Calculate usable battery capacity based on rated miles values

[Yinn]

I'm sorry..could someone explain charge efficiency and charge efficiency losses to me as they pertain to this? I feel like a dunce..

Here's what I think I know...
My solar panels absorb (generate? what's the right word?) 1mWh/mo. But only 900kWh/mo makes it to my house for a 90% efficiency.

I assume it's similiar in that once at the inverter/panel/grid, let's say I pull 100kWh from the panel to charge my car. Only 90kWh makes it to the HPWC. So my car is only juiced to 90kWh. If I wanted a full 100kWh charge, I would actually be using 111kWh at the panel...or needed to absorb 123kWh at the solar panel.

So my charge efficiency is 81% and my losses are 23kWh. Is that correct?

If so, I'm still not sure how they pertain to the ratings.

 

[ran349]

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.

No, you didn't do the calculation wrong. The EPA number comes from an independent test, so I wouldn't expect it to exactly match the Tesla numbers. I would consider all of those numbers ballpark values with all the unknowns involved.

 

[rowdy]

I think most of the confusion is coming from the early 90 packs right after the change in chemistry. These cells which were crap and suffered from rapid degradation (there were many threads on dropping range at the time from early 90D owners). Hence wk057 getting 85.8kWh from the BMS. Note that newer 90 packs are showing ~88kWh typically.

All current packs (100, 90, 75) have the same cells. The cells appear to be mature (none of the early degradation issues that plagued early 90 packs).

If you want to compare current packs then you need to use the same cell capacity rating 3.5Ah.

Initial capacity (no buffer)
100 (8256 cells) - 104.0kWh
90 (7104 cells) - 89.5kWh
75 (6216 cells) - 78.3kWh

Most degradation appears to occur in the first couple of charges. So real world figures you should expect a few kWh less from the ideal initial capacity.

 

[ran349]

I'm sorry..could someone explain charge efficiency and charge efficiency losses to me as they pertain to this? I feel like a dunce..

Here's what I think I know...
My solar panels absorb (generate? what's the right word?) 1mWh/mo. But only 900kWh/mo makes it to my house for a 90% efficiency.

I assume it's similiar in that once at the inverter/panel/grid, let's say I pull 100kWh from the panel to charge my car. Only 90kWh makes it to the HPWC. So my car is only juiced to 90kWh. If I wanted a full 100kWh charge, I would actually be using 111kWh at the panel...or needed to absorb 123kWh at the solar panel.

So my charge efficiency is 81% and my losses are 23kWh. Is that correct?

If so, I'm still not sure how they pertain to the ratings.

Your calculations are correct based on the numbers you are using. The EPA ratings are only concerned with the efficiency from the wall to your car, so your solar panel calculation does not pertain to that. If you wanted an accurate number for efficiency, you would need to measure how much energy you used from the wall and how much actually got added to the battery. I get close to 82% efficiency from the wall to the car, but that includes vampire losses I accumulated throughout the day, which I don't use in my calculation.

 

[wdolson]

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.

They call it an "anti-brick" buffer because li-ion batteries get damaged when you drain them down too far. They turn into bricks.

Additionally you can never get the last drop of power out of a battery. There comes a point where there is still energy in the battery, but you can't get enough current to flow to get that energy out. You can still measure a bit of a voltage, but put a load on the cell and you won't get much of anything.

 

[wdolson]

Calculations done on how much energy gets from a power plant to turning the wheels on your car have estimated around 65%. That's going to vary depending on how far you are from the power plant, the infrastructure used to transmit the power, the charger on the car, internal efficiency of the car, etc. But that's the estimate.

This was comparing EVs to fuel cells. I think fuel cells were only about 20% when splitting water to make hydrogen.

What your efficiency will be depends on where you draw the boundaries of the system. For a fair comparison, you need to make sure the boundaries are the same for all comparisons.

 

[Olle]

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.

Yes I am aware of those buffers to protect the pack. But what about the the limited packs? Would these, when you hit 0 miles, trigger an artificial "car is shutting down now"let you call the flatbed truck or unlock the limit an let you approach the "real" buffer?

