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Pack Behavior: CAC climbs with spring/summertime heat increase

Discussion in 'Roadster' started by wiztecy, Apr 10, 2014.

  1. wiztecy

    wiztecy Active Member

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    So I'm seeing this trend and feel like its pretty confirmed by others, that during the spring/summer the CAC rises significantly in the spring/summer and tends to drop significantly in the fall/winter.

    Any idea why this happens?

    My pack 3-4 days ago was sitting at 155.17 CAC. Temps started rising in the mid-80s where I'm at in California from the low to mid-70s where we'd been hugging. My return home trip the pack temps when ambient temps were in the 70's or below would be anywhere from 26C to 28C. Withing the past 4 days the pack temps increased to 32C where I initiate the cool-down cycle as soon as I arrive home. During the rise of temps and before I had to shoot off before a full std. mode charge was completed and didn't allow balancing to occur, 2 days in a row. I noticed that my pack was staying out of balance and was taking more than 1 hour to balance, so I allowed 4 hours extra to balance on 3 consecutive days. That seemed a little odd, since I've never had my pack take so long to balance (other than when it was replaced and rebalanced). I then looked at the CAC and saw it was climbing, so the longer balancing process started to make sense now given I had balanced 2 refurbished Roadster packs and learned their behavior.

    The odd thing here, on the two times I balanced the new packs the packs indicated they were fully balanced. But the CAC still showed significant incremental increases. The odd observation I'm seeing here is that there appeared to be a correlation between the pack telling / showing me its out of balance, also the time it was taking to balance, and then the increase of CAC which happened in parallel.

    Right now, 3-4 days later my pack is at 156.20 CAC. During the fall/winter I could see my CAC decreasing where it landed around the 155.17 area. It popped up a little, but nothing major and not a huge increase in a short period of time.
     
  2. dhrivnak

    dhrivnak Active Member

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    #2 dhrivnak, Apr 11, 2014
    Last edited: Apr 12, 2014
    I see a slightly different behavior. My CAC is relatively constant but my range has dropped off about 5 miles each winter only to return in the summer. I attribute this to average watts/mile being higher in the winter as I run the heat and defogger. My trips also tend to be shorter as well. Then summer when my efficiency climbs my range also ticks up.
     
  3. thefortunes

    thefortunes Member

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    I know we've discussed this in previous threads, but this is interesting to me because I do not see my ideal range changing with my wh/mi. Looking back at my data (taken daily - I'm kind of a nerd), there is no correlation between my previous wh/mi and ideal range. The only correlation occurs between CAC and ideal miles.

    I haven't been through enough warm/cold cycles to form any supportable opinion there, although my CAC variation does not seem to correlate with temperature. In fact my highest CAC reading to date occurred on a -7degF day.
     
  4. wiztecy

    wiztecy Active Member

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    I'm at 156.52 CAC now and sure its still going to climb. I see my CAC adjust twice during my charge sessions. Once after my charge and second after it's sat after a pulldown after a charge. I keep my CAC around 50-60 CAC over the weekend since I use my Zero electric motorcycle and Metro Electric SkateBoard for around town commutes. Will report back next week on how it moves up/down. I think this type of behavior is good to track to get a better understanding on the pack's behavior.

    So as with behavior, I think pack's may act differently when refurbished/new vs. after 4+ years. Don't know the real answer for that but I figure is for us to find out and discover.
     
  5. Alan

    Alan Member

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    I too have seen good things happen to both range & CAC as the weather warms up.

    For the CAC to recalculate it seems to need a long drive without the car powering down. Suspect if you drive from fully charged to near empty in one go it can measure / calculate more about the capacity than from several short trips.
     
  6. dhrivnak

    dhrivnak Active Member

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    Below is a graph of my ideal miles by date. As you can see I drop off some each winter only to gain most of it back in the summer.
    Range_Graph.JPG

    I also suspect that ideal range is correlated with your range as I tend to have higher ideal ranges when I drive easy. For me the R2 is .44 so if I am remembering my statistics 44% of my ideal range is affected by how easy I am on the last few drives.
    Range_ideal_range.JPG
     
  7. thefortunes

    thefortunes Member

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    #7 thefortunes, Apr 14, 2014
    Last edited: Apr 14, 2014
    Here is my Ideal Range (standard charges only) and CAC over the last 7.5 months.

    I have nowhere near the ideal range volatility that you show (my range moves between 180-188, while CAC moves between 150-157).

    Interestingly, it also appears that changes in my range lag a short time behind changes in my CAC.
     

    Attached Files:

  8. wiztecy

    wiztecy Active Member

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    #8 wiztecy, Apr 16, 2014
    Last edited: Apr 16, 2014
    ESS is still climbing strong with the CAC and pack still out of balance after 4 hours of balancing per day. I'm now at 156.73 CAC, doing a full std mode charge now and will give it more time to balance. I really need to do some cool graphs like you guys for sure!

    As for the ideal mile range / CAC, I also don't see them following the same line as the pic above. My CAC is climbing but my ideal miles are not. After this CAC has stabilized I'll do a range mode charge and do the one key drive to recalibrate the ideal miles to the CAC.

    It interesting the CAC to ideal miles. My original pack had a lower CAC, around 152-154 and it use to charge consistently up to 188 ideal miles. This pack with a higher CAC appears to come in lower, sometimes as low at 184 ideal miles. Curious why two packs will yield two variations in ideal miles.
     
  9. djp

    djp Roadster 2.0 VIN939

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    It's most likely the balancing. An out of balance pack with high CAC could should lower ideal miles than a balanced pack with low CAC.
     
