Mid Range Battery Math

[hugh_jassol]

Maybe @AlanSubie4Life can set me straight on some things... but here goes:

Given info:

• From new batt capacity of 63.8 kWh (per the Tesla EPA test)
• Buffer of 3.5 kWh
• Percent display does not include buffer (i.e., 100% = 60.3 kWh)

If use the average Wh/mi*projected_range / batt_SOC from the energy screen... I get an estimated 100% pack at ~58kWh.

If I now add in the buffer (3.5kWh), I believe that should give me what the car thinks my current battery capacity is (58+3.5 = 61.5 kWh).

This would mean that my current degradation (I'm at about 10k miles) is around (1-61.5/63.8) or 3.6%, which seems expected. Since the car at new (and for the first ~6 mo of ownership) showed a range of 264 mi @100% - this should put my range right around 255 at 100%. However, in my display at 100% I see 245 mi.

If I use my 58 kWh number and the discharge constant (226 Wh/mi), I get a 100% range of 256 mi (seems to be consistent with my calculation in the previous paragraph).

If I use the charge constant (237 Wh/mi), I get 244, which is what I see in the display. However, this would imply that the buffer is not included in the range display. I was under the impression that Tesla included the the buffer in this range display (from @TimothyHW3). Could this be the case they changed it? Or am I doing something incorrectly here?

If it were the case the Tesla changed from including to NOT including the buffer in the range display, then maybe that could explain the recent confusion?

 

[ewoodrick]

Internal buffers aren't included, they aren't even specified.
Don't try to make detailed since out of the range, it changes with software versions.

 

[hugh_jassol]

Internal buffers aren't included, they aren't even specified.
Don't try to make detailed since out of the range, it changes with software versions.

I purchased a cable for ScanMyTesla... as soon as it makes its way to me I should be able to confirm some of this

 

[AlanSubie4Life]

Buffer of 3.5 kWh

On the MR the buffer may be smaller than 3.5kWh. I would guess maybe 3kWh when new (4.7% of battery size). I don't know. Need to see the results from a CAN bus reader. Yours might be 2.8kWh now.

average Wh/mi*projected_range / batt_SOC

Yes, this formula does work...though it was not immediately apparent to me that it was 100% equivalent, which it is...

Full Battery Cap = Avg Wh/mi * Proj Range / Batt SoC% = Avg Wh/mi * Proj Range / (Rated Miles Remain)/(Full Miles Remain)

= Avg Wh/mi * Proj Range / (Rated Miles Remain / (Full Battery Cap / Charge Const))

= Full Battery Cap * (Avg Wh/mi * Proj Range / Rated Miles Remain) / Charge Const

= Full Battery Cap (includes buffer)

Because Avg Wh/mi * Proj Range /Rated Miles Remain = Charge Const (That's how we calculate it.)

QED

However, in my display at 100% I see 245 mi.

When new, your battery has at a minimum: Charge Const * 264 rmi = 62.57kWh (Charge const is about 237Wh/rmi) As you say, the EPA test gets 63.8kWh - I cannot explain that discrepancy, just FYI. The discrepancy is something that makes me think my formulas may have an error, for brand new batteries anyway - but has not been explained yet. It's possible there is a buffer below the usable buffer which gets reduced over time until a certain point, whereupon you start losing rated miles. No idea! Would have to have enough data points over time from a brand new car, especially when at near zero % SoC, and then look at it after it ages, to see what happens.

Anyway: You get 58kWh using your method.

Also, 245rmi * 237Wh/rmi = 58.1kWh

(You can also use the formula with the discharge constant and add the buffer):

226Wh/rmi * 245rmi + 2.8kWh = 58.2kWh (difference is probably due to rounding errors on values used).

That's loss of 7% of your battery capacity including the buffer. Since I suspect they reduce the buffer size proportional to the loss of capacity, it is also 7% reduction of your battery capacity not including the buffer.

was under the impression that Tesla included the the buffer in this range display (from @TimothyHW3)

I think you're perhaps misinterpreting what @TimothyHW3 said in that video. In any case, I don't believe that Tesla includes the buffer in this display, because at 0% SoC and 0 rated miles, you still have the buffer available - yet this screen would say 0 projected miles remaining. If you had 3 rated miles remaining, and your last 30 mile efficiency was 237Wh/mi, this screen would project you to have 3 miles remaining. But once you get your CAN reader, you'll find that with 3 rated miles remaining, you have more like 2.8kWh + 3rmi *226Wh/rmi = 3.5kWh which will likely get you more than 3 miles if you do not use too much power and cause automatic shutdown due to droop.
Summary: I believe this projected range meter is wildly pessimistic when you get to very low SoC, because it does not include the buffer (this is probably good, since you don't really ever want to use the buffer, as you can shut down the car if you press the accelerator too hard).

