Motor-battery combo performance differences S60, S85, and P85

[chrisdl]

jcaspar:
I understand your thinking. In my posts, please always read "drivetrain" as the motor-gearbox-inverter combination however. It is installed as one block and I've seen it referred to as "the drivetrain" in other posts. For me it makes it easier to distinguish between just the motor and the motor-inverter combination. I've replaced motor in my original post by drivetrain to make it clear that I'm not only talking about the motor. I guess it's still not clear. But anyway, now you know my personal semantics, FWIW

 

[Otmar]

4) Battery is charged at 360 VDC at supercharger
=> 96 3.6 V battery cells in series in a single bank

I think that this assumption is not accurate for both sizes. I have watched both the 60 kWh and 85 kWh models on the Supercharger and the finish voltage is quite a bit higher on the 85 kWh model. This would indicate that the 60 kWh model has fewer than 96 cell bricks in series.

While Model S supercharged at Refuel, I noticed a 60 kWh model tapering current to 32A at 352V while the 85 kWh model was finishing at 37A and 402V. These were values indicated on the dash.
402V / 96 cells is 4.19V and that makes sense for top of charge.
352 V at the same cell voltage would indicate that the 60 kWh unit only has 84 bricks of parallel cells in series.

 

[chrisdl]

Good info, Otmar. I'll check it out and update. Thank you!

EDIT:
I've updated the calculation, based on Otmar's input. Thanks! (see UPDATE 2)

 

[JRP3]

I think your conclusion is accurate. To further refine your calculations, the number of cells in the MS pack has been previously reported to be 7104, in a 96-series, 74 parallel configuration. The cells are 3.1 A-h, so at an average discharge voltage of 3.6v, would be about 11.2 W-h per cell. http://www.teslamotorsclub.com/showthread.php/17590-Model-S-Battery-Pack-Cost-Per-kWh-Estimate/page8

Problem is 11.2 x 7104 = 79564.8 Wh, or 79.6 kWh, not 85 kWh. If the 7104 number is correct then they must be using the 3.4ah cells which would be 12.24 Wh per cell and 86592.96 Wh, or 86.6 kWh for the pack.

 

[Porter]

Problem is 11.2 x 7104 = 79564.8 Wh, or 79.6 kWh, not 85 kWh. If the 7104 number is correct then they must be using the 3.4ah cells which would be 12.24 Wh per cell and 86592.96 Wh, or 86.6 kWh for the pack.

I'm not sure if I have the math right, but I thought to estimate pack kWh, it would 74 (cells in parallel) * 3.1 (cell Ah) * pack voltage = kWh. We know there are only 3.6 V (nominally) per cell and that there are 96 cells in series. This would equal a nominal pack voltage of 345.6 V. We also know that the pack has a label on it that says 400 V. We also know that this type of cell can have a peak charging voltage up to around 4.19 (or less) before it starts to hurt lifetime. As Otmar said above, this would be about 402 V. I'm guessing this is partly an issue of specmanship, rounding and marketing in terms of what the actual voltage is as well as the actual capacity.

At "nominal" voltage (3.6 V) this would only be 79 kWh. But at peak charging voltage (4.19 V), this would be 92 kWh. Its likely the real number is somewhere between the two. The other possibility is that the 85 kWh is based on usable vs actual. If you take the 6-7% off (safety margin) of the 92,273 number, you get pretty close to the 85kWh. Thoughts?

 

[qwk]

I'm not sure if I have the math right, but I thought to estimate pack kWh, it would 74 (cells in parallel) * 3.1 (cell Ah) * pack voltage = kWh. We know there are only 3.6 V (nominally) per cell and that there are 96 cells in series. This would equal a nominal pack voltage of 345.6 V. We also know that the pack has a label on it that says 400 V. We also know that this type of cell can have a peak charging voltage up to around 4.19 (or less) before it starts to hurt lifetime. As Otmar said above, this would be about 402 V. I'm guessing this is partly an issue of specmanship, rounding and marketing in terms of what the actual voltage is as well as the actual capacity.

At "nominal" voltage (3.6 V) this would only be 79 kWh. But at peak charging voltage (4.19 V), this would be 92 kWh. Its likely the real number is somewhere between the two. The other possibility is that the 85 kWh is based on usable vs actual. If you take the 6-7% off (safety margin) of the 92,273 number, you get pretty close to the 85kWh. Thoughts?

