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CAC value can jump up and down

Discussion in 'Roadster: Technical' started by Marius, Jun 8, 2014.

  1. Marius

    Marius Member

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    My roadster did 130.000 km in march of this year. I went for the yearly maintenance to Tesla with a standard range of 278 km and a CAC value of 146. The day after I got it back with a CAC value of 141 and a standard range charge of 269 km. I have asked Tesla what did happen and they could not tell me more than that the pack was tested and OK and did not understand the drop in value.

    The following days I did one range charge and went to a long drive immediately after charging finished. After that I kept it on electricity for a few days but nothing changed. What has happened? Next time I will tell Tesla not to touch my pack as clearly maintenance jeopardises the state of the battery?

    Now 2 months later I did end up and an ODO of 148.000 and did roughly some range charges followed by long trips and notice every 2 weeks a gradually climbing of the CAC value. Today I am on 143,97. I do drive easy average 100km per hour and most of the time long highway trips.

    What is happening?

    Is the degradation of the pack not lineair? Just yesterday I did a race on a circuit burning 270km range in 120 km speeding and a few times getting into power limiter. Surprisingly the CAC ends up at 144.

    So is it fair to conclude:
    1. Tesla do not touch my pack anymore
    2. Do a range charge every 2 or 3 weeks followed by immediately emptying the pack by a long trip?

    It got me thinking that outside temperature has an impact. However last year I had a standard charge value of 293 km in june and 20.000 km later in september degradation montly to 278 km.

    What is the best way to move forward in respect of battery life time?
     
  2. djp

    djp Roadster 2.0 VIN939

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    I doubt the capacity loss is related to anything that Tesla service did. Your capacity is tracking the Roadster battery study very closely, which predicts a Standard range of 264 km after 130,000 km.

    http://www.pluginamerica.org/surveys/batteries/tesla-roadster/PIA-Roadster-Battery-Study.pdf

    The CAC is measured when you do a single long drive (on one key turn) from >85% SOC to <20% SOC and then charge back up in Standard mode. The service visit probably triggered a re-measurement of the CAC, which finally caught up to the correct value.

    The best way to extend battery life is to avoid high SOC and heat. Only do a Range charge when you need it, don't let the car sit at high SOC, and run a cool down cycle after a long drive.
     
  3. Marius

    Marius Member

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    It is now 6 months later and I have reached a total of 181.000 km with a standard range value of 270 km and a CAC value of 143,02.

    This means that degradation of the pack in 51.000 km is about zero!!

    I am positively surprised but can not understand why and what.

    Is it a fact that batteries tend to degrade quickly in the beginning and then staying at about 88% capacity for longer time?

    Maybe there is an impact in the fact that for 2 months now I have a problem in the airco. According SC my airco is leaking which explains the fact that it only charges at half rate during standard charge mode. For this I switched for the last 2 months to performance mode charging in which I manually switch to standard at 250km using the OVMS app, meaning the last 20km is charged at standard mode and remains plugged in at standard mode allowing balancing.
    I am still waiting for news about a new airco system ordered 2 months ago so in winter time I can use this work around. Temperature of the pack after long driving 33 degrees Celsius and during charging gradually dropping to 20 degrees Celsius due to ambient temperature in Holland of 5-10 degrees Celsius in winter time.
     
  4. qwk

    qwk Model S P2681

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    CAC drift is normal. A deep discharge(especially from a range charge), triggers a recalculation, so that's why there was a drop. Over time, with shallow cycles the CAC drifts back up slowly and reflects a higher number than what it should be, because the calculation isn't as accurate with just a little of the pack being used. What it boils down to, is the CAC is only accurate after the pack is balanced, and there is a full battery cycle performed. All other CAC measurements are just wishfull thinking.

    I would bet that if you do a range charge, drive to ~20% of battery remaining on one key turn, and then charge back to full, the CAC will be back down, and most likely below the number that the SC measured at that service visit.
     