The formula: Charge Const * Full Rated Miles = Battery Size only applies, I believe, when you are at 100% charge. If you try to use it for calculating remaining capacity it will not agree with the BMS readout (and the error will be larger the lower the SoC).

Let us know when you get your reader:

1) What is the FullkWhNom? (My guess ~58kWh for you)
2) What is the buffer size on your MR? (My guess is 2.8kWh)
3) What does the readback look like when you get to, say, 5% SoC? 0.05*58.1kWh = 2.9kWh remaining, or 2.8kWh + 0.05*245rmi*226Wh/rmi = 5.57kWh? (Fairly sure it will be closer to 5.6kWh.)

If you provide any screen captures, be sure to include how many rated miles (not SoC %) you had remaining corresponding to each picture. It's the only way to make sense of things.

 

[hugh_jassol]

That's loss of 7% of your battery capacity including the buffer. Since I suspect they reduce the buffer size proportional to the loss of capacity, it is also 7% reduction of your battery capacity not including the buffer.

Bummer...as this would be well outside the expected loss for ~10k miles, at least according to the commonly thrown around expected loss numbers...

It's an agonizing wait for my OBD adapter cable... still waiting for it to leave Germany I think.

 

[HelixSpiral]

It took about 4 weeks from order to delivery for mine.

 

[TimothyHW3]

Bummer...as this would be well outside the expected loss for ~10k miles, at least according to the commonly thrown around expected loss numbers...

It's an agonizing wait for my OBD adapter cable... still waiting for it to leave Germany I think.

There is a new provider in the US for that cable adapter - it is cheaper too.
HRN-CT20T11 | Geotab GO | Geotab Adapters | GPS Fleet Tracking

Would be interested to see what the values for MR are.

 

[hugh_jassol]

There is a new provider in the US for that cable adapter - it is cheaper too.
HRN-CT20T11 | Geotab GO | Geotab Adapters | GPS Fleet Tracking

Would be interested to see what the values for MR are.

I ordered my cable Dec 8... last tracking update was through a US Postal Service facility in New Your City on Dec 18. Hope it gets here soon - I have time off during the holidays to use it!

I will post my mid-range numbers as soon as possible.

 

[hugh_jassol]

@AlanSubie4Life @TimothyHW3

Ok the MidRange numbers are in. Cable arrived this week, and today I installed it in my LEMR...

Nominal Full Pack: 57.8 kWh
Energy Buffer: 2.6 kWh
SOC: 89.5 %
Range Displayed on main screen: 217 mi

Screenshots attached.

One thing that jumps out at me here... the cell imbalance 3310 mV seems WAY high, right?

Let me know what other data I could take...
View attachment 493482

 

[AlanSubie4Life]

@AlanSubie4Life @TimothyHW3

Ok the MidRange numbers are in. Cable arrived this week, and today I installed it in my LEMR...

Nominal Full Pack: 57.8 kWh
Energy Buffer: 2.6 kWh
SOC: 89.5 %
Range Displayed on main screen: 217 mi

Screenshots attached.

One thing that jumps out at me here... the cell imbalance 3310 mV seems WAY high, right?

Let me know what other data I could take...
View attachment 493482View attachment 493483

Looks like my “guesses” were very close to correct given you now have 242.5 miles at 100% (means my “guesses” would now be 57.5kWh for battery capacity and 2.7kWh buffer size using the above formulas). Maybe the buffer is only 4.5 or 4.6% of the full capacity on the MR rather than 4.7%. No idea.

So at least that all basically checks out. 8% degradation/loss of capacity.