You are most likely correct. I don't think Tesla reports the battery Kwh number like other manufacturers. It's likely closer to actual usable capacity.

 

[JRP3]

It's true if you up the nominal cell voltage the Wh per cell goes up, and hence the pack kWh as well, and Tesla may indeed be using a higher nominal voltage in their pack rating. This is somewhat inaccurate though because it doesn't reflect usable energy, as soon as you put a load on the pack the voltage immediately sags below resting voltage, so actual voltage x amp hour numbers drop.

 

[stopcrazypp]

It's true if you up the nominal cell voltage the Wh per cell goes up, and hence the pack kWh as well, and Tesla may indeed be using a higher nominal voltage in their pack rating.

But isn't the nominal voltage in cells picked such that that if you multiply it by the mAh number you actually get the capacity (Wh) you can draw from the cell? You can't just arbitrarily raise the number unless the cell type actually supports those higher voltages (by actually operating in a higher voltage range).

To get a bump from 79.6 kWh to 85kWh from voltage alone, you need cells with a nominal voltage of 3.84V. Even the "high voltage" LG D1 cells have a nominal voltage of 3.75V (they have a higher charging voltage of 4.35V rather than the standard 4.2V).

 

[JRP3]

Right, that's why I said it's inaccurate if that's the way they are rating the pack, but remember actual operating voltage changes with load, so there is some fudge factor involved. If the 7104 cell count is correct then the 3.1ah cell doesn't really work, and if the 3.1ah cell is correct the 7104 cell count doesn't really work, or it's not really an 85kWh pack.

 

[Porter]

But isn't the nominal voltage in cells picked such that that if you multiply it by the mAh number you actually get the capacity (Wh) you can draw from the cell? You can't just arbitrarily raise the number unless the cell type actually supports those higher voltages (by actually operating in a higher voltage range).

Again remember we are talking about the nominal "pack" voltage of a very large pack and not the individual cell voltage. Nominal is not actual. There are lots of variables that go into what "actual" could be. Here is how one battery supplier describes it:

"The lithium battery output voltage varies during discharging. The nominal voltage is measured at the mid point between full charged and fully discharged based on a 0.2C discharge (where C is the rated capacity of the cell in mAh). A single lithium battery cell nominal voltage is usually shown as either 3.6V, 3.7V or 3.8V. The actual voltage profile during discharge depends on the cell chemistry being used, the discharge rate, the temperature and the age of the cells or battery being discharged. A single cell lithium battery cell maximum voltage is 4.2V, cut off voltage will be 2.5V ~ 3V."

A large pack that has a moderately low discharge rate and a robust thermal management system could have a higher overall nominal voltage than the individual cell. Still, I doubt its the 4.17 V that would be calculated from using 400 V - that sounds more like the max voltage. I'm have very high confidence in the 7,104 cells in 74P 96S configuration and the 3.1 Ah battery. By the fact that there is a label on the pack that says 400V and 85kWh, the rest of the specmanship and rounding is a bit speculative.

 

[stopcrazypp]

Right, that's why I said it's inaccurate if that's the way they are rating the pack, but remember actual operating voltage changes with load, so there is some fudge factor involved. If the 7104 cell count is correct then the 3.1ah cell doesn't really work, and if the 3.1ah cell is correct the 7104 cell count doesn't really work, or it's not really an 85kWh pack.

That's true, but the standard test is 0.2C and it doesn't get much better when you go lower and it gets worse at higher draws. That's where the 3.6V comes from the NCR18650A for example (frequently used for reference), just look at the discharge chart:

There's practically no way you can get 85kWh out of a 79.6kWh pack that used 3.6V*3.1Ah cells as a basis with a 7104 cell count.

- - - Updated - - -

Again remember we are talking about the nominal "pack" voltage of a very large pack and not the individual cell voltage.

Pack voltage in relation to cell voltage is simply a function of how many blocks are in series. We are pretty much 100% certain the series number is 96 and this is supported by the charging voltage calculations 4.19V*96 = 402V.

