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Tesla million mile battery

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  • #16
    Originally posted by TheLurch View Post
    Right, but unless i've read that wrong, the percentages are defined relative to an output voltage (the voltage will change as the battery nears empty or full). What i don't understand is how to relate those voltage changes to how much additional charge can still be extracted from the battery at that point.
    I really appreciate you taking the time with me. I not only don't understand the relationship between voltage changes and charge remaining, I'm not even sure who to ask. And more than that, when I was growing up, we believed in saying the pledge of allegiance, brushing your teeth after every meal, and 2.1V for a wet cell battery. I can see 4.2V from a pair of them in series, but why that, or 4.3V for that matter, would be part of the specs for these Li-ion batteries is a mystery.

    If there's a formula, that'd be bonus. I grok formulae just fine.

    But now that you highlight that graph, the red vs. blue lines does really drive home the relationship between battery performance and how much you stray from intermediate discharge states.
    Red, blue, and black, with the black being the authors' preferred comparison with their NMC532.

    C is simply a measure of how fast you charge it. C/3 is a pretty slow charge; 1C is what would be considered a normal charging rate for a battery of this size. Batteries tend to perform better if you charge them more slowly, as the ions have more time to integrate into the electrode in an orderly way.
    Okay, I really don't like this. Why would they choose to publish C/3 at 40ºC rather than 1C, like everything else, unless they did both and found they needed C/3 to beat the best of the Ecker et al. cells.

    Looks like cheating.

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    • #17
      Originally posted by lee_merrill View Post
      Interesting indeed, now we just need a million-mile car to go with it. Though electric vehicles are substantially simpler than their gasoline-powered counterparts, so maybe a million-mile car would be possible?
      A million-mile car is not only already possible, it's happened - the current record holder being a Volvo in North America that's been used for a several hundred mile daily commute for decades and has clocked up over 3,200,000 miles.
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      • #18
        Originally posted by Juvenal View Post
        Okay, I really don't like this. Why would they choose to publish C/3 at 40ºC rather than 1C, like everything else, unless they did both and found they needed C/3 to beat the best of the Ecker et al. cells.

        Looks like cheating.
        Yeah, it does. The Ecker data is all at 35ºC; one wonders why the new data wasn't obtained at that temperature for a direct comparison.
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        • #19
          Originally posted by One Bad Pig View Post
          Yeah, it does. The Ecker data is all at 35ºC; one wonders why the new data wasn't obtained at that temperature for a direct comparison.
          All the other data shows decreasing performance as temperature increases, so this bias is in favor of the Ecker cells.

          There's one more bias in the overcharge of 4.3V vs. 4.2V, but I don't know which way that leans.

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          • #20
            Originally posted by Juvenal View Post
            All the other data shows decreasing performance as temperature increases, so this bias is in favor of the Ecker cells.

            There's one more bias in the overcharge of 4.3V vs. 4.2V, but I don't know which way that leans.
            I think you're going overboard in imputing bias here. It's perfectly reasonable (and generally important) to identify how two experiments differed in order to consider things like the degree to which they can be compared, or whether it counts as a reproduction, etc. But there are plenty of reasons for experiments to differ that don't involve someone trying to make their results look better than previously published stuff.

            Labs have limited time and resources. It could be that the lab was most interested in conditions that are different from those tested by other labs. or they don't have the same equipment available for testing, or they tried to do a number of experiments but the one that was a direct reproduction of previous results failed for some random reason. It could even be as simple as a grad student not knowing the literature and setting up the experiments that looked interesting to him. In this case, since we're talking long-duration battery testing, it's not an easy thing to go back and redo an experiment later if your realize you don't quite have the data you wanted.

            So, there's plenty of non-nefarious reasons the experiments don't precisely mimic what's in the literature. It's a reason to be cautious about interpreting the results of these experiments in comparison to others, but it's not a reason to assume the researchers arranged that intentionally.
            "Any sufficiently advanced stupidity is indistinguishable from trolling."

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            • #21
              Originally posted by TheLurch View Post
              I think you're going overboard in imputing bias here. It's perfectly reasonable (and generally important) to identify how two experiments differed in order to consider things like the degree to which they can be compared, or whether it counts as a reproduction, etc. But there are plenty of reasons for experiments to differ that don't involve someone trying to make their results look better than previously published stuff.

              Labs have limited time and resources. It could be that the lab was most interested in conditions that are different from those tested by other labs. or they don't have the same equipment available for testing, or they tried to do a number of experiments but the one that was a direct reproduction of previous results failed for some random reason. It could even be as simple as a grad student not knowing the literature and setting up the experiments that looked interesting to him. In this case, since we're talking long-duration battery testing, it's not an easy thing to go back and redo an experiment later if your realize you don't quite have the data you wanted.

              So, there's plenty of non-nefarious reasons the experiments don't precisely mimic what's in the literature. It's a reason to be cautious about interpreting the results of these experiments in comparison to others, but it's not a reason to assume the researchers arranged that intentionally.
              Some of the differences will create better results, some worse, and it's important to note which is which if we're to make fair comparisons. I'm not suspicious of the temperature regimes, and only slightly concerned by the top voltage, and in any case, the differences are clearly posted.

              But the C/3 thing looks bad on its face, and even more so when you factor in limited time and resources. Working with a slower charge rate consumes more of both.

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              • #22
                Originally posted by Juvenal View Post
                Some of the differences will create better results, some worse, and it's important to note which is which if we're to make fair comparisons. I'm not suspicious of the temperature regimes, and only slightly concerned by the top voltage, and in any case, the differences are clearly posted.

                But the C/3 thing looks bad on its face, and even more so when you factor in limited time and resources. Working with a slower charge rate consumes more of both.
                Or not, if you consider this from the POV of researchers asking the question of "how long can we possibly keep this battery chemistry working?" If that's the question they wanted to answer, then running at least some of the samples at low charge rate—which we know allows batteries to handle more cycles—makes perfect sense.
                "Any sufficiently advanced stupidity is indistinguishable from trolling."

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