The cell imbalance makes no sense - unless one of the blocks of batteries is a dead short somehow. I wonder if they changed the addressing or CAN bus stuff on the fly and now Scan My Tesla is incorrect. Obviously cell volt min is way off too - unless there is a dead short. Cell volt mid is just the average of max and min it appears. Given your battery voltage is 394V and the max voltage is allegedly 4.108V, it seems unlikely you have a short. (394/96 = 4.104 which is the average cell voltage, so it is not possible to have a short, have that 4.108V be the max voltage, and have that average cell voltage - 4.108*95 = 390.26...so that 96th cell has to be more than 3.7V.).

No suggestions currently on what else to read back. (EDIT: Suggestions below.) It would be awesome to get readbacks from someone with a brand new car (any variant) if you know someone.

For easy reference:
Lines & Constants

The 226Wh/rmi discharge and CAN bus constant in my estimate here has never been measured or read back from a LEMR AFAIK. Maybe it is closer to 237Wh/rmi/1.045 = 227Wh/rmi. This value is just dependent on what % of usable battery capacity that Tesla chooses to allocate to the buffer. I assumed 4.7% but it may be 4.5% for LEMR.

To calculate it, we would need CAN bus readbacks at two different SoCs, along with the displayed rated miles for each of those two readings. I guess you could do that - reread again at 90% and again at around 50%. But you would need to be sure the battery is the same temperature for each reading to avoid error (leave sitting overnight with similar nighttime temps, before each reading maybe). Another thing you could try is do a long fast drive, immediately supercharge to 90%, and with a nice warm battery immediately do a readback. I think you will find available energy “fullkWh” is higher when you do the reading with a fully warmed battery (one that has been warmed up, preconditioned, supercharges at maximum rate, etc.). It is chilly in LA right now which probably influences the fullKWh reading by a percent or two.

 

[HelixSpiral]

Also, screen shot the "ALL" tab instead of just the "BATTERY" tab. There is additional info there. The rated miles remaining/battery percentage displayed on the car's screen is calculated using the "Estimated/ideal remaining" value not the "Nominal remaining". That is the value that changes with temperature.

 

[TimothyHW3]

@AlanSubie4Life @TimothyHW3

Ok the MidRange numbers are in. Cable arrived this week, and today I installed it in my LEMR...

Nominal Full Pack: 57.8 kWh
Energy Buffer: 2.6 kWh
SOC: 89.5 %
Range Displayed on main screen: 217 mi

Screenshots attached.

One thing that jumps out at me here... the cell imbalance 3310 mV seems WAY high, right?

Yes, please always post the ALL tab, not the battery tab, with all the pages there. And how do you charge? It seems that you almost never charge on DC/Supercharger - according to the app you probably did only 2-3 sessions, is that right?

Cell imbalance can't be right, it is deffect. You can't have 0,020mV on the min cell. Or if it is correct, then it means you have a dead cell so early into the car. I would check with Tesla and have them show you the cell level of all the cells just in case. This is the first time I see a dead cell show up on SMT, but if it turns out to be true I would push them to give you a new pack.
[email protected]
Maybe you can shoot Amund an email and ask him if it is a bug or whatnot. Maybe he doesn't have data on Mid Ranges you might be the first one.

How many miles do you have on the car? If I do the math based on kWh I estimate about 10,000 miles, maybe 15000miles, did you have that much?

 

[ran349]

@AlanSubie4Life @TimothyHW3

Ok the MidRange numbers are in. Cable arrived this week, and today I installed it in my LEMR...

Nominal Full Pack: 57.8 kWh
Energy Buffer: 2.6 kWh
SOC: 89.5 %
Range Displayed on main screen: 217 mi

Screenshots attached.

One thing that jumps out at me here... the cell imbalance 3310 mV seems WAY high, right?

Let me know what other data I could take...
View attachment 493482View attachment 493483

Based on your data, and assuming that the SOC% is correct(ideal remaining = nominal remaining), then your charge constant integer would be 238.

 

[AlanSubie4Life]

then your charge constant integer would be 238.

I think I got some definitive data at some point from the Energy screen calculation showing it was 237Wh/rmi. (That is why I have no “~” in my tracking post.) However, not doing it myself and obviously those calculations require a fair amount of precision and enough significant digits. 238Wh/rmi is totally possible. But, it would be good to get screen captures of the energy screen at ~90% SoC showing the three numbers (rated range, projected range, recent efficiency), for each of the 5/15/30 miles (or whatever km) settings. Ideally when displaying in units of km. With Wh/km recent values above 100Wh/km for each of those three recent efficiency settings. Then we could reconfirm.

