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Calculate usable battery capacity based on rated miles values

Also, why does BMS State of Energy vary from state of charge (and rated miles) on the low end?

Basically what I care about is which number I should place greater confidence in. I've been in a situation where the IC reports 5 miles of range (2% SOC) left but CAN is showing 4% SOE and something like 3.1 kWh remaining. Which do I believe?

So how does one explain people driving miles past 0%? I know it has been documented a few times over the years in different models. Is it just some rounding or other error in how it is shown, so in reality even though it is showing 0%, it is really 0.4% or something like that?

My guess would be that the explanation lies in the deviation of SOE and SOC as suggested in my post quoted above. Although, I have yet to test this and it likely doesn't apply equally to all cars.
 
Amending my first post to reflect reported capacity from a 100 kWh pack (102.4 kWh total, 98.4 kWh usable). Details in this thread: Pics/Info: Inside the Tesla 100 kWh Battery Pack

Great info. Thanks!

The P100D had an EPA "Combined range voluntarily lowered to 315 miles" (305.9 city & 346.9 highway). Is this the reason why your capacity calculation was less than actual?
 

David99

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Jan 31, 2014
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Yeah, the BMS wont have knowledge of the true top capacity without a near full discharge and a near full charge. Inside the full voltage range it uses the current shunt and cell voltages to estimate SoC. Extrapolating out to 100% from lower capacities will generally result in gross errors in either direction (usually resulting in a lower value in cars more than a few months old or so and higher in newer cars that haven't had much calibration at all). You can still get an idea by extrapolating proportionally (range / percent), but I wouldn't take it as an accurate number and give it at least a +/- 5% error margin. The car generally tends to have a better estimate of range closer to 0% and closer to 100% overall. So extrapolating from 5% or 95% to 100% is likely more accurate than 80% to 100%, for example.

This is exactly what Tesla wrote in an email to a concerned customer who saw his range drop. Thanks a lot for explaining this from a more technical point of view!
 

David99

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Generally if you charge to a lower SoC often the error will tend towards displaying less rated miles available as the pack will tend to underestimate capacity. However, I've seen it both ways and it will overestimate as well. Either way, as you discharge the pack and expected voltages don't match up with power usage the BMS will just adjust towards the correct value as things progress. You should never hit 1 mile left and not have any juice. You *could* hit 0 and still have a little, but it's unlikely. The BMS does do a decent job estimating range these days < ~15 miles or so.

You mentioned many times there is no extra capacity passed zero. Just to clarify, when you refer to zero, you mean the actual 0% state of charge on the battery, not when it shows zero miles left. We know Tesla prevents the battery to be discharged to true 0% to protect the battery. I and many others have been able to drive beyond the point when the car showed zero miles left. When I reach 0 miles and 0% (on the dash) the car still shows aprox 4% SoC on the CAN bus. How much of that you can use seems to depend on several facts and yes there is no guarantee the car let's you use any of it, but I just want to clarify when you refer to 0 (zero) you mean the actual true 0% SoC, correct?
 
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@David99 you are getting at the same point I made earlier. I believe on cars where SOE > SOC at the low end, it makes sense that we should follow SOE. This is because SOE does not include bricking protection.

Also, why does BMS State of Energy vary from state of charge (and rated miles) on the low end?

Basically what I care about is which number I should place greater confidence in. I've been in a situation where the IC reports 5 miles of range (2% SOC) left but CAN is showing 4% SOE and something like 3.1 kWh remaining. Which do I believe?
 
Wait a minute... I have the S70, with the 75 battery pack but I didn't do the software upgrade. So I have only 65.9 kWh? If I look at all the figures, this is the worst scenario. It says the normal 75 has 72.6 kWh usable, so an upgrade would give me 6.7 kWh extra.

However, since I'm software limited, does it mean I can charge the battery to 100% every time without doing it any harm?
 

wdolson

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Jul 24, 2015
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I did some calculations a while back on the actual battery sizes going from the cell capacity up. I adjusted my figures to come as close as possible to Jason's numbers, but couldn't quite match them perfectly. It looks like the 70 pack has more cells per module than the 60 pack did for the math to work right.

If you could point out anywhere I made any errors I'd appreciate it.
2017-01-25_001.png


2017-01-25_001.png
 
@wk057:
1. when you looked at the software limited 60 and 70 packs, did you see how much of the software limitation that was placed bottom and top of SOC respectively?
2. In regards to bottom SOC limitation on these software limited packs, do you know if that bottom limitation temporarily unlocks if you hit 0 miles? or is it programmed to shut down the car just like the non limited packs?
3. I have the limited 70. 65900 Wh/295 Wh/mile =223.4 miles. But it always shows the advertised 230 Rated Miles at full charge. How could this be?
 

wdolson

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Jul 24, 2015
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I think you used either big packs numbers not from here or mulfunctioning calculator :p, because I get 71,3125 and 75,075. Also I suspect modules and cells ar binned so diffrence of at least 0,5 kWh would be normal

I think I rounded off too much, but I still have some math that doesn't work for the new pack. Elon has said the new 100 KWh pack uses the same cells as the 90 KWh pack and wk057 has said there are 516 cells per module. Using the calculated values for the NCR18650Gs that work for the 90 and 75 packs, that comes up with only 99.7 KWH for the 100 KWh pack, but wk057 found the pack has a rating of 102.4 KWH. That would imply a new cell with a slightly higher AH rating. Unless I screwed up my math again.

