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Rated Miles displayed after unplugged cold soak

Discussion in 'Model S: Battery & Charging' started by bluetinc, Feb 17, 2013.

  1. bluetinc

    bluetinc Member

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    So after my trip yesterday up to Delaware, I had a spirited drive back down to DC last night before allowing it to cold soak overnight. I think this case is similar to what Broder did after his charge in Milford, where he would have had the battery nice and toasty, directly before a cold soak.

    I just happened to feel adventurous this last night/this morning. (and a little more confident do this with the newer access that I get using the S REST API with some extra information that didn't feel like I had before). I parked my S unplugged, with about 40 miles of rated range, then left it unplugged overnight. It cold soaked down into the mid 20's overnight, and range showed 4 miles when turned on. The battery level was 19 percent at this point in the morning. I then slowly drove the car for 9 downtown city miles then 2 highway, during which the battery level slowly decreased from 19 percent to 12 percent, clearly the pack was warming itself during this time using about twice the energy that I would expect had the pack already been warm. I then plugged the car in to charge.

    At the same time, my rated range display went down from 4 to 0 then to "charge now", and then worked it's way back UP to 6 miles of range left.

    I think from this that it is clear that using the "rated miles" left to understand the battery state does not provide a linear relationship.

    During this time, I also watched the battery display, which always displayed the percentage full from the battery. Normally I'm use to seeing the battery go from green, to yellow, to red. Even while the car was demanding to be "charge now" my battery was still yellow and not red (I believe because the true energy in the battery was never that low).

    If you look at the graphs from Broders trip, you can see a similar case where only a few percent or the batteries energy was used up over night (~5%) but the range was brought from about 90 to 25.

    I'm not sure if this is exactly the desired behavior. I'm fairly sure what is happening is that the car is trying to account for the extra energy it's going to need to warm the battery pack up, but perhaps it's a bit too much? I'm also not sure if being overly conservative is the wrong way to go on this, but I would like to understand it a bit better.

    Peter
     
  2. 100thMonkey

    100thMonkey Member

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    the range estimator on the Leaf is, not so affectionately called, the "geussometer". Many Leaf owners talk about simply taping over it. I actually find it useful, but only if I can level out the energy usage. Any estimation is really a guess based on how you have been driving, and projecting that consumption rate over the remaining charge. If you keep the consumption rate relatively level, the prediction gets much more accurate. The fact that the S has to heat the battery throws this whole calculation into even more murky water. devising a calculation to help estimate how much range the battery warming eats up is going to be pretty tricky, since depending on temperature, it's going to be different, and trying to factor those fluctuations in with fluctuations in driving and you are tasking the computer to do something nearly impossible, tell the future. Any calculation will end up being a guess, essentially incalculable. this sounds dismal, but we've learned to deal with some fairly dramatic fluctuations driving gas cars, we just keep lots of extra gas in the tank and don't fret over the minutia... hence plugging the car in whenever possible, especially overnight, especially in the winter time. it's just one of the unique characteristics of EV ownership, though frustrating to those of us that want to nail all this down with objective figures. to me it's a small price to pay for all the benefits of no gas and a fantastic ride!
     
  3. Norbert

    Norbert TSLA will win

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    The REST API gives you SOC in percent?

    And just to be more specific on this detail, the SOC was 12% when you plugged in at the end, and the "rated range" display at this point was at 6 miles?

    It would guess your battery wasn't completely warmed up yet, and that you would have recovered yet more range.

    It seems about 4% SOC corresponds to a rated range of 0, and 100% SOC corresponds to about 265, roughly.

    So under normal circumstances 12% SOC would correspond to about (12 - 4) / (100 - 4) * 265 = 22 miles. (If the relationship is linear under normal circumstances.)

    Under these circumstances: Minus the remaining battery heating energy. So perhaps at the point when you plugged in, the algorithm assumed that 22 - 6 = 16 miles worth of range, in energy, would still be necessary for completely heating up the battery.

    In the beginning of the morning you had 19% SOC, which would usually correspond to (19 - 4) / (100 - 4) * 265 = 41 miles.

    BTW, that's about as much as it displayed when you plugged it in, so maybe at that point the battery hadn't lost much charge yet, only heat energy (depending on how SOC is calculated). So based on the lower temperature, it calculated 4 miles, which might mean it expected needing 36 miles range for warming up the battery. You drove probably more than 12 rated miles while loosing (19% - 12%) = 7% SOC. 7% difference would usually be 7 / (100 - 4) * 265 = about 19 miles. Which means, under these assumptions, it used, by coincidence of similar numbers, (19 miles - 12 miles) = 7 miles of rated range for heating. As I calculated above, it expects 16 miles to still be necessary, so together (16 + 7) = 23 miles for b. heating. In the beginning it was expecting more than that, 36 miles. Perhaps the data available in these circumstances is very inaccurate and it calculates the worst case?