 

[Yinn]

Yes I am aware of those buffers to protect the pack. But what about the the limited packs? Would these, when you hit 0 miles, trigger an artificial "car is shutting down now"let you call the flatbed truck or unlock the limit an let you approach the "real" buffer?

Could you imagine if Tesla just left it constantly charged at 75kwh capacity? When you run out a screen pops up with two options.

1. Dial roadside assistance.
2. Insert credit card to continue.

Like those arcade racing games where after the timer expires!

 

[miimura]

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.

You don't want to make a 90kWh pack from 14 modules because it has the disadvantage of a lower pack voltage. Since DC charging is current limited, you would want to use the highest available voltage for a given pack size. Anyway, I expect that they will discontinue the 90kWh pack and just make 100's and 75's, as the standard packs until they change over to 2170 cells. The reason that I think they will keep the smaller pack on the original module layout is that there appears to be twice as many coolant connectors in order to reduce the snaking of the coolant loops through the cells. Having a single loop going through the whole module and taking up a little more space is likely cheaper per kWh than the new layout. The new layout is optimizing absolute cell packing in the same envelope, not optimizing cost. They can also leave the automation in place for building the original modules and increase production capacity with new automation that is set up for the new module layout.

 

[wdolson]

You don't want to make a 90kWh pack from 14 modules because it has the disadvantage of a lower pack voltage. Since DC charging is current limited, you would want to use the highest available voltage for a given pack size. Anyway, I expect that they will discontinue the 90kWh pack and just make 100's and 75's, as the standard packs until they change over to 2170 cells. The reason that I think they will keep the smaller pack on the original module layout is that there appears to be twice as many coolant connectors in order to reduce the snaking of the coolant loops through the cells. Having a single loop going through the whole module and taking up a little more space is likely cheaper per kWh than the new layout. The new layout is optimizing absolute cell packing in the same envelope, not optimizing cost. They can also leave the automation in place for building the original modules and increase production capacity with new automation that is set up for the new module layout.

A 90 pack made with only 14 modules would have some disadvantages from the lower voltage and poorer performance than the current 90, but it would probably have a little better range because it would be lighter.

 

[miimura]

A 90 pack made with only 14 modules would have some disadvantages from the lower voltage and poorer performance than the current 90, but it would probably have a little better range because it would be lighter.

Are you sure about that?? 14 modules from a 100kWh pack would have 7224 cells while a 90kWh pack has 7104 cells. How can more cells weigh less?

 

[wdolson]

Are you sure about that?? 14 modules from a 100kWh pack would have 7224 cells while a 90kWh pack has 7104 cells. How can more cells weigh less?

There are fewer modules and you don't have the weight from that. I believe the cooling system for the new modules is a little more compact and lighter too.

 

[Boatguy]

This is interesting data, but additional data from my car, both static and empirical paints a slightly different picture, though unfortunately less attractive.

First, usable capacity.

• Refresh Model S Dual Motor, non-Performance: 285 Wh/Rated Mile

• Model S 90D: 294 rates miles * 285 Wh/mi = ~83.8 kWh usable

1) Your numbers for the S90D don't pencil out. You reported a usable capacity of 81.8, but the math computes to 83.8. So which is off, the usable battery capacity, or the Wh/RM?

2) The energy app on my S90D has the "rated" line at 300Wh/mi. The EPA range is 294mi which implies the usable battery is 300wh * 294 = 88.2kWh which we know is not the case. Further, reverse engineering the math, they use the 300Wh/mi to calculate the projected range based on current driving conditions (5/15/30mi average).

3) Empirically I've measured the rate at which Rated Miles (RM) are subtracted. Over a few thousand miles I've divided kWh consumed by RM consumed, this should side step vampire and other consumption. And I did this on fairly long legs of 100-230mi each. The average is always in the range of 272-274 Wh/RM. On my most recent 1,823mi trip I lost 1 RM for each 273Wh consumed. This is higher consumption than the 285Wh/RM that you quote in you original post.

Based on that, the original usable battery capacity was 273Wh * 294 = 80.2kWh, of course lower than what you extracted from the BMS.

