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Seeking Pack Size VS. Degradation and Cost Analysis/Info

Discussion in 'Technical' started by gkalexdc, Apr 4, 2012.

  1. gkalexdc

    gkalexdc Member

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    I am hoping that someone here can answer the inter-relationship about buying too large of a pack, and then not using its full potential all the time. Example;
    If you drive only 60 miles every day round trip, but have a larger pack size, say a 65 KwH, do you end up under utilizing it and lose its capacity at a higher expense than just having a smaller pack (say the 40) and just do more frequent charging. There has to be a cost/ degradation/size analysis graph around, or an explanation for a lay person, (physician) like me to get the science from. My personal concern is, if you buy a (85Kwh) 300 mile range (ideal I know) and only use it for 70 mile days, is there a point of limited returns as to size, and how long that pack will last over time versus what your cost would be on a smaller pack, that may get more charge cycles, degrade faster. I hope I am making sense. Hard to put into words accurately. Thanks in advance. :confused:
     
  2. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    The way I understand it is the larger the pack, the longer the battery will last. There is basically no such thing as too large of a pack. Batteries like those found in the Tesla last the longest when they have small duty cycles. So if you fully charge and then fully discharge the battery it will have a set number of cycles. Say 500 for ease of imagining. If you charge to 80% and discharge to 40% the cycles might go up to 50,000. If you go from 70% to 50% there might be 5,000,000 cycles.
     
  3. gkalexdc

    gkalexdc Member

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    Ok, BUT.............. If you Cycle 70% to 50%, and have 5,000,000 (e.g.) and 8 years goes by, and you have the $20,000 extra invested in this pack, then degradation takes the pack....not the cycles did you waste your upgrade, versus its degradation. So....is there a meeting place of size, cycles, degradation? I intend to have the ability to drive long, but know there is a place logically that all of these factors add in/ subtract and line out. BTW, thanks for your reply to my thread. I learned a lot just now.
     
  4. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    No doubt there is a meeting place, but only if you consider cost in the calculation. Here's the best explanation I've seen about battery degradation.


     
  5. KangarooAustralia

    KangarooAustralia EV Enthusiast

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    For those people like me who don't always remember what acronyms stand for:
    SOC = State of Charge
    State of charge (SOC) is the equivalent of a fuel gauge for the battery pack in a battery electric vehicle (BEV). The units of SOC are percentage points (0% = empty; 100% = full).

    For more info see:
    State of charge - Wikipedia, the free encyclopedia
     
  6. Robert.Boston

    Robert.Boston Model S VIN P01536

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    This sounds like you're thinking of older rechargeable batteries that had "memory" -- if you didn't deep discharge them from time to time, the "bottom of the battery" became inaccessible.

    Tesla's batteries do not have this "memory" issue.
     
  7. hcsharp

    hcsharp Active Member

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    Re-reading your post and Robert.B's response re memory, I think you're asking if the larger battery would last enough longer than a smaller one to pay for itself (ignoring convenience)? The answer is: Very unlikely. There is no graph like you're asking for because there are just way too many variables to be able to come up with anything remotely accurate. 70 miles/day would be no where near enough miles for the 85kwh pack to pay for its premium over the 40kwh pack by longevity savings. Nobody knows what a replacement 40kwh pack will cost of course, but my guess is that you'd have to be discharging it to near zero at least once a week before it would wear out in half the time of the 85kwh pack.

    The main reason to buy a 60 or 85kwh battery is convenience and performance. And BTW, you will charge the larger pack just as often as the smaller one except when taking long road trips.
     
  8. gkalexdc

    gkalexdc Member

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    Well, Robert, I was actually thinking that having a bigger pack, and only going 60 miles a day most of the time. The question really is, not the cycling , but the aging of this pack over time, and having paid a premium of $20,000 over a smaller pack, and having it degrade in 8 years, and having lost the investment of a bigger pack due to degradation. Where does size, range, cost, and degradation come to a point practically, and economically. I would hate to have paid the extra $20 K , only use smaller ranges , with a once in a while longer trip, and end up having a degraded pack ( because of age).
    My question seems hard to put into words, but Jerry33 said that bigger packs have higher cycle counts if you use the smaller ranges of recharge, rather than deep cycle dischargjng and recharging.
    So, I have a question about that. IF you buy a bigger pack, say the 300 mile range, have more cycles because of its SIZE, then ultimately does this pack END its useful life at 8 years? I dont think we have enough data to know what these packs do. So. .... my question about size and age, and cost came about. Considering degradation, is it smart to buy BIG just to lose that investment. Higher pack replacement later, and higher cost initially. If you buy a smaller than 300 mile (85KwH), say the 65KwH is that smarter to use, and just use more recharging..... Its a tough question. I was just hoping there was a thread where this was discussed, or someone had some great opinions. All are welcome.
     
  9. hcsharp

    hcsharp Active Member

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    Why would a larger capacity pack have more cycles? I don't think you understand how these cars work. Or I'm misunderstanding you.

