul 14, 2019 at 2:23 PM
    I should add that it’s also possible (likely!) for regen that Tesla limits the rate to values significantly less than the curve that is possible when supercharging. Since regen happens all the time, perhaps you don’t want to be constantly maxing out the battery charge rate.

    One of these days I need to use the VBOX to generate acceleration vs. speed curves and estimate the max regen power Tesla allows at each speed. I think someone has done this before, but no idea what the answer is.
     
  12. ZOMGVTEK

    ZOMGVTEK Member

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    I get dots on my regen bar at 90% as well. It usually goes away after driving a bit. I just charge to 85% and usually don’t see the dots.
     
  13. Tony_YYZ

    Tony_YYZ Active Member

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    Ditto. Even in the high 60s I never get limited Regen when charging to 90%.
     
  14. justsomeguy

    justsomeguy Member

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    I noticed this recently too. The limited Regen occurs down to even 80% soc
     
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  15. mswlogo

    mswlogo Active Member

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    #15 mswlogo, Jul 15, 2019 at 5:11 PM
    Last edited: Jul 15, 2019 at 5:21 PM
    I’m seeing it also. All the way down to like 82% I might see 3 dots and it was about close to 80F today.

    I’m switching back to charging to 80%

    I wonder if it’s folks with AWD are noticing the lower SOC for limited Regen since it can potential have higher output.
     
  16. WilliamG

    WilliamG Member

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    I have about 7 or 8 dots on the regen bar on my P3D+ at 90%, and have done since I got the car. As I get closer to 80% I get full regen.
     
  17. darth_vad3r

    darth_vad3r Member

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    #17 darth_vad3r, Jul 15, 2019 at 6:47 PM
    Last edited: Jul 15, 2019 at 7:42 PM
    thumbs up. I'm a sucker for wolfram alpha links, I just used some here this weekend :) ... and I was wondering some of this when reading your first post, so glad you followed up with this ... but I still have a question about the limits of regen power input to charge the battery ... does the tesla regen generate DC current directly to the battery? I mean, does it need to go through some other circuitry that's also potentially subject to warming or capacity limits until warmed up?

    re: 0.09g, I'd read elsewhere of regen force limited to about 0.2g? Not sure if that's accurate. Let's see .. a bit more hunting ... stop when I find charts posted by @Zoomit here :) ... which show 0.09g is "low" and 0.16g is "standard" for the Model 3.

    I think the limits applied to supercharging profiles which are under predictable conditions (charging continuously with zero discharging) are probably different than the limits applied to the "regen profile" due to the requirements of handling very bursty charging and discharging cycles as freqent as every couple seconds. I mean, *you* know you want to 'regen' charge for the next 10 minutes down a long hill, but the car doesn't know you want to do that, and you might need to step on the gas any moment, then regen again, then gas [EDIT: accelerator, *sigh*], etc... so I think it has to take extra conservative measures over and above those constraints used under 'known' predictable conditions of supercharging. [EDIT: which is also what you said in your 3rd post, LOL]
     
  18. AlanSubie4Life

    AlanSubie4Life Active Member

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    It has to convert from AC to DC in the inverter/converter (probably shares some circuitry). For sure these units have some power limits but not sure they ever limit regen. I would imagine they would generally work better when cool, as most semiconductors do. They also have much less thermal mass than the battery.

    My guess is that the battery is the limiting factor here in any case.

    That 0.09 was just the g force you’d get at 90% charge - assuming you can regen just as much as you can supercharge at that SoC. I’m quite sure you can get a lot more regen than that when it is not limited.

    I think so too. Again, really need to get an acceleration vs. velocity plot and then can calculate the regen power at every point.
     
  19. mobil-ai

    mobil-ai Member

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    Agree. I charged to 100% and noticed during city driving that regen was very weak, almost hard to notice - car would very gradually coast when I eased off the accelerator, like a gas car. I guessed it could be because the battery did not need further charging or the current requirement for charging was too low. Drove around a bit and I could see the regen gradually improving as battery charge dropped. The regen seems inversely proportional to how charged the battery is. I have found regen to be "normal" when battery is at 80% or less.
     

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