Another thing I found when running the numbers is they can make a 90 KWH pack that more closely matches 90 KWh by using 14X of the new modules that go into the 100 KWH pack. Both using the numbers from the NCR18650G and the possible new cell come up with a higher KWH rating than the current 90 KWH pack.

Once production of the 100 KWH pack gets up to speed, they might bump of the range of the 90 KWH pack, or drop the 75 KWH pack entirely. They may do this when the Model 3 is introduced to keep a bigger gap between the Model 3's top of the line and the bottom of the Model S's line. They could always do a software restriction on the pack to offer something smaller than 90 KWh.

2017-01-25_004.png
 
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Yinn

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Nov 15, 2016
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Behind you
  • All RWD Cars (non-Performance and Performance): 295 Wh/Rated Mile
  • Pre-refresh Model S Dual Motor, non-Performance: 290 Wh/Rated Mile
  • Refresh Model S Dual Motor, non-Performance: 285 Wh/Rated Mile
  • All Model X Dual Motor, non-Performance: 320 Wh/Rated Mile
  • All Model S Dual Motor, Performance: 310 Wh/Rated Mile
  • All Model X Dual Motor, Performance: 333 Wh/Rated Mile

Quick notes: Rated miles are EPA miles. I'm unsure what systems are used in other parts of the world. Internally on the cars everything in miles and uses these numbers then calculates the values for other regions using these as a base.

I don't have any window stickers, but these seem to different from FuelEconomy.gov which gives these figures for the 2017 models. I'm not sure if they changed over time or why it differs from the internal firmware.

Model X 75D: 360wh/mi for 238mi range
Model X 90D: 370wh/mi for 257mi range
Model X P90D: 380wh/mi for 250mi range
Model X 100D: 390wh/mi for 289mi range.

It allows customization, but I left it at default EPA figures. I tried doing a max 100% local and 100% highway, and the lowest those figures go to is 350wh/mi for the MX 75D. Those seem awfully high though. It would imply the 100D carries a 112kw pack. Either way, any insight on why the internal rated system is so different from the EPA's own website?

**I use ~350wh/mi on a refresh S60D for range nowhere close to 217mi, my own numbers are more in line with your findings with ~180mi out of a 100% SOC @ 350wh/mi.
 

ohmman

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Feb 13, 2014
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North Bay, CA
I don't have any window stickers, but these seem to different from FuelEconomy.gov which gives these figures for the 2017 models. I'm not sure if they changed over time or why it differs from the internal firmware.

Model X 75D: 360wh/mi for 238mi range
Model X 90D: 370wh/mi for 257mi range
Model X P90D: 380wh/mi for 250mi range
Model X 100D: 390wh/mi for 289mi range.

It allows customization, but I left it at default EPA figures. I tried doing a max 100% local and 100% highway, and the lowest those figures go to is 350wh/mi for the MX 75D. Those seem awfully high though. It would imply the 100D carries a 112kw pack. Either way, any insight on why the internal rated system is so different from the EPA's own website?

**I use ~350wh/mi on a refresh S60D for range nowhere close to 217mi, my own numbers are more in line with your findings with ~180mi out of a 100% SOC @ 350wh/mi.
Yeah, those are way off. I don't know why those are published that way. Clearly someone very bad at math put that together.
 
@wk057:
1. when you looked at the software limited 60 and 70 packs, did you see how much of the software limitation that was placed bottom and top of SOC respectively?
2. In regards to bottom SOC limitation on these software limited packs, do you know if that bottom limitation temporarily unlocks if you hit 0 miles? or is it programmed to shut down the car just like the non limited packs?
3. I have the limited 70. 65900 Wh/295 Wh/mile =223.4 miles. But it always shows the advertised 230 Rated Miles at full charge. How could this be?

The 85, 90, and 100 have a bottom range buffer of 4.0 kWh. The 60 I believe has 2.5 kWh. There is no way to "unlock" this buffer even if you hit 0.
 
I don't have any window stickers, but these seem to different from FuelEconomy.gov which gives these figures for the 2017 models. I'm not sure if they changed over time or why it differs from the internal firmware.

Model X 75D: 360wh/mi for 238mi range
Model X 90D: 370wh/mi for 257mi range
Model X P90D: 380wh/mi for 250mi range
Model X 100D: 390wh/mi for 289mi range.

It allows customization, but I left it at default EPA figures. I tried doing a max 100% local and 100% highway, and the lowest those figures go to is 350wh/mi for the MX 75D. Those seem awfully high though. It would imply the 100D carries a 112kw pack. Either way, any insight on why the internal rated system is so different from the EPA's own website?

**I use ~350wh/mi on a refresh S60D for range nowhere close to 217mi, my own numbers are more in line with your findings with ~180mi out of a 100% SOC @ 350wh/mi.

Those numbers include charging efficiency losses and that is why they are higher.
 

ohmman

Upright Member
Global Moderator
Feb 13, 2014
11,156
21,743
North Bay, CA
Those numbers include charging efficiency losses and that is why they are higher.
Charge efficiency appears to be in the 90% range, if I'm not mistaken. So, scaled:

Model X 75D: 360Wh/mi (324Wh/mi) for 238mi range - 77112Wh usable pack equivalent
Model X 90D: 370Wh/mi (333Wh/mi) for 257mi range - 85581Wh usable pack equivalent
Model X P90D: 380Wh/mi (342Wh/mi) for 250mi range - 85500Wh usable pack equivalent
Model X 100D: 390Wh/mi (351Wh/mi) for 289mi range - 101439Wh usable pack equivalent

The numbers still seem high across the board, but especially for the 75/100. Or I did something wrong.
 

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