Existing LEMR screen captures

These screens result in calculation results just below 237Wh/rmi. Obviously a few more screen captures with as large numbers as possible are good to reduce rounding errors, etc.

 

[hugh_jassol]

Yes, please always post the ALL tab, not the battery tab, with all the pages there. And how do you charge? It seems that you almost never charge on DC/Supercharger - according to the app you probably did only 2-3 sessions, is that right?

I only supercharged once just to “try it out” otherwise I have a Tesla wall charger at home in my garage. However, the car was home delivered to me from a local delivery center, so it may have been supercharged there when they drove it to me (it had 37 mi on it when it got to my house)

Over the next day or so I will have some more time to play with it and collect the more data. Stay tuned...

Cell imbalance can't be right, it is deffect. You can't have 0,020mV on the min cell. Or if it is correct, then it means you have a dead cell so early into the car. I would check with Tesla and have them show you the cell level of all the cells just in case. This is the first time I see a dead cell show up on SMT, but if it turns out to be true I would push them to give you a new pack.
[email protected]
Maybe you can shoot Amund an email and ask him if it is a bug or whatnot. Maybe he doesn't have data on Mid Ranges you might be the first one.

Yeah, that was some garbage data. I don’t know if the BT connection was weak (doubt it) or the fact that I was collecting the data on a Kindle Fire 7, which is very slow... but about every 4-5 refreshes of the screen show bogus data and the rest of the time it all makes sense.

How many miles do you have on the car? If I do the math based on kWh I estimate about 10,000 miles, maybe 15000miles, did you have that much?

10,200mi

 

[TimothyHW3]

Great, so 10,000 was spot on.

Do you have the correct cell imbalance now? What is it?

 

[hugh_jassol]

Great, so 10,000 was spot on.

Do you have the correct cell imbalance now? What is it?

6 mV. Although one of my screen shots says 4 mV. I am assuming that just normal variation.

 

[hugh_jassol]

But, it would be good to get screen captures of the energy screen at ~90% SoC showing the three numbers (rated range, projected range, recent efficiency), for each of the 5/15/30 miles (or whatever km) settings. Ideally when displaying in units of km. With Wh/km recent values above 100Wh/km for each of those three recent efficiency settings. Then we could reconfirm.

Attached. All images and screenshot were taken within about 60s. I haven't had time to do the discharge test yet. I notice that the Scan My Tesla "SOC UI" is not what is displayed in the UI... I'm guessing that's not what it means?

 

[AlanSubie4Life]

Attached. All images and screenshot were taken within about 60s. I haven't had time to do the discharge test yet. I notice that the Scan My Tesla "SOC UI" is not what is displayed in the UI... I'm guessing that's not what it means?

No, SoC UI is not what shows on the UI. Not sure what the formula is for it.

I am surprised - these numbers work out to closer to 236Wh/rmi. All very close to 237Wh/rmi of course - not anything to worry about. But 236Wh/rmi may be closer to correct.

What your rated miles at full at the moment? Still 246 rated miles?

That would work out to 246rmi*236Wh/rmi = 58.1kWh. I guess if you are actually closer to 247rmi @100% it would all make sense. (About 58.4kWh)

 

[TimothyHW3]

Attached. All images and screenshot were taken within about 60s. I haven't had time to do the discharge test yet. I notice that the Scan My Tesla "SOC UI" is not what is displayed in the UI... I'm guessing that's not what it means?

SOC UI is a value read by the CAN. It is an internal calculation of nominal full pack*SOC UI = nominal remaining.

SOC is calculated by Amund and is Nominal full pack * SOC % = expected remaining. What the car shows.
I think he uses nominal remaining in some versions, but I found it to be wrong, it has to be expected remaining.

Please note that the car uses expected remaining to calculate the SOC and not nominal remaining. Most of the time when the battery is warm these two values, nominal remaining and expected remaining will be the same, but very often they are not.


So it seems your battery is ok after all. If we take the 62kWh as a reference new - 2.6kWh Buffer we have about 1-2% Degradation. This is what most Teslas Model 3 have in the first few months.