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I think my car is lying to me...
- Thread starter supratachophobia
- Start date
Wasn't that your position four weeks and sixty posts ago?
I don't think Tesla is giving you false information. I do think that the information the car gives can be imprecise and hard to correctly interpret.
But they are, if this data is to be believed. Data they provide. The car literally says that on a full charge, I can drive 274 miles based on information they and the EPA agreed on. But on a full charge, I can only drive 250 miles. And that range is based on their rules of consumption, not mine.
So rated mileage is based on pack capacity when new and EPA Wh/mile. Over 160k miles, the range has degraded a little under 10% (assuming you did get 274 miles when new). Additionally, the rate of mileage loss appears to be getting worse faster.
So either the pack is degrading (some degradation is expected) or the car's electrical or mechanical systems are more lossy than when new.
Have you had alignment checked recently? Are suspension and hubs in good shape? No loose underbody trim pieces or luggage racks (just throwing it out there
)Also, did you charging method change at the time if the degredation change point?
Just rescanned the thread. Do you have access to individual sub-module voltages? The statement
Makes me wonder if you have lost a cell (or it has gone resistive) in the pack so the car is operating based off of total pack voltage/ coulomb counting but then is cutting out due to a set of cells hitting the voltage floor. IIRC the packs are 74p so one cell lost should only be <2% capacity shift. Still, it may be something Tesla could check for you.
So rated mileage is based on pack capacity when new and EPA Wh/mile. Over 160k miles, the range has degraded a little under 10% (assuming you did get 274 miles when new). Additionally, the rate of mileage loss appears to be getting worse faster.
So either the pack is degrading (some degradation is expected) or the car's electrical or mechanical systems are more lossy than when new.
Have you had alignment checked recently? Are suspension and hubs in good shape? No loose underbody trim pieces or luggage racks (just throwing it out there
Also, did you charging method change at the time if the degredation change point?
Just rescanned the thread. Do you have access to individual sub-module voltages? The statement
Makes me wonder if you have lost a cell (or it has gone resistive) in the pack so the car is operating based off of total pack voltage/ coulomb counting but then is cutting out due to a set of cells hitting the voltage floor. IIRC the packs are 74p so one cell lost should only be <2% capacity shift. Still, it may be something Tesla could check for you.
Great question, there does to be some variance in cell readings but nothing that is flagging an error with Tesla. And even some wonky temp readings on module 19 that cause supercharging to slow down. But that doesn't seem to concern them either.
And this has never been about consumption. I know we can all get hung up on "my car isn't going as far as it used to". That's EV ownership.This is about Tesla telling an owner they can achieve a result under certain conditions, but then moving the goal mid-play despite meeting those conditions.
If it was 12% over 160k miles, id be a lot less concerned. This is 12%+ with 60k miles and 2.5yrs.
Here is a different spin on things.
This thread (and most regarding energy capacity and degradation) talk about energy out of the battery (kwh).
Now, bear with me. The batteries don't deliver kwh linearly. What I mean by that is you have a battery that has a fairly consistent Amp Hour capacity. During discharge the open circuit voltage of the battery decreases. At the same time the internal resistance of the cells increases.
So to deliver the same power (kw) the current needs to increase due to lower cell voltage as SOC decreases. Compounding things, the internal resistance of the cell increases as you discharge. So your current goes up due to lower cell voltage and internal resistance.
Based on all this, the first 50% of the cell capacity (in amp-hours) will get you more energy than the second 50% if your power requirement stays constant.
Now we know that Tesla takes a very analytical approach to the remaining range. Everyone else uses a GOM (guess o meter based on driving style etc) so all of this is more obscured. Could it be that in their effort to be accurate they are reflecting the actual energy in the pack and not the energy available with losses?
Another way to look at it is: if the user reduced power requirements to achieve more or less a consistent amp draw as the state of charge declined the first half of the pack would contain the same energy as the second half of the pack at least in theory. (There are more complex cell dynamics that this concept ignore)
This is not realistic since we might have the cruise set at 75 so as the voltage and internal resistance work against us the power requirement stays the same. With an accurate BMS with no fudging (which I don't claim to be true or false here) The first half of the battery will decline in SOC at a slower rate than the second half.
Those of us that cut our EV teeth with lead acid batteries really understand this because the internal resistance curve is much steeper. Li_Ion has a much flatter internal resistance curve.
This thread (and most regarding energy capacity and degradation) talk about energy out of the battery (kwh).
Now, bear with me. The batteries don't deliver kwh linearly. What I mean by that is you have a battery that has a fairly consistent Amp Hour capacity. During discharge the open circuit voltage of the battery decreases. At the same time the internal resistance of the cells increases.
So to deliver the same power (kw) the current needs to increase due to lower cell voltage as SOC decreases. Compounding things, the internal resistance of the cell increases as you discharge. So your current goes up due to lower cell voltage and internal resistance.
Based on all this, the first 50% of the cell capacity (in amp-hours) will get you more energy than the second 50% if your power requirement stays constant.
Now we know that Tesla takes a very analytical approach to the remaining range. Everyone else uses a GOM (guess o meter based on driving style etc) so all of this is more obscured. Could it be that in their effort to be accurate they are reflecting the actual energy in the pack and not the energy available with losses?
Another way to look at it is: if the user reduced power requirements to achieve more or less a consistent amp draw as the state of charge declined the first half of the pack would contain the same energy as the second half of the pack at least in theory. (There are more complex cell dynamics that this concept ignore)
This is not realistic since we might have the cruise set at 75 so as the voltage and internal resistance work against us the power requirement stays the same. With an accurate BMS with no fudging (which I don't claim to be true or false here) The first half of the battery will decline in SOC at a slower rate than the second half.
