Help understanding Wh/mi (EPA vs. displayed)

[DB 2]

Your contention that my soc would be 4.6% would only give 84.5kw * 0.046 = 3.84 kwh. The buffer value in 0x382 is 4.0. My numbers , 3.4 kwh / 84.5 kwh * 100 = 4.02 %, more closely support that this number is percent, not kwh. Besides, a 4 kwh reserve would make the actual range even less than 259 miles, i.e 257 miles instead of the 270 quoted epa miles.

Please just try the 50% charge test I suggested next time it is convenient. It is very easy and will go a long way to proving who is right. I'll try not to sound too cocky, but I really don't have much doubt about what you will find.

Preview of coming attractions: I am just about certain that the energyBuffer value is not a percentage or strictly speaking a kWh quantity either, but rather a type of factor that results (when equal to 4.0) in a kWh value between 3.75 and 3.85 for packs between 60 and 100 kWhs. I need to do a little more testing before I am ready to say more.

 

[TIppy]

Please just try the 50% charge test I suggested next time it is convenient. It is very easy and will go a long way to proving who is right. I'll try not to sound too cocky, but I really don't have much doubt about what you will find.

Preview of coming attractions: I am just about certain that the energyBuffer value is not a percentage or strictly speaking a kWh quantity either, but rather a type of factor that results (when equal to 4.0) in a kWh value between 3.75 and 3.85 for packs between 60 and 100 kWhs. I need to do a little more testing before I am ready to say more.

Car is at service to have a non-presenting door handle fixed. I'll try it when it comes back.

 

[DB 2]

A well calibrated BMS with a well calibrated current shunt will have no energy available once you hit zero rated miles.

@wk057

In fact, you have it exactly backwards. A well calibrated BMS with a well calibrated current shunt will always result in energy still available when you hit zero rated miles.

Now that I have your attention, let's be sure we are talking about the same things. First, I assume by "zero rated miles," we are talking about "on the dashboard UI." Next, I only know software 8.X. Anything is possible with earlier software. Also, I can only examine the system in mild weather, although I doubt that really makes a difference.

From your post above, as well as previous posts on this subject, I have the impression that you think of the BMS and the UI as two relatively sophisticated, more or less co-equal systems. Perhaps I have drawn the wrong conclusion. Hypothetically speaking, if the BMS reports 3.5 kWh remaining and the UI reports 0 rated miles remaining, whom do you believe? (Not really hypothetical; it has happened to me repeatedly).

My understanding of the relationship between the two is different, and I have significant quantities of hard data to support that understanding. To me, the BMS is master and the UI is servant, whose job is only to present BMS data in a format useful to the driver, using a relatively simple formula. Any apparent discrepancy between the two is an illusion, created by the programming and priorities of the UI. A major purpose of the UI is to minimize the chances of a forced shutdown while still showing miles (or %) available on the UI. It does this by reaching 0 miles remaining while the BMS still estimates that power remains before shutdown (approximately 3.8 kWh, at least on most cars, including mine, a new 60 recently unlocked).

I mentioned hard data supporting this understanding. That consists, among other things, of video simultaneously showing real-time decoding of CAN3 message ID 0x382, and the rated mile gauge from the UI. This video shows that every change in the UI gauge is coincident with a 0.1 kWh change in idealEnergyRemaining from the BMS. (I know, ideal, surprised me too). Dozens upon dozens of examples, both charging and driving. Always simultaneous, and always consistent with a formula that results in 0 rated miles on the UI when idealEnergyRemaining reaches about 3.8 kWh. The formula is accurate to the hundredth, and even thousandth, of a rated mile (thanks @llavalle)

I have also tested the % SoC remaining setting on the UI, and it also changes simultaneously with the BMS, but it uses nominalEnergyRemaining instead of ideal (even more surprised).

One caveat: 99.9% of my data comes from only one car.