    With respect to the questions you're asking, yes we have a lot of data. Assuming I'm understanding you. The larger battery will last significantly longer than the shorter one even if you drive both of them exactly the same number of miles at the same speed every day of its life.

    The return on your investment has much more to do with convenience and performance as I stated above. There will also be a return based on longer life, but a 40kwh replacement pack will be cheap enough in 5 yrs that you could probably buy 2 or 3 of them for the same price as one 85kwh pack in 2012. You would have to really drive your car HARD to wear out 2 or 3 smaller packs in the same time as one 85kwh pack would last. So the 85kwh pack is a bad investment unless you want longer trips and better performance. Hope I explained it better.
     
  10. smorgasbord

    smorgasbord Active Member

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    If you're looking at costs, then the smaller battery almost always wins. The issue is range and performance.

    My analysis shows that for most people, the big battery's limited shelf life is going to swamp cycle life in terms of overall battery life expectancy, thus increasing its cost/mile. Here's my reasoning:

    First, assume that no matter what the pack size, the cycle life is the same. This may not be true, as the different packs use different density cells that may additionally support more cycles - but let's ignore that for now. Remember, a cycle is a full charge to full discharge and back to full charge. If you go from 100% SOC to 50% SOC and then back to 100% SOC, that's a half a cycle.

    Second, let's look at Model S battery warranty: 8 years for both - 100K miles on the 40kWh.

    Third, let's adjust the mileage per cycle from claimed to real since battery warranty mileage will be actual. I'll say 80%, so 160 miles is really 125 miles and 300 miles is really 240 miles.

    100K miles divided by 125 miles/cycle is 800 cycles. Tesla is probably being conservative with the warranty, so let's say real life is 1000 cycles.

    40 kWh goes 125 miles on a cycle, or 125,000 miles for the life of the battery. Cost is $20K. That's $0.016/mile.
    85 kWh goes 240 miles on a cycle, or 240,000 miles for the life of the battery. Cost is $40K. That's $0.17/mile.

    So, even though the bigger battery costs twice as much, it only costs about the same per mile. But, that assumes the shelf life of the batteries in infinite, which it isn't. If it were, then if you drive 10,000 miles a year, the 85kWh battery would last you 24 years. That ain't gonna happen.

    Let's assume Tesla's 8yrs/100K miles warranty on the 40kWh properly expresses the relative effects of cycle and shelf life. That works out to 12,500 miles/year. Drive more and cycle life matters more - drive less and shelf life matters more.

    But, on the big battery, to hit 1000 cycles in 8 years you need to drive 30,000 miles/year. That's one reason why Tesla offers an unlimited mileage warranty - most people won't drive that much. The other is the cycle life on that higher density chemistry is probably better, too, but that only helps if you're driving more than 30,000 miles a year, since otherwise the shelf life effects take over.

    So, how can the big battery pay off? Let's try an extreme case: You do 30,000 miles/year but you drive the same distance every single day of every week or every month of every year. You do 84 miles a day every day, except Xmas, or 30,000/year. That also means you can get by with a 100 mile range per charge. Let's say after 8 years/1000 cycles a Model S battery is at 70% of its original capacity. That means the 85 kWh battery will take you 173 miles (instead of 240) after 8years/240K miles. So, you don't have to stop using it. Now, once batteries hit this amount of degradation, the rate of degradation increases, so it's not linear. So, instead of 4%/year, it's some higher number that gets higher each year. Let's say you get another 3 years before the range is too small. That means you've done about 330,000 miles on your $40K battery, or $0.12/mile. There, you've beaten the 40kWh battery. Of course, during those 13 years you would have replaced the 40kWh battery twice - and each time it would have been bigger and cheaper - meaning less money per kWh, so in the end you're probably still out more money with the big battery. And that's without considering the time value of the $20K you put into the big battery up front.

    So, unless you want the increased acceleration, in terms of cost per mile you're better off with the smallest battery that has the range you need after 8 years/100Kish mile. And that's even if you do lots of miles per year, because that'll mean you'll be replacing that battery with an even cheaper per kWh battery sooner (you just gotta watch the range).
     
  11. gkalexdc

    gkalexdc Member

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    These are VERY informative answers !! I feel lucky to have found this forum, and you all are very intelligent. Your explanations are getting the job done here.
     
  12. gkalexdc

    gkalexdc Member

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    It seems I couldnt get the "Science lesson Quote to copy, See Jerry33's Science lesson above !!
     
  13. tennis_trs

    tennis_trs 2010 2.0 Roadster Sport

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    Looks like it should be $0.16/mile.
     
  14. hcsharp

    hcsharp Active Member

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    That wasn't Jerry's science lesson.:rolleyes:
     
  15. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    I did my best to give you the credit.
     
  16. gkalexdc

    gkalexdc Member

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    This was JUST the explanation I was seeking . Thanks for your analysis. I do wish that a pack was infinite in its shelf life....the operative is WISH. I just would like to be able to drive ELECTRIC to Austin from Houston and/ or not have to drive like grandma in my LEAF, which I have now. The Tesla is superior for sure.
     

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