    The fact that it went down to zero, and then up again, makes it look like there is a bug beyond assuming the worst case. But perhaps that is only because it always puts away 4% SOC (or so) as reserve. And if it displayed the complete range including reserve, it would look more intuitive? And/or it always subtracts the energy needed to fully heat up the battery, even if the remaining range doesn't allow going as far as it would take to heat the battery to normal temperatures.

    I think it would be easier to tell if you had created another (or more) data points by driving further. This is the first time I'm trying to make such calculations, so I hope that I didn't make false assumptions, and that it added understanding instead of being confusing. In any case, more is needed to understand this. Perhaps they will fix it before we figure out what is happening.


    I would expect that you can still recover range until the range corresponding to SOC is about the same as the rated range displayed, which was not yet the case (22 miles vs 6 miles).
     
  4. bluetinc

    bluetinc Member

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    Yes, it appears that there is a battery SOC in the data coming back, but, it's not clear to me if this offset a bit thereby hiding the "reserve" of the car or not. It also appears that this is the number that is used to show the battery bar when you click the battery status on the 17". It definitely shows about 90% when you charge to standard, and 100% when charging max range.

    There may very well have been even more "recovery" coming, in fact Norbert, the numbers I ran match yours, but the next leg of the drive was going to be a bit over 10 miles till the next charger and I was leaving the "dense" area of chargers. I'd prefer not to really run the car below 0 during my testing :)

    If I get a chance to do this again, I would set up a quick logging program to log everything for all to look at, but I'm not sure when I'll get a good chance to do that.

    And just to be sure it's clear, I don't feel that I actually lost very much energy overnight during the cold soak. I'm not upset about this in the slightest. This is just to better understand the car :)

    I think my next step will be to run the car down to almost 0 miles remaining while really warm and see what the car says the battery percentage is. Maybe I already have a picture of that somewhere around, I'll look later today.

    Peter
     
  5. Robert.Boston

    Robert.Boston Model S VIN P01536

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    More data for the mill:

    After a lot of driving yesterday, I partially charge up to 31 miles of rated range then shut off charging for the night at about 11pm. It was in the low 20s last night, car outside; at 6:36am the car reports 11 miles of rated range, an apparent loss of 21 miles ~ 6.4 kWh. I start charging again, 201v 40A. At 7:30am, the car reports 40 miles of rated range, an an apparent gain of 8.8 kWh, or 9.8kWh/hour. The plug only provided 7.24kWh of power in those 54 minutes, though, and some of that was undoubtedly was charging loss, so perhaps 6.5kWh of effective power added to the battery. That suggests that of the 6.4 kWh of "lost" power overnight, approximately 3.3kWh was not lost, but only hidden because of the cold-soaked battery.

    Note that the car was plugged in during the night, but not charging. It's possible that the car pulled some power on its own. Yesterday morning, when I went to start up a range charge to start the long trip, I found that the car was topping off its charge already.
     
  6. RDoc

    RDoc S85D

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    It would be interesting to know at what temperature the battery starts drawing power to keep itself warm and if it's different depending on whether the car is plugged in or not. From various discussions it seems like the main net cold soak loss is just battery heating, both maintenance during the night and getting up to operating temperature. The experiences at low temps while on 110V seem to show very little net charge gain at all, so presumably most of the power is going into maintaining the battery minimum temperature. It's hard to say what the split in your case was between overnight temperature maintenance and any initial reheating to accept the charge.

    My plan for our X, when we get it, is to try very hard to at least be plugged into 110V on cold nights. Hopefully the Logan charge points won't all be ICE'd when we escape the winter. :smile:
     
  7. bluetinc

    bluetinc Member

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    From my work with batteries, I expect that they used at least two set points. One lower temperature (maybe -10C) for cold soaking, and a secondary minimum charge temperature which is certainly above 0C. This would mean the numbers for energy used to warm the pack, while plugged in and charging, are not applicable to energy used during an unplugged cold soak.

    Peter
     
  8. RDoc

    RDoc S85D

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    Those are my assumptions as well, but I haven't seen any discussions of the lower temp cold soak behavior so I suspect they only use the upper temperature for overnight maintenance. If it were maintaining the battery at -10C or so, the car would likely warm the battery up to around 0C before driving since it's my understanding that drawing significant current much below 0C is a bit fraught. Perhaps that's a mode for use when parked for long periods, e.g. at an airport, that will be unveiled when a more capable charge timer application becomes available.
     

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