 

[llavalle]

The formula is accurate to the hundredth, and even thousandth, of a rated mile (thanks @llavalle)

You're welcome but I have no idea how I contributed here

edit : NVM, just looked at that other thread you linked. I need caffeine today!

 

[animorph]

@wk057

In fact, you have it exactly backwards. A well calibrated BMS with a well calibrated current shunt will always result in energy still available when you hit zero rated miles.

Now that I have your attention, let's be sure we are talking about the same things. First, I assume by "zero rated miles," we are talking about "on the dashboard UI." Next, I only know software 8.X. Anything is possible with earlier software. Also, I can only examine the system in mild weather, although I doubt that really makes a difference.

From your post above, as well as previous posts on this subject, I have the impression that you think of the BMS and the UI as two relatively sophisticated, more or less co-equal systems. Perhaps I have drawn the wrong conclusion. Hypothetically speaking, if the BMS reports 3.5 kWh remaining and the UI reports 0 rated miles remaining, whom do you believe? (Not really hypothetical; it has happened to me repeatedly).

My understanding of the relationship between the two is different, and I have significant quantities of hard data to support that understanding. To me, the BMS is master and the UI is servant, whose job is only to present BMS data in a format useful to the driver, using a relatively simple formula. Any apparent discrepancy between the two is an illusion, created by the programming and priorities of the UI. A major purpose of the UI is to minimize the chances of a forced shutdown while still showing miles (or %) available on the UI. It does this by reaching 0 miles remaining while the BMS still estimates that power remains before shutdown (approximately 3.8 kWh, at least on most cars, including mine, a new 60 recently unlocked).

I mentioned hard data supporting this understanding. That consists, among other things, of video simultaneously showing real-time decoding of CAN3 message ID 0x382, and the rated mile gauge from the UI. This video shows that every change in the UI gauge is coincident with a 0.1 kWh change in idealEnergyRemaining from the BMS. (I know, ideal, surprised me too). Dozens upon dozens of examples, both charging and driving. Always simultaneous, and always consistent with a formula that results in 0 rated miles on the UI when idealEnergyRemaining reaches about 3.8 kWh. The formula is accurate to the hundredth, and even thousandth, of a rated mile (thanks @llavalle)

I have also tested the % SoC remaining setting on the UI, and it also changes simultaneously with the BMS, but it uses nominalEnergyRemaining instead of ideal (even more surprised).

One caveat: 99.9% of my data comes from only one car.

So if the UI shows 0 miles and the BMS shows 3.8 kWh how much more will the car let you use? You seem to be saying this is still available to the driver. If this is the antibricking buffer and it allows you to use it then it does not seem to be doing its job. How many miles were you able to drive after hitting 0 miles on the UI? wk057 is warning that we can't rely on the car running past 0 miles on the UI. You have shown that the battery has energy remaining when the UI says 0 miles, but that is not the same as being able to use that energy to keep driving. I have seen reports on TMC of drivers getting more miles after 0 miles on the UI, but also others being shut down at 0 miles.

 

[TIppy]

@DB 2,

Here are my results for charging the car to 50%:

Full pack: 82.2 kw
expected available 42.9 kw
nominal available 43.1 kw
ideal available 43.3 kw

So, 0.5 = (available - min) / (full - min), or min = 2 * available - full.

These are the minimum kw values for the the three available kws:

expected min: 3.6 kw
nominal min: 4.0 kw
ideal min: 4.4 kw.

If you use the nominal remaining energy, you get exactly 4.0 kw as the reserve buffer.

I guess they don't really know the actual available energy, because the greater the power you draw from the pack, the less energy available.

 

[TIppy]

Of course, the units should have been kwh in my post above.

 

[TIppy]

@DB 2,

I also was able to drive below zero miles remaining. I just tested this. Driving at about 25 mph around my neighborhood, I drove an additional 12.8 miles and still had 1.3 kwh left of the 4 kwh buffer. The car indicated about 210-211 kwh / mile as I was driving. (4000 - 1300) / 12.8 = 210.9 kwh / mile. If I had used the entire 4kwh buffer, I could have driven 19 miles.

