I don't disagree with you at all, it's just beside the point. Due to degradation my car charges 25% slower than it used to. Just sharing my findings.
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I don't disagree with you at all, it's just beside the point. Due to degradation my car charges 25% slower than it used to. Just sharing my findings.
FlyingCookie
Member
I did check mine and they open and close as they should. I also monitor the battery temperature (through the CAN bus). I've also had Tesla check my logs after I had a slow charge rate and they could never find any errors or anything out out of the ordinary. I strongly believe the slowdown is administered by Tesla as the battery gets older.
David,
Nice to hear that it's not that. One other thought on how Tesla might be calculating the taper. I know you showed your graph of the charge rate against %charge. What if the formula they use uses remaining capacity of the battery rather than %charger to decide on the taper. It would almost make sense for them, they would have one formula that could work for all the various battery sizes without worrying about keeping separate specialized ones. With your capacity loss, it almost feels like those curves might overlay after being re-normalized, )I would do it but I don't have the raw data you do.) -Peter
Nice to hear that it's not that. One other thought on how Tesla might be calculating the taper. I know you showed your graph of the charge rate against %charge. What if the formula they use uses remaining capacity of the battery rather than %charger to decide on the taper. It would almost make sense for them, they would have one formula that could work for all the various battery sizes without worrying about keeping separate specialized ones. With your capacity loss, it almost feels like those curves might overlay after being re-normalized, )I would do it but I don't have the raw data you do.) -Peter
hacer
Active Member
That would depend on the trip length I suppose. So if my trip is less than 6 hours of driving then any extra time is not a benefit but instead is inconvenient unless it fits well with a meal break. For longer trips I'd say it would be an annoyance if the SC stops increased by over 20 min.
But the OP drives between 600 and 1000 miles in a single go. In the first case, I'd say 45 minute charging stops instead of 35 is beneficial. In the latter case I would simply advise against driving 1000 miles without a quite long rest break in the middle or a second driver who can genuinely sleep during the first half, otherwise you're probably a danger to yourself and others.
How is a longer charge time forced by the car beneficial? That's like saying Tesla should never have further developed the battery packs and stuck with the A pack that was limited to 90 kW. You might have a different preference in frequency and duration of taking breaks on road trips, but how would that justify the car forcing everyone to take a longer break? That makes no sense. If I need a longer break, I do so on my terms and my preferences.
I do not have a complete log of all 600+ Supercharge sessions over the entire time I owned the car. If I remember correctly the decrease has come gradually over time.
That is a good point. I'm 100% sure that it is a contributing factor. As I wrote, to charge the same amount of energy I now have to charge to a higher percentage which means going into a lower rate. But it would only account for part of the time. I have about 9% battery degradation, the reduction in charge speed is 25%, though.
If I find some time I will try to scale the 'old' curve to compensate for the capacity loss, but again, the numbers don't seem explain the entire difference. The other thing that is obvious is how the shapre of the curves are different. When the car was new, the max power of aprox 116 kW would be sustained for the first 25%, then it drops slowly. The old curve starts at full power only for a very short time and starts to slow down faster. The shapes of the curves are not the same. The taper is more aggressive now.
The typical charging algorithm for a lithium NCA battery like Tesla usually has the charger pumping out constant current until the cell voltage reaches a pre determined threshold (up to the max cell voltage specification if you're designing to the limit). This happens pretty quickly in a lithium cobalt chemistry. Once max voltage is reached this signals the end of the constant current phase and the voltage is then held constant. Current will decay quickly as the battery saturates. I'm sure Tesla's supercharger algorithm is more complex in terms of its execution but it still probably follows those basic tenets. Two things that we are all are probably observing regarding DC charging as our batteries degrade:
- If Tesla is limiting the constant current rate to a fixed C percentage then it stands to reason that as battery amp hours degrade, the max current (and hence max kW observed) goes down.
- I'm trying to reach back to my schooling on the issue but I seem to recall that as internal resistance climbs (as it does when these chemistries age) the time spent in constant current mode decreases. This means the battery goes into the saturation mode sooner, i.e. it tapers more aggressively.
When charging a battery like this you don't really "calculate the taper". In the saturation mode where the charge power tapers down, the typical method is to hold a constant voltage and let the cell(s) absorb whatever current their internal resistance allows. The current naturally decays as the cells reach max state of charge. Tesla might be doing something more exotic but probably not. The taper is not usually something you force other than setting a max voltage and watching out for alarm conditions like battery temp and so forth.
