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Battery degregation and pack balancing

Discussion in 'Model S: Battery & Charging' started by lmurphy137, Dec 27, 2014.

  1. lmurphy137

    lmurphy137 Member

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    Just to clarify, I am not obsessing over the rated range calculations. I just saw something while charging that I hadnt read about yet so I figured I'd post it. i am on a trip that takes me outside the protection of the supercharger network so I did a max charge off of a supercharger and something slightly odd happened. The indicator hit 100% and the max range was 239 rated miles (p85 with 20k miles). What was odd was it kept charging. It charged for another 20 mins and the display didn't change and it was pulling down 40 amps at about ~400v. It went on for about 20 mins and the current went down to 3A and I unplugged it. The whole time it said it was charging. During the drive we got about 11 miles(@~340 whr/m) before it read 238 so I think that since I baby the battery the estimate on capacity is off. It seems like everyone that has 99% orginal capacity also do large charges and deep discharges which we all know is hard on the battery. The normal degregation for my car has the total capacity at 252 which could be really close. We ended up driving it down to 60 miles left on this trip (201 miles) so we made it there without an issue, I iust never heard about the car continuing to charge after "hitting" 100%
     
  2. SeminoleFSU

    SeminoleFSU Member

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    It was probably balancing. It does this by allowing certain clusters of cells that are in blocks to bleed off, then it applies a little more juice to the whole pack as the clusters with higher capacity are bled off. That's my understanding anyway. I could be wrong
     
  3. kennybobby

    kennybobby Member

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    Balancing might be a possibility but i doubt it in this case.

    You were doing a max charge so it was trying to stuff as much charge as possible into the cells. It put in about an extra 5280 W-hrs during the 20 minutes of 40A @ 400V, and then you burned off 3740 W-hrs during those first 11 miles. So it did give you some additional miles to your range which might be important when trying to stretch between charging stations (max range).

    For daily charging its probably not worth the extra 20 minutes of waiting just to get 11 more miles off of the top of the cell where the voltage sags so quickly under load anyway...
     
  4. ZBB

    ZBB Emperor

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    No - it was balancing. Superchargers start dropping the amperage during the taper. Once it hits what looks like full, it's still tapering. The amperage will tail off over those 20 minutes down to 1A and then end the charge. I've watched my car do that on a superchqrger a few times (monitored with the mobile app...)...
     
  5. FredTMC

    FredTMC Model S VIN #4925

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    Yep. Ditto here
     
  6. kennybobby

    kennybobby Member

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    #6 kennybobby, Dec 27, 2014
    Last edited: Dec 27, 2014
    Well then how do you guys explain the extra 11 miles?

    Plus balancing can only account for 100 mA due to the 39 Ohm bleed resistors on the BMU boards.

    i understand about tapering off of the charge during CC/CV procedure of Lithium cells, usually cutoff at C/20, this is well known.
     
  7. Dwdnjck

    Dwdnjck Member

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    Rated range Gauges aren't that accurate
     
  8. AmpedRealtor

    AmpedRealtor Active Member

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    That's not balancing. That is tapering. Balancing occurs in the background and transparently to ensure all cells are "balanced" to the same charge levels. Balancing is handled by the BMS (battery management system) and has no feedback to the user via displays. It's something that just happens. The taper, on the other hand, is readily observed. The reason the car tapers is to ensure each cell is filled to the maximum without risk of damage to the cell.

    - - - Updated - - -

    The first time I ever charged to 100%, I also had an extra 10 miles driven before the range display changed by 1 mile. I think it is simply an algorithm issue. Since then, every time I've charged to 100% the range starts reducing immediately. I think it was an estimation error and nothing more. No free lunch.
     
  9. Zarwin

    Zarwin Member

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    I did my first range charge in months and mine did the exact same thing, got down to 1 minute left and charged another 30 minutes without the rated miles changing. Honestly I can't remember if the mileage dropped right after I started driving but when I charged back up to 90% the next time I had an extra 8 miles. The previous charges to 90% showed 221 miles and after that extended range charge with the mileage and time anomaly a 90% charge showed 229 miles, so I assumed my pack was out of balance a bit and the range charge corrected it to some degree and that was the reason for the extra 30 minutes of charge.

    Edit: this was on a L2 charger, not a supercharger
     
  10. lmurphy137

    lmurphy137 Member

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    Estimation error seems the most likely. It was new to me, thanks!
     

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