On Tesla's epa report, they say they discharge the battery pack to 208 volts. Mine was still at 291.4 volts.

 

[TIppy]

wh / mile.

 

[llavalle]

On Tesla's epa report, they say they discharge the battery pack to 208 volts. Mine was still at 291.4 volts.

208 seems very unlikely. that would be 2.16V per cell on a 85/90....

 

[TIppy]

208 seems very unlikely. that would be 2.16V per cell on a 85/90....

Plenty of other typos in that document. This could be one, too. If it was for a low voltage pack, it would still be 2.48v per cell, or a pack voltage of 237 volts. I guess the person could have been dyslexic, and it should be 280 volts. That's probably close to what my battery voltage would be after another 1.3 kwh of discharge. That would be 2.92 volts per cell, and leave a little capacity to get down to the customary 2.75 volts per cell.

 

[TIppy]

The report for the model 3 filed in July of this year still has a 208 volt cutoff for the 78 kwh battery. It also mentions that the rear motor is a 190 kw permanent magnet motor, not an induction motor. I guess they wanted that extra 5% efficiency.

 

[llavalle]

The report for the model 3 filed in July of this year still has a 208 volt cutoff for the 78 kwh battery.

Perhaps this is the value under load ?!

 

[TIppy]

Perhaps this is the value under load ?!

My battery still has an internal resistance of 0.063 ohms at 292 volts, so it would require 1325 amps to pull the voltage down to 208 volts. The model 3 doesn't pull more than 650 amps at max power.

 

[TIppy]

Perhaps this is the value under load ?!

Read an article about the roadster battery, and it said they limit the lowest per cell voltage to 3.0 volts and then shut the car down. For an 85, 90 or 100 kwh pack that would be 96 * 3 = 288 volts. So it looks like the 208 volts is just an obvious typo that keeps getting cut and pasted to the latest forms. After using 2.7 kwh of the 4kwh reserve buffer, the average cell voltage was 3.034 volts. So it looks like the cell voltages would be 3.0 volts at the bottom of the 4kwh buffer, and then the car would be shut down. It doesn't appear to be the brick with the lowest voltage as one of my bricks was at 2.998 volts and the car was not shut down.

 

[llavalle]

[...] average cell voltage was 3.034 volts [...] It doesn't appear to be the brick with the lowest voltage as one of my bricks was at 2.998 volts and the car was not shut down.

Are you saying you have more than 36 millivolts unbalance (average of 3.034 with min of 2.998)? That's an awful lot. I've never seen more than 4-5mV on my pack. Granted, I never went below 10-15km of range but even then, I still had 4 or 5mV difference between the lowest and highest group of cells.

Have you charged over ~93% SOC lately? From past experience, the balancer only kicks in if the SOC is over %93 SOC.

 

[TIppy]

Are you saying you have more than 36 millivolts unbalance (average of 3.034 with min of 2.998)? That's an awful lot. I've never seen more than 4-5mV on my pack. Granted, I never went below 10-15km of range but even then, I still had 4 or 5mV difference between the lowest and highest group of cells.

Have you charged over ~93% SOC lately? From past experience, the balancer only kicks in if the SOC is over %93 SOC.

It had been awhile when I did this test, but I think the fact that the slope of the discharge curve increases quite a bit at the low end had a lot to do with the bigger delta. After a recent charge to 100 %, my delta is 4.2 mv. But the important point is that the less than 3 volts for that brick did not trigger the car to shut down. So as the epa report meant to indicate, the trigger event is when the pack voltage drops to 288 volts. This implies that the epa range includes the 4kwh reserve.

 

[llavalle]

It had been awhile when I did this test, but I think the fact that the slope of the discharge curve increases quite a bit at the low end had a lot to do with the bigger delta.

Right. Makes sense indeed.