Those of us that cut our EV teeth with lead acid batteries really understand this because the internal resistance curve is much steeper. Li_Ion has a much flatter internal resistance curve.
The car only has 60k and 2.5 years on it? Wow. Might end up hitting warranty criteria.
The module 19 temp reading may indicate a deeper issue maybe bad connection causing voltage offset?
With lead acid, discharge rate has a large impact on total energy. With lithium, the effect is much less. So with SOC based on amp hours it will report the right level, but as you say, the Ah per mile ans SOC per mile will increase as the pack voltage decreases. Even running a constant amp draw will not yield the same kWh for the first 50% as the second due to the voltage decline of the pack (might be in your ignored cell effects).
But all this is secondary to the number of miles a full pack gets you, which seems to be the root concern.
What warranty criteria? The S&X have no gradual degradation warranty.
While I don't know what it is, the 8 year infinite mile warranty should have a capacity criteria. If the trend (12% in 2.5 years) continues it would be at 64% in 8 years.. If there is a bad cell/module/group that would trigger it on it's own.
So it sounds like we more or less agree that energy available declines per % SOC as we discharge the pack.
This is the part where I am very curious what BMS does to determine kwh capacity. Two large aging affects of batteries are increased internal resistance and reduced capacity.
I have two questions based on this that I have not seen a solid answer on.
1) So, we could have the same capacity (in AH) in a cell but with increased internal resistance due to aging lower range. This could be why some people report needing to decrease their wh/mi to achieve the same actual mile/rated mile ratio as the pack ages.
2) This is my second question: I believe I have seen WKO mention that he does not believe the BMS tracks internal resistance of the cells. So, If the driver drives the car really fast all the time. Due to the internal resistance affect on voltage the BMS will see less kwh per cycle than a pack driven with lower consumption but with the exact same SOH (State of health). So, is it possible that driving behavior does affect the BMS's estimate of pack capacity? We know that the car uses a fixed wh/m so no GOM. But, does the BMS account for the average pack current when it estimates pack capacity? Or, does that driver that has high consumption affect the BMS's estimates?
Maybe it should but it doesn't. It specifically says that gradual degradation is not covered. The model 3 has a 70% warranty.
Roger that, I was going for a threshold endpoint in the future "end up hitting", not a rate of decay early on. Annoying in the interim, but possible. If 3 is at 70%, they might match that. Or there is an issue that will trigger replacement.
Edit, after actually looking it up... I suppose it would depend on what gradual means, non-normal decay rate may allow for a claim....
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But is it using nominal or usable to calculate that remaining? That is my real question.
Pretty Graph Time!
This should be fairly self-explanatory, but I'll try to explain anyway. For the 90kwh batteries, this is what is going on behind the scenes: The car starts out reporting to the driver that it can travel XXX miles on it's current charge of 100%. It gets this number by assuming that it can use the Nominal Pack capacity to do XXX range. But we all know that the number used for Nominal includes the 4kwh brick protection and can't be used for driving. So the car has another number, Usable Capacity, which has that 4kwh subtracted. This is supported by the fact that 0% on your dash corresponds with Usable Capacity in the BMS also being 0%.
At some point below 90% and ~80% (closer to 80), something magical happens. The car stops using Nominal Capacity to calculate range remaining, and instead uses a number between Nominal and Usable. That is, until it gets down to 20%, by which time, it's only using Usable. Notice how the the distance between the two lines gradually, and linearly, gets closer together as SOC drops.
View attachment 317809
It might be tough to see since I shrunk the data-set down for ease of use, but refer to the rate in change on the small graph; the lines are flat between 100 and ~80, and also between 20 and 0.
View attachment 317813
I'm going to continue to make bold statements and say that Tesla knows about this huge "What's your 90%?" thread and that is why their magic only starts to happen below 90%. They want their best foot forward so-to-speak, so as not to raise concerns from 90kwh battery owners that their range has degraded so severely.
This should be fairly self-explanatory, but I'll try to explain anyway. For the 90kwh batteries, this is what is going on behind the scenes: The car starts out reporting to the driver that it can travel XXX miles on it's current charge of 100%. It gets this number by assuming that it can use the Nominal Pack capacity to do XXX range. But we all know that the number used for Nominal includes the 4kwh brick protection and can't be used for driving. So the car has another number, Usable Capacity, which has that 4kwh subtracted. This is supported by the fact that 0% on your dash corresponds with Usable Capacity in the BMS also being 0%.
At some point below 90% and ~80% (closer to 80), something magical happens. The car stops using Nominal Capacity to calculate range remaining, and instead uses a number between Nominal and Usable. That is, until it gets down to 20%, by which time, it's only using Usable. Notice how the the distance between the two lines gradually, and linearly, gets closer together as SOC drops.
View attachment 317809
It might be tough to see since I shrunk the data-set down for ease of use, but refer to the rate in change on the small graph; the lines are flat between 100 and ~80, and also between 20 and 0.
View attachment 317813
I'm going to continue to make bold statements and say that Tesla knows about this huge "What's your 90%?" thread and that is why their magic only starts to happen below 90%. They want their best foot forward so-to-speak, so as not to raise concerns from 90kwh battery owners that their range has degraded so severely.
DB 2
Member
Nominal capacity when full and nominal capacity remaining (from the BMS) do not include the "anti-bricking buffer," which the car will prevent you from entering. However, they do include an "anti-run out of gas" buffer of 3.8 Kwh that is not included in rated miles or SoC% shown on the dashboard. But it can be used (if you're feeling lucky.) See this (old) post for my evidence. Last time I checked, the BMS doesn't have a "usable capacity" variable. It must be calculated.