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Regen limitation in cold weather

Discussion in 'Model S: Battery & Charging' started by PhilBa, Oct 14, 2013.

  1. PhilBa

    PhilBa Active Member

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    #1 PhilBa, Oct 14, 2013
    Last edited: Oct 14, 2013
    I know that when you range charge, the car places a regen limit and under cold weather scenarios a regen limit is also imposed. I'd like to understand cold weather limiting better.

    I am seeing somewhat inconsistent limiting. I some times stay over at my fiancee's house and lately the nights have been getting below 50F. That seems to be about the point where regen limiting kicks in. Generally, my SOC is around 75% (~200 miles on an 85K battery). However, several times the temperature has been well below 50F (44F the last time this happened) but no regen limiting was shown. Each time the car sat outside overnight and I left in the morning.

    While it's not like the end of world, I'd like to better understand how the MS decides to impose a regen limit.
     
  2. Zextraterrestrial

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    It could be that when the temp is at 44 but not limiting, the temp has just fallen at the dawn hour and the battery hasn't 'soaked' in it for long. I am getting limited between 44 and 48F currently. If you don't want to be limited in the morning I think the best way is to set charge timer to charge for the hour before you leave for work or wherever you are going.
     
  3. ModelS1079

    ModelS1079 Member

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    Or, similarly, use the app to start charging when you awaken to another always-beautiful Tesla Morning. As we can not adjust the charge limit with current firmware, this sometimes requires a toggle to max range charge, and then a toggle back to preferred SOC/Charge Limit when entering the car. As long one does not actually fill a max charge, the Regen limitation will have dissipated within 20 min or so of starting the morning charge. While you get your own charge from a cup of coffee, your Tesla gets it's own. All fair.
     
  4. PhilBa

    PhilBa Active Member

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    Wish I could charge at her house! Heck, I'd be happy just to be able to park in her garage (no snarky comments, please!). I've looked at the hourly temperature for her location and usually the lowest has been low 40s around 3AM and I leave around 8. The last time there was no regen, the low was 40F at 3AM.

    I guess the battery has some pretty significant thermal mass. Maybe the presence of rain has some effect - it would increase the rate of heat loss. Perhaps that's the variable I'm missing. Though the rain won't directly increase the battery cooling, maybe it is enough to cool the body of the car.
     
  5. Zextraterrestrial

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    maybe a breeze would change cooling a bit too?

    no 120V you can use?
     
  6. PhilBa

    PhilBa Active Member

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    Yeah, probably a number of factors come into play.

    I could use an extension cord but don't really need the power and the benefit is less than the hassle of putting the cord away in the morning.
     
  7. Doug_G

    Doug_G Lead Moderator

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    It's not how cold the weather is... it's how cold the battery is. So it would depend on the temperature profile the car was exposed to. Even if it's cold when you leave work, if it was warm all day the battery would probably still be warm.

    Wind has an enormous effect. The car cools down much more quickly on windy days. Just sucks the heat out of the car.
     
  8. Denarius

    Denarius Active Member

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    Turning on the climate control will also preheat the battery (using battery power obviously).
     
  9. Dreamin

    Dreamin Member

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  10. mknox

    mknox Well-Known Member

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    I assume you mean the cabin and not the battery. Wouldn't you need some sort of evaporative effect for wind to cool metal?
     
  11. PhilBa

    PhilBa Active Member

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    @Dreamin, thanks for the link. I did a search on regen in titles but somehow missed that one. The pack has a fair amount of thermal mass so will be behind the ambient temp as the environment warms up. In the case where it came back, I bet your pack was close to the limit and driving through colder air dropped it down.

    @mknox, Cold air blowing across a surface still removes more heat than same temp still air though evaporative cooling is much faster. In still air, there will be a higher temp boundary layer, particularly on the bottom of the pack.

    So, from this and the other thread, I see a number of things that affect pack temperature and thus when the firmware decides to regen limit:
    - air temperature
    - rain
    - humidity
    - wind
    - ground temperature
    taken together, it makes sense that determining regen limitation seems somewhat random.

    Clearly, SOC is involved in this process. However, my pack SOC was never higher than about 75%. I am guessing that there is a maximum charge rate that is a function of pack temperature and SOC.
     
  12. Doug_G

    Doug_G Lead Moderator

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    Not at all. Cold air blowing across a warmer surface. It's the reason your computer has a fan.
     
