To be fair, 93% will vary depending on how fast the pack is being charged. Supercharging will reach CV much sooner than 120V charging, for example.
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Here is a graph from a recent 100% charge tracked with VisibleTesla
You can see when it reaches 100% (the middle red line). The blue line at the bottom is the charging Ampere. The battery reaches 100% and only a little later does the current drop which is the sign of changing from constant current to constant voltage charge. I have tracked many 100% charges and the current never started dropping before the battery reached 100%. In most cases the current actually stayed constant for a while after reaching 100%.
It's just a little surprising because that's not the way a Lithium battery is charged. It is charged at constant current until it reaches it's final voltage (at that point it's not 100% charged though), then the charger switches to constant voltage charging. The current continues to drop while the voltage stays the same. This is the way the last few percent are carefully added. What I see when tracking the Model S charger doesn't line up. It continues to do constant current even when it says it's at 100% and only after a few minutes after reaching 100% it switches to what looks like constant voltage charging. Neither the battery percentage goes up nor the rated miles after that, though. Yet after the battery level reached 100% the charging process continues to add about 3.5 kWh into the battery. Just odd. Anyone care to explain? wk057?
You can see when it reaches 100% (the middle red line). The blue line at the bottom is the charging Ampere. The battery reaches 100% and only a little later does the current drop which is the sign of changing from constant current to constant voltage charge. I have tracked many 100% charges and the current never started dropping before the battery reached 100%. In most cases the current actually stayed constant for a while after reaching 100%.
It's just a little surprising because that's not the way a Lithium battery is charged. It is charged at constant current until it reaches it's final voltage (at that point it's not 100% charged though), then the charger switches to constant voltage charging. The current continues to drop while the voltage stays the same. This is the way the last few percent are carefully added. What I see when tracking the Model S charger doesn't line up. It continues to do constant current even when it says it's at 100% and only after a few minutes after reaching 100% it switches to what looks like constant voltage charging. Neither the battery percentage goes up nor the rated miles after that, though. Yet after the battery level reached 100% the charging process continues to add about 3.5 kWh into the battery. Just odd. Anyone care to explain? wk057?
Do you have battery pack voltage to go along with SOC and charge current?
At 40A it's quite possible that the car doesn't get to constant voltage until it reaches "100%" - quoting "100%" because it's simply what the BMS is calling 100%, not what the cells can actually be charged up to.
The actual battery voltage isn't available while charging from AC. It only shows on a DC charger.
I highly doubt the BMS would be confused about what 100% state of charge is, but I agree, what the Model S shows to the user as 100% is probably not the true 100% of the cells. But then, I can see it pumping in 3.5 kWh more which would be the equivalent of about 10 extra miles. The 'rated range' doesn't go up, though. Maybe it's adding these extra 10 miles as a buffer to the bottom. I really don't know. Just guessing.
The constant voltage mode is reached during AC charging, even at 20kW, before the amperage actually needs to drop off since the current is relatively low. Keep in mind that even 20kW charging is only a 1/4C charge rate, and these cells will happily accept > 1C charge at most points. So looking at an amperage curve during AC charging isn't going to tell the whole story.
Also, believe it or not, Tesla's 100% on the dash is in fact pretty darn close to actual 100% for the cells. I'd estimate within 1% of what you'd charge them too with a 4.2V CV stage. Calibration issues could cause 100% to not quite be 100% on the dash, but generally it's pretty close.
As for that specific graph, I'm guessing that you hadn't 100% charged for a while? The fact that the taper doesn't happen until after 100% points to a calibration issue. The dash readout takes into account energy in/out as a factor as well as voltage, so it's often off by a bit if you never hit 0% or 100% often.
For supercharging, the taper curve isn't CC/CV like normal lithium ion charging. It's a temperature focused curve. Tesla knows how much heat they'll be able to sap away from the pack under charge and how they need to slow the charge current to stay below a threshold. There is actually room for improvement there, since the temperature thresholds are different when the cells are already partially charged, but Tesla seems to just map the same curve regardless. There may be reasons for that which I'm unaware, but I think there could be some improved charging speed near the higher half of the SoC range if you arrived with a cool (not recently under supercharge thermal load) pack at a supercharger with say, 50% SoC, the cells can handle a little over 1C charging at this point before they heat up like they would earlier in the supercharge curve.
Back on topic, though, the CV portion of a supercharge doesn't happen until nearly the end of a charge (99-100%) since the amperage just appears to be a curve mapping plus loads in the car. I haven't checked with CHAdeMO, but I assume that's probably the same although limited by the charger amperage.