  5. Doug_G

    Doug_G Lead Moderator

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    Remember that CAC is an estimate of the pack condition. It stands for Calculated Amp-hour Capacity. It's not an absolutely perfect indicator.
     
  6. smorgasbord

    smorgasbord Active Member

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    Yeah, well, the biggest hit my pack took was when the car was in Tesla's hands for weeks. They don't keep the cars plugged in, they're often outside in the sun, etc. I had and have since been babying the battery. It's gotten a bit better since (it's been a half year), but hasn't recovered to where it was before Tesla got the car.

    The hard lesson is to not let Tesla have your car for longer than they need and/or keep reminding them to plug the car in as much as possible. Unfortunately, most service centers only have 1 or 2 Roadster chargers (and in Costa Mesa they only have UMCs!), so they can't keep all the Roadsters they have plugged in all the time.
     
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  7. ecarfan

    ecarfan Well-Known Member

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    Surely any Tesla Service Center could at least keep Roadsters in for service plugged into a 110V?
     
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  8. Doug_G

    Doug_G Lead Moderator

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    110V plug...

    Edit: what ecarfan said.
     
  9. slcasner

    slcasner Member

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    On the contrary, doing this sequence might cause the CAC to increase. If the car has only gone through shallow discharges for an extended period of time, the battery management system doesn't know where the bottom of the battery really is and the estimate may be pessimistic. Going on a road trip with range charges and discharges down to 20% or so lets it recalibrate. That was my experience:

    Before: 168 standard mode IM, CAC 140.12
    After: 179 standard mode IM, CAC 149.15
     
  10. Marius

    Marius Member

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    That will explain it. Original at 130.000 km I had 278 km and CAC 145, messed up during maintenance to 269 km and CAC 141 and now at 181.000 km I have 270 km and CAC 143. Recently I did some long trips to empty 20% SOC.
    Anyway a very minor degradation.
    I am curious what the future will bring.
     
  11. m0rph

    m0rph Member

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    #11 m0rph, Feb 15, 2015
    Last edited: Feb 15, 2015
    I was surprised when I went through my logs the other day..
    The car started it's active life in May 2012 and had a CAC 158.08. Until November 2014 it had only covered 12000km and was often not used for weeks. The CAC went little up and down, between 159.6 and 157.4, and was at 158.02 when I bought it. In the last 2,5 months I have driven 15000km+ and today it is still holding strong: 157.68 at 28000km. It was even 158.3 last week.

    The reason I went through my logs, was that I did a range charge and found my car still charging at 406km which is 99% SOC according to logs! :cursing:
     
  12. Marius

    Marius Member

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    #12 Marius, Feb 15, 2015
    Last edited: Feb 15, 2015
    So you should be happy. No degradation in 30.000 km.
    I reported a sudden drop from 145 to 141 after service at 133.000 km, but now I am at 187.000 km and CAC still on 141,72.
    I see in Germany on the tff forum TalkRadius who is on 230.000 km and a CAC of around 130 and a standard charge on just 230 km.
    On the other hand on the same forum was Hans who got a pack with only 100 km left after 200.000 km. He got another pack and passed 400.000 km already. So 200.000 km with the new pack.
    I do not know the secret, but am convinced that the pack also ages in time. This means that driving 60.000 km per year makes the pack in good state on higher kilometers and is better than park it in the garage for longer time. In US, guys reported CAC of 140 on 1.5 models being 6 years old and low mileage.
     
  13. m0rph

    m0rph Member

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    If I watch my logs, aging is not yet visible.. It might become a progressive thing over the years though. Also sitting in a (heated) garage didn't do it any harm. My belief is that the battery maintenance is of more importance than anything else. My car is always charged at 13A, is always plugged in and balancing regularly, is driving from garage to garage, in a soft climate, easy driving at avg of 175 kWh,... I'm curious to see how the CAC will evolve when I'll start charging at 32 and 63A. I also read about CAC rising with higher temps in the summer..
     