4) My 100% RM is now 282 which is about 4% less than the original 294.

(Interestingly though, this has not happened in my BMW i3 which has about 20,000 miles. BMW calculates miles based on recent conditions, not with a constant, so this is significant daily variation depending on what happened the prior day. But over two years there has been no obvious deterioration of capacity, and possibly a small increase.)

So there are three numbers in play here, 300Wh in the Energy app, 285Wh that you extracted from the BMW, and the empirical 273Wh; a significantly large range.

One possibility is that Tesla dynamically changes the Wh/RM to mask the true battery deterioration. They take a little off the reported RM, then reduce the RM more rapidly. Here's how that would work:

Energy App: 294 * 300Wh = 88.2 kWh usable battery

This is clearly just marketing deception for somebody sitting in a showroom and looking at an S90D that they are being told has a 90kWh battery.​

Original per BMS: 294 * 285Wh = 83.8 kWh usable battery

Empirical data after 10Kmi: 282 * 273Wh = 77.0 kWh usable battery

Rather than show current RM of 77kWh / 285Wh = 270RM which would be 8% less at 10K miles than when new, they lower the reported RM a bit (4%), and then increase the rate at which they subtract RM (another 4%) so that the indicated RM still has some link to to the SOC.

At the very least, the projected range in the Energy app should not be used as a basis for estimating remaining odometer miles. Even if you drove at exactly 300Wh/mi, you could never achieve the projected range.

I've had five email exchanges with Tesla on the subjects raised above and they flatly refuse to address any of the issues, finally punting the entire thing to the Service Dept, as though there were something in my car that needs to be fixed. That leaves the explanation I've pieced together above which is an assertion that they actively and significantly misrepresent the capacity of the battery when new (in the energy app), and during ownership (dynamic adjustment of Wh/RM). It's not quite the VW diesel fraud, but it smells like there is some active misrepresentation going on.

It's very disappointing and I hope someone will have an explanation that shows Tesla is being more honest that this analysis suggests.

 

[AWDtsla]

That leaves the explanation I've pieced together above which is an assertion that they actively and significantly misrepresent the capacity of the battery when new (in the energy app), and during ownership (dynamic adjustment of Wh/RM). It's not quite the VW diesel fraud, but it smells like there is some active misrepresentation going on.

It's very disappointing and I hope someone will have an explanation that shows Tesla is being more honest that this analysis suggests.

This has been my conclusion for quite a while too. Dataloggers like teslafi point this out to me everyday, while saving me from having to do most of the mathematics. This shouldn't be that complicated. The car should report a number that everyone can validate as being true, and this topic should be over.

Still looking for data on EPA testing procedures for range. I believe they were gamed.

 

[David99]

@Boatguy
thanks for all the info. i have had similar experience. especially the 300 Wh/m in the energy graph not matching the rated range. jason's findings in the firmware conflic with the energy graph as well.
I think the rated range number is artificially manipulated to keep owners at peace. But at the same time the fact stanfs that battery capacity cannot be acurately measured when the battery is charged and discharged partially like it happens in our cars. I think we should seriouly let got of the rated range numbrr as a measure for battery capacity. It is clearly not accurate. the car reports the battery capacity on the CAN bus. regardless of weather and temperature, ths number does not change much at all. at the same time, the reported capacity on my car has dropped about 1 kWh over the last 8 months, yet the rated range at 100% has not changed. so clearly the car does some serious rounding. i think Jason is right, we should take the rated range as a rough estimae, nothing more.

 

[SageBrush]

If the 'static rated Wh/mile' number is not available, the available EPA numbers work pretty well:

1. 1. 33,700 / highway MPGe gives highway Wh/mile including charging losses
2. Looks like charging losses are estimated at ~ 13%,
3. So highway Wh/mile is (1.) / 1.13

Then usable battery is range * (3)

E.g.,
The software upgradeable 60D is rated for 218 miles and 106 MPGe on the highway by the EPA
33,700/106 = 318 Wh/mile including charging losses,
or 318/1.13 = 281.4 Wh/mile on the highway
218*0.2814 = 61.3 kWh

 

[TonyWilliams]

They call it an "anti-brick" buffer because li-ion batteries get damaged when you drain them down too far. They turn into bricks.