Interesting. Do you see any benefit to introducing a chill mode to cool the battery before SpC and increase the thermal buffer thus allowing the pack to sustain higher rates later into the charge curve?
Supercharging: Chill Mode
Technically it would work. There are some lower limits to what would be effective so it probably wouldn't be a significant enough gain to be useful and/or noticeable. I think they'd be better off just doing multiple supercharge curves for various starting SoC rather than just sticking with the one they're using.
Edit: I can confirm that the cells much more readily accept high charge rates when they're cold... but I think this may have it's own set of degradation drawbacks.
That makes sense. I have a single charger so the charge rate is only 1/10 C (or something around that). And yes, I haven't done a 100% charge for a while before the one I graphed.
Also good to know that what Tesla shows us as 100% is actually true 100%. I always wondered if Tesla had set the 100% point slightly below the cells true 100% for longevity reasons.
That was my idea. Go to a CHADeMO charger and use it to charge to 100% and watch the voltage (on a DC charger the pack voltage is shown). Although I think they are designed to cut off as soon as the car reports 100%. They don't want EVs to sit at these stations for longer than necessary.
Sometimes I really wish Tesla would have a 'geek mode' for those owners who want to look into the technical details.
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That's interesting! I would have guessed cells would charge better at a higher temperature in the same way as they perform better at a higher temperature when discharged. I learn something new every day.
On that topic, not sure pre-cooling would be that helpful. It would just allow slightly quicker charging for the first minutes and then the battery heats up while Supercharging. A more aggressive cooling would help faster charging throughout the entire charging session.
Geek mode = read-only diag screen coming in 7.1. Nah, just kidding, but it would be cool.
As mentioned in another thread, I would *pay* for such an option. If they had an on screen enter your credit card number and be charged $500 to unlock a read only diagnostic screen (with cell voltages and stuff like that) I would be the first to do it.
mknox
Well-Known Member
You know, I probably would too. In the past, I have always purchased the shop manuals for my cars from the manufacturer. (The last set for my Cadillac CTS came in a set of 6 volumes each the size of a NYC phone book!).
Two days ago I charged to 100% in anticipation of a longer drive, but my plans got canceled. I let the car sit overnight at 100%. Guess what? Several hours after charging - I'm thinking 3-4 hours - my range went from 252 miles to 254 miles. My 90% charge is now back to 228 and where it was several weeks ago before I charged to only 70% for a week. This is the 2nd time now that I've seen a range bump after letting the car sit for a few hours after charging to 100%.
AR, I thought it was bad for the battery to sit after a full range charge.
In my case it recovered miles so I'm not complaining, but I don't normally let the car sit at 100%.
glhs272
Unnamed plug faced villian
Still getting 182 to 183 rated miles at 90% state of charge on my 60kwh pack. 60K miles on the odometer.
ZBB
Emperor
Nice!
I reported mine one here last week... I bottomed out at 167 about 2 weeks ago (it fell all summer -- even with 90% charging and only 1 road trip with supercharging (and 2 100% charges, although not at Superchargers). Its bee creeping back up, now at 171.
But its been quite a while since I've seen over 180 for a 90% charge... Highest in the last year was 178...
Yep, balancers kicking in and doing their work. 2 rated miles is ~600Wh. If one cell group was out of balance that means it only needs to bleed 6.25Wh from it to recover 2 out of balance rated miles. The bleeders are ~100mA, so it'd take ~12-14 hours to bleed this much. I'd guess it wasn't quite off by 2 rated miles and the rest was being rounded down previously or part of a larger calibration error.
I have a 2012 P85 with under 36K miles. For the first couple of years until early 2014 the only option was to charge the car to 90% Daily or 100% Range charge. When I used to charge the car to 90% I had a Rated range of about 245 for Daily and 265-267 for 100%. When the software update gave us the option to charge at a lower SOC I started charging to about 180 Rated Daily range. After a while when the car had about 14K - 15K miles the Rated Daily dropped dramatically to 213 - 214 miles and 100% dropped down to about 245-246. After reading the forums since the middle of last year I have been charging to 90% and if I drive only 20- 30 miles in a day I wait until the Rated range is down to 130 -150 miles and then plug it in to charge to 90%. After a few months my Rated Daily went up to about 226 miles but in the last couple of months it has gradually dropped down to 222 miles. Maybe I need to do a full Range charge like Amped to regain some of the loss?
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