  13. mknox

    mknox Well-Known Member

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    Right. I remember all this from school, but that was a long time ago :confused: I actually thought the computer fan was to remove the heat from a confined space so that the heat source could radiate more effectively into a cooler environment. I figured the battery was already exposed to the great outdoors and that wind wouldn't have much of an effect. We do put fans on the radiators of larger distribution transformers, which are also outdoors, so there's that. I think the concept of "wind chill" and "Real Feel" on the weather forecasts is what was misleading me, since those values do depend on the evaporative effect of wind against skin.
     
  14. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Wind chill does not depend on "the evaporative effect of wind against skin." When your body is cutting blood flow to the skin to preserve core temperature, there is no, or almost no perspiration, happening...
     
  15. Doug_G

    Doug_G Lead Moderator

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    Right. Air is actually a pretty good insulator. It only transfers heat efficiently when it is in motion. Natural convection will move air to a certain extent; forced air is much more effective. As is cold air blowing across your vehicle due to wind.

    As Cottonwood says, you don't need moisture on your skin for wind chill. The temperature difference to the air does all the work. Sweating is only needed for additional cooling in high temperatures, especially when there is no wind.

    I'm surprised you haven't noticed that ICE vehicles also chill out a heck of a lot faster on a windy day than a calm one.
     
  16. mknox

    mknox Well-Known Member

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    Okay, at the risk of exposing even more of my ignorance, this is what I found on "Humidex" and "Wind Chill". It seems to be a measure of how things feel to humans and not an actual change in temperature. It's expressed as an equivalent temperature in dry air.

    Q: If a piece of steel, like a signpost, is sitting out in the elements at night (i.e. no outside heating influences, like the sun) at an ambient temperature of, say, 30 degrees, does the temperature of the steel drop under windy conditions vs. still air? My (admittedly weak) understanding is that it does not. In fact, since the space shuttle gets white hot under "windy" reentry conditions, I assumed that the friction of moving air would actually warm an object.
     
  17. Doug_G

    Doug_G Lead Moderator

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    No. If the signpost is at ambient temperature, then wind will have absolutely no effect on it.

    If the signpost was in the sun and got hotter than the ambient air, then wind absolutely will cool it down.

    The space shuttle moved at hypersonic velocities. The heating was primarily due to shock wave heating ahead of the craft. This is an entirely different situation. It was bleeding off a massive amount of energy. Yes in theory the friction of wind would create a tiny amount of heating, but that is so small an effect as to be unmeasurable.

    Basic thermodynamics - two objects with different temperatures placed in contact will tend to equalize. Cool air against warm metal will warm up the air and cool down the metal. Air however is not terribly conductive thermally. It is after all a gas and not very dense. However, if you create a pocket of warm air against an object and then blow it away, that will remove heat. And it will replace it with new cold air, which will heat up, and then get blown away. Of course in the real world this is a continuous process, but you get the idea.
     
  18. PhilBa

    PhilBa Active Member

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    Back to battery/regen questions. Last night, I had my car parked outside. Low temp was 41F according to my weather app. This morning at 9AM when I started driving it was 44F. The car showed a fairly low regen limit (line was at 15KW-ish though it was probably more like 12KW). After less than 1 minute of driving an alert popped up that said something like "Warming Battery, performance will improved soon". I'd never seen that before and have parked outside overnight in similar weather. The alert went away after about 6 minutes.

    Also, the regen limit line took a long time to go away - about 12 miles/15 minutes or so. It did move up once to about 30KW or so.

    Is this normal behavior? I am seeing the regen limit line a lot more lately though the weather doesn't seem like it's gotten colder.
     
  19. Zextraterrestrial

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    The regen limiting is a lot more aggressive on the newer firmware versions than it was last year. I also have been seeing it last a bit longer while driving before it drops the limit.
    I haven't had the 'battery warming' yet this season but was limited quite low the other morning since I hadn't charged and it was ~ 36F when I left for work
     
  20. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    This is all completely normal. After an overnight cold soak at 26˚ F (-3˚ C), I saw the regen limit at only a few kW. It took 25 miles or so of driving to have the regen limit go away completely. It may have been several steps, but the regen limit seemed to improve smoothly over those 25 miles.

    I am on a steep hill side and my garage is 3/4 underground with the above ground parts well insulated, so it stays at 45-50˚ F all winter without heat. The Model S will spend the nights in the garage for the rest of the winter.
     

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