Additionally you can never get the last drop of power out of a battery. There comes a point where there is still energy in the battery, but you can't get enough current to flow to get that energy out. You can still measure a bit of a voltage, but put a load on the cell and you won't get much of anything.

That bottom of the cell voltage is when any cell string hits 2.5v. Because the packs are top balanced, at the bottom of the charge, the voltages can get significantly out of balance, so that 2.5v is the trigger to shut it down.

The best way to make should you don't exceed that 2.5v threshold when you're trying to stretch that last mile with "0" showing is to be VERY ginger with the accelerator.

One day, a few years ago, I attempted to stretch to get to a charger (after passing a perfectly good one 20 miles behind me). But, I felt cocky, because I could watch all the cell voltages, and as long as I didn't let any cells get below 2.7/2.8v, I thought I had it made with just a mile or two to go.

Of course, what I didn't know was that the charger that I was going to was on top of a hill. I tried going up the hill very slowly while not allowing any accelerator application that would make those cells dip too much in voltage. The car stopped several hundred feet short when I couldn't make the car move, and any more pedal movement dropped the cell(s) to 2.5v... pop, as the battery main contactor opened.

AAA was their in about 15 minutes. They asked where I wanted to go... "over there".

So, what would the pack voltage be? With a small battery (40-60-70-75) car with 84 in series, something around 250v, and the big battery car with 96s would be around 275v. What's important to know is that when the cells reach the low 3 volt area, the voltage drops like a rock past the "knee".

 

[AWDtsla]

If the 'static rated Wh/mile' number is not available, the available EPA numbers work pretty well:

1. 1. 33,700 / highway MPGe gives highway Wh/mile including charging losses
2. Looks like charging losses are estimated at ~ 13%,

Charging losses are not 13%. More like 8%, according to the numbers reported real-time which could be fudged but it also matches the spec'd efficiency, and the test value from @wk057 IIRC.

Anyway, using the EPA number doesn't do that much for me. I was just thinking today about my past performance cars, their rated MPG, and what I achieved in them with my enthusiastic driving style. Now the problem is I didn't validate their MPG readouts very well, but assuming they were correct, the difference between claimed consumption and achieved consumption in my P90D seems to be quite large AND I've been driving my car less enthusiastically trying to get the consumption down. Realistically the "savings from gas" I have are because of free charging, not because of electric drive.

 

[wk057]

I'll state again... and again and again: The value used by the car as the static Wh/Rated-Mile *DOES NOT EVER CHANGE*. The only thing that affects this number is the physical configuration of the vehicle, as specified in the first post in this thread, which doesn't change unless you do something unusual like a battery upgrade (to different capacity group) or motor upgrade (non-PD to/from PD.) It is possible Tesla has modified it from much older firmware, but the values I've reported have been the same since ~4.x versions.

Finally have some new data about 100 kWh pack: Model X P100D: 342 Wh/rated mile.

I will get the value for the Model S P100D next time I have that car online (forgot to grab it), but some math suggests it would be ~314 Wh/rated mile.

 

[stopcrazypp]

This has been my conclusion for quite a while too. Dataloggers like teslafi point this out to me everyday, while saving me from having to do most of the mathematics. This shouldn't be that complicated. The car should report a number that everyone can validate as being true, and this topic should be over.

Still looking for data on EPA testing procedures for range. I believe they were gamed.

I should point out EPA test procedures do not care about the range display. They can be as optimistic or as pessimistic as the automaker desires (for example the Leaf one is way more optimistic). The test stops as soon as the car can't keep up with the cycle anymore. It doesn't matter if the range display says it has 0 miles, 10 miles, or -10 miles left.

There is also no standard for battery capacity display, or even a requirement to display it. You can look at the Leaf battery capacity indicator for example, it's way less granular and it has been documented that Nissan fudged with the numbers to make it seem better. Tesla's rated range display is already one of the best in the industry in terms of granularity.