I fully understand that is how normal LiIon charging works, but setting aside the beginning of the charge where it is clearly constant current of around 330 Amps, and the end of the charge where it is clearly constant voltage near 400V, the bulk of the charge has an increasing voltage, and a decreasing current (well below what we know if available at the beginning of the charge). We also know that Tesla is charging the batteries as almost 1.5C at the beginning of the charge which is outside of the standard charge profile. They do this with their active cooling, up to a point, and then the 1.5C rate drops. I don't think you can apply the typical method to Tesla's charge cycle as they are clearly not following it unless they are simply in an alarm condition for most of the charge. -Peter
I've never watched the current during a DC session; that's neat info. I guess Tesla's constant current phase is micromanaged to try to get the C rate as high as possible. I wonder if they have some way to account for deltas in internal resistance as the cells age and that is what drives different charge curves for older packs? I guess you could back into an internal resistance by observing the development of cell imbalances. Fun stuff.
As has been demonstrated many times, Tesla figures this stuff out (the development phase for adults) as they go along...AFTER the vehicle has been sold to customers. They think this is OK, and many buyers agree.
I suspect the non-fanboi customers that are entering the fold are going to be much less accepting of not being able to define what they are buying and what it will and will not do during their ownership than the zealots have been.
You will likely have options in 12months that will allow you to chose between 'old and slow' and 'fast and loose' development methods. I understand opinions will vary (at least until a few accidents of serious nature gain media attention, or Elon's messiah-praise interviews change to interrogations)...but I'm in the camp that is counting the days until the grown-ups sell EVs
I suspect the non-fanboi customers that are entering the fold are going to be much less accepting of not being able to define what they are buying and what it will and will not do during their ownership than the zealots have been.
You will likely have options in 12months that will allow you to chose between 'old and slow' and 'fast and loose' development methods. I understand opinions will vary (at least until a few accidents of serious nature gain media attention, or Elon's messiah-praise interviews change to interrogations)...but I'm in the camp that is counting the days until the grown-ups sell EVs
hacer
Active Member
It's like when your mom forced you to eat your vegetables: you didn't like it, but it was good for you. Longer periods of sitting is bad for your health http://annals.org/aim/article-abstr...havior-mortality-u-s-middle-aged-older-adults
I agree that it is less convenient to charge slower because it gives you fewer options. I was just trying to point out the upside. It's a glass-half-full sort of thing.
When I got my car the was no supercharger in Harrisburg, PA (and many other now existing super chargers). I took a trip to up-state New York from Maryland along the then feasible route with superchargers. This is a trip I make about once a year. Nowadays the new superchargers provides a path that is about 20 miles shorter, with less traffic congestion. Although my car may charge slower, the whole trip is faster. I'm guessing you might have similar experiences.
But i don't want to derail your thread, so I'll throw out some ideas about charging rate: The batteries are made of many modules in series. each module contains 6 series segments of 72 parallel cells. If you have an 85 kWh pack, there are a total of 6912 cells in the battery. Tesla warranties the battery, not the cells. Each cell has a fuse to protect the battery (and cell) in the event of cell failure. If one cell were to fail (more than likely I would guess in 100k miles and 4 years), you would lose 1/72 of the capacity (plus the ordinary degradation not causing cell fuse to blow). Although it is only 1 in 6912 cells missing, the capacity loss is still 1/72 because the battery is only as strong as its weakest series element. If you discharge the 71 cells to their low-voltage limit, you won't be able to discharge any of the other series segments further even though they aren't at their low voltage limit. You also have limited the maximum charging current to 71/72 of what it was before the cell failure. The good news is that one cell failure in any of the other series segments won't matter once one string has a failed cell. However, two in the same parallel segment lowers your peak charging current to 70/72. So in these cases Tesla has no choice but to reduce charging rate of the battery just to keep the cell rate the same. In addition, the series resistance of each cell increases with age, so that also requires a reduced charge rate in order to maintain the original thermal charging profile.
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In other discussions it was brought up that the charge port might be the limiting factor. Frankly the data speaks against it, but nevertheless I spent an hour cleaning all contacts in my charge port. The visual difference was very obvious after the cleaning. I went to the Supercharger to test it and the charge rate is almost perfectly identical to what it was before the cleaning.
Did your charger handle get hot? this may be one of the problems with supercharging causing the reduced charging speeds.
David,
I've been following your YouTube videos for a while and your observations in this thread are very interesting.
Since you're able to read the CAN data from your car, are you able to see the charge rate requested by the car while it's Supercharging? If so, this would tell you if the car is requesting a lower rate vs. the Supercharger providing a lower rate than the car is requesting (due to dead charging modules, overheating, etc.).
I've been following your YouTube videos for a while and your observations in this thread are very interesting.
Since you're able to read the CAN data from your car, are you able to see the charge rate requested by the car while it's Supercharging? If so, this would tell you if the car is requesting a lower rate vs. the Supercharger providing a lower rate than the car is requesting (due to dead charging modules, overheating, etc.).
It was 70 outside and the handle was and kept cool the entire time. So definitely not a heat issue.
I'm not as smart as Jason or others who have actually decoded the data. I'm not aware of the right data packets that would show that amount of detail. It would be very helpful to have that info! This way we could determine if it's the car that is request a lower rate or the station unable to provide more. Clearly there must be some data communication going on that controls all of this.
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