  14. wiztecy

    wiztecy Active Member

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    #14 wiztecy, Feb 15, 2015
    Last edited: Feb 15, 2015
    I honestly believe that charging faster is better for the Roadster as long as the ESS temps are kept normal / ideal temps. i.e.: around 26c. My theory is that I'd want to bring the cells up to voltage avoiding time that any charge or stress can be put on them. And also if I have my pack sitting at 50-60% SOC, its less time sitting taking a charge at a higher SOC to get to the 82-84% SOC, which I need for my daily (std. mode chg.) commute needs. But this benefit is only marginal. Anything below 85% down to 35-40% SOC is nice to the ESS.

    I have a 70amp HPWC for the Roadster but can't connect it up until I get my service upgraded, so its sitting in storage. I've been using my MC240 that came with the Roadster which charges at 30amps and its been very relivable as well as suiting my daily charging needs. Its also nice to the electrical grid and the most efficient amps to charge at.

    However [email protected] is where the most efficiency is as well as being nice to the electrical grid as Tom Saxton has found.

    Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog

    [TR="class: row_header"] [/TR] [TR="class: groupdiv"] [/TR]
    Charge Rate Wh per Std % Wh per Ideal Mile120V - 12A 807 414
    120V - 16A 723 371
    240V - 16A 589 306
    240V - 24A 544 282
    240V - 32A 527 274
    240V - 40A 512 266
    240V - 48A 524 272
    240V - 70A 516 268

     
  15. dhrivnak

    dhrivnak Active Member

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    Am I reading the chart right? To me it looks like 40 amps is better than 32 amps. Granted the differences are not great but 40 amps is the point where your watts/ideal mile is the lowest.
     
  16. Marius

    Marius Member

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    #16 Marius, Apr 12, 2015
    Last edited: Apr 12, 2015
    In older posts I have mentioned the sudden drop 1 year ago in CAC from 146 to 141 in 1 day and simultaneously ideal range from 278 to 269 km. For 1 year I did not notice any degradation during 60.000 km driven. Last monday I experienced during charging with 32A a sudden drop from CAC 141 to 135 and simultaneously ideal range from 269 km to 254 km. Temperature was normal 12 degrees Celsius, I had driven normal 100km per hour driving speed, batterietemp was 24 degrees Celsius. The pack is in balance which I checked in the diagnosis screen, all sheets with a 0 and cell voltage varies between 4,03 and 4,05 volt.

    Apparently aging goes in steps? I am now on 199.000 km so let's hope the CAC stays on this value now for another 60.000 km. The drop in range makes live more difficult in some instances on a daily trip with more than 400 km and no possibility to do destination charging.
     
  17. marco2228

    marco2228 Roadster Signature #34

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    My Roadster has a slightly different CAC after every charge/discharge.. Went gradually down in the winter from 152 to a bit below 146 and now is increasing again.
    4,03 vs. 4,05 doesn't look like an entirely balanced pack. On the ESS -> SOC screen, where you can also see the CAC, on the top right you see a MIN and a MAX percentage (SOC). What are these values?
     
  18. Marius

    Marius Member

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  19. marco2228

    marco2228 Roadster Signature #34

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    Ok, that's strange.
    Just had a look on my service screens.
    MIN 84%
    MAX 86%

    My Roadster has always been balancing, but right now it's not.

    Maybe it stops balancing, when it reached a delta of 2%? But that doesn't make sense, does it? Why not balance until 0.x %?
     
  20. djp

    djp Roadster 2.0 VIN939

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    @Marius, that looks in line with my min and max SOC. I think the balancing algorithm stops when the difference between min and max is narrowed to 2%.

    Can you download your ahr.log to see if one brick is dragging down the rest of the pack? Try a few snapshots at high, mid and low SOC.
     

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