  10. wiztecy

    wiztecy Active Member

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    So the pack finally balanced out two days ago but still climbing each day with the CAC. Its now at 157.07 CAC. Again I see it recalculating twice per day. Once after a full std. mode charge where SOC is at 84-85% and second after I pulled the pack down to 72-74% SOC from my drive into work after the charge.
     
  11. ibcs

    ibcs Member

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    My pac CAC shows 146.81 at 24,362 miles 60 degrees outside temperature. Standard charge shows ideal miles of 173. Is that normal? Trying to learn as much about the roadster, but I was surprised by CAC of 157.

    Thanks,
    ---Kent
     
  12. wiztecy

    wiztecy Active Member

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    #12 wiztecy, Apr 20, 2014
    Last edited: Apr 20, 2014
    My higher CAC is due to a pack replacement 1 year ago while at the end of my factory warranty, had an auxiliary power supply failure for which in the 1.5's is buried inside the ESS. They replaced the ESS with a refurbished one and it never reached the original CAC nor ideal miles after break-in / balancing. So they replaced that one with my current one which I'm happy with. This one climbed all the way up to 160 CAC but dropped down to 155.23 in the winter, and it climbed back up again. ...hence for the thread to get a better understanding for everyone.

    Your CAC to ideal miles appears to be on par. However depending if you had two sheets get pulled down hard (which I saw on my original ESS) that'll cause you to have a low CAC at an earlier period which I think most likely may be your case and others I've observed. There are other Roadsters that have 23-29k on them and they still report 155-157 CAC and get 185-188 ideal miles in std mode. Those of course wouldn't have the two sheets pulled down. This again is a theory on your pack as well as others who have a lower CAC but not super high miles. I saw my pack graphed nicely in detail and in bar-graph format that gave CAC / amp hour information per sheet as well as per brick. The two sheets pulled down were managed together by the BMB, for that the BMB manages 2 sheets except for one BMB that has light duty of managing only 1 (11 sheets total in the Roadster's ESS). You can pull your logs and run the VMS parser which will give you some information on the Min, Max, and Avg brick CAC of the pack, that's as close as we can get for publicly available tools to get a picture on the Pack's health as well as why.
     
  13. dhrivnak

    dhrivnak Active Member

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    #13 dhrivnak, Apr 20, 2014
    Last edited: Apr 20, 2014
    If you look at (and preferably add to) the Roadster Owner Battery study Plug In America I think you will find yourself very close to the average line.
     
  14. ibcs

    ibcs Member

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    Thank you for the quick responses. I will make sure that I'm registered on Plug in America.
     
  15. wiztecy

    wiztecy Active Member

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    CAC has climbed to 157.73 and appears to still be growing, pack is balanced (grid all 0's), std. mode miles increased from 184 to 186 after sitting for 3 hours.
     
  16. wiztecy

    wiztecy Active Member

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    I have a theory why the CAC climbs in the summer/hotter months and drops in the colder months. It appears the spread between the Average, Min, and Max is smaller in the summer than in the winter. I'm thinking that when the bricks begin to spread and drop, in particular in colder months, the CAC gets recalculated and becomes lower since each brick can't be used to its full V capacity. Same when temps increase and the spread becomes less, the CAC gets recalculated again. Its known that Voltage does increase when temperature increases as well as the opposite when cold. And when it gets cold the stronger bricks hold a higher V where the weaker ones drop away.
     
  17. m0rph

    m0rph Member

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    How did it evolve after that?
     
  18. tomsax

    tomsax Member

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    Here's my graph of CAC history back to the beginning of time.

    TJv3cOK.png

    The top (green) graph is the CAC. The lower graph shows miles driven each day. The huge drop down to 147 Ah marks where the battery pack got replaced in November, 2010, due to three sheets "failing the bleed test." It took until the following July for the CAC to peak as the car got to know the new battery.

    The big drops in CAC seem to correlate well with big road trips. The big spike in July of 2014 was the 1,944-mile drive to Monterey and back for TMC Connect. The CAC seems to drift slowly up when I'm not taking the car on big trips. It's especially noticeable with the big drop for 156 to 151 near the middle of graph (June 17-25, 2012) when we did a road trip to the Bay Area (CAC drops from 155.9 to 151.5), then slowly rises to 152.9 when we were back home and driving more normally. Then from July 27 to August 4 we did a trip to Spokane (600 miles round trip) and it dropped again to 151.2.

    There are also big drops in cooler temperatures, from 151.2 to 149.5 on March 6-10, 2013, and from 152.9 to 150.2 in early November, 2013, both trips to Portland (360 miles round trip).

    Between those two Portland trips, there's an area where the CAC goes up around the same time as some distance summer driving. Go figure.

    I made this graph from the daily records in the permanent section of the log file. VMSParser -p -r DAY -t <filename.tar>

    For more detail, see the large version of the chart.
     
  19. wiztecy

    wiztecy Active Member

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    I'm now at 150.13 CAC and charge up to 182 ideal miles, sitting after the charge / balance has completed for 2-3 hours. Ideal miles will fluctuate between 178-182.

    The CAC took a significant drop this year, but looking at the logs it all looks normal. Rest of the bricks in the sheets are within a normal tolerance so I'm not worried. Lowest brick gets rotated out which is what I want to see. I'm expecting to see my CAC climb back up into the summer months as well as having the opportunity to do some basic recalibration with longer drives.

    I currently have 40k on the Roadster and 20k on the ESS.
     
  20. Doug_G

    Doug_G Lead Moderator

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    I've noticed moderate drops in CAC coincident with "moderate" road trips. I suspect that is a correlation.
     

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