And yes, nominal is not actual, but it's a good approximation of the actual cell capacity when multiplied by the mAh rating of the cell. And as I pointed out, you need a raise of nominal (per cell) to 3.84V to support a 85kWh pack with the given 74P96S configuration (using 3100mAh cells). And high voltage cells in the industry top out at around 3.75V (highest I've seen are Samsung cells at 3.78V) and those are obtained with a 4.35V charging voltage (which we know the Tesla pack doesn't use).

By the fact that there is a label on the pack that says 400V and 85kWh, the rest of the specmanship and rounding is a bit speculative.

We have people who charged the 85kWh pack and the 402V refers to the peak voltage (it's what shows at the end of the cycle), not the nominal voltage. The label likely specifies the maximum voltage the pack might be at (I'm going to guess it's a warning label, not a specification label).

I'd like to point out this thread that gives some insight as to how the 60kWh pack at least is configured:
http://www.teslamotorsclub.com/show...ack-Pics/page2?p=449465&viewfull=1#post449465

 

[evmile]

For your next project can you figure out what is overheating when you drive the Model S at a race track?

 

[brianman]

For your next project can you figure out what is overheating when you drive the Model S at a race track?

+ 1

 

[JRP3]

For your next project can you figure out what is overheating when you drive the Model S at a race track?

I suspect it's the inverter for the motor, but since I believe the motor and inverter share coolant, if you just put a CO2 misting system on the heat exchangers you can probably solve the problem whatever the cause. At least till the CO2 bottle runs out.

 

[mnx]

Those who have tracked the roadster say it's the motor that hits temperature limits first. While the CO2 cooling may help with that, JB mentioned to me in an e-mail that:

There are a number of components in the powertrain
that all have similar power limits (as designed!) and these are based on
hardware limits. With EV's today there are substantial cost and
performance penalties to design them for extremely high continuous power
ratings. In particular there are big tradeoffs in the battery system

I don't know what that means exactly but if I had to guess even if you managed to avoid thermal limits, the Model S would reign in power if you draw too high a power level for longer periods of time.

I'm very interested in this subject since I'm thinking about participating in some Time Attack events next year. They are typically only 2-3 laps at a time, so if I could extend getting full power out of the Model S just a bit longer it would help greatly.

 

[JRP3]

I think the S has separate heat exchangers up front for the pack and the motor/inverter, and they can share fluid to warm the pack in the cold. Potentially you could put a chiller on one heat exchanger and see what happens, then move it to the other, or mount two of them to cover all bases. It would be good to put a thermometer on each one and see if power reduction kicks in at a specific temperature. It is possible that after a certain amount of time at a certain level of current draw Tesla would cut back power anyway regardless of temperature, which would lead me to think it's the pack that is the limiting factor, and that they are avoiding long high C rate draws on the cells.

 

[neroden]

Next question: how do the three variants differ on *regen*, when the motor's a generator and the battery's charging... they should all perform slightly differently, eh?

 

[stopcrazypp]

Don't have the time to look at it thoroughly (and also to dig up the Roadster thermal limits for reference), but this thread will probably provide the most insight as to the overheating limit:
http://www.teslamotorsclub.com/showthread.php/22172-Model-S-Overall-Thermal-Management

From the 162°C indicated for the motor though, it seems that might be the area (same with Roadster).

 

[chrisdl]

Thanks for all the feedback, people. It all makes sense. I'm not sure how to adjust the calculation to make it more accurate, but I guess that the conclusion wouldn't be any different. That said, I'll be happy to improve it at any time.

Regarding the thermal limitation: considering that the biggest losses at high power are in the battery due to internal resistance of the cells, that is most likely the bottle neck when racing on the track. When the battery temperature gets too high, the system cuts back the maximum current drawn from the battery to prevent the battery cells from overheating. Battery cells overheating could lead to thermal runaway which, in turn, could lead to fire.

If my theory is correct, it would mean that you can race an S85 longer without overheating than a P85.

For more details see point 8) in my OP.

If I feel up to it, I may do a more detailed calculation based on the efficiency of the inverter, the motor and the resistive battery losses.

 

[mnx]

If my theory is correct, it would mean that you can race an S85 longer without overheating than a P85.

That has been shown from anecdontal evidence on the forums. Some owners with S85's say that have gone ~ 4 laps of a track with no power limit, while P85's are good for 1 lap at the track. Of course not all tracks are the same so we have no direct comparisons but it makes sense.

However I disagree with your conclusion. The S85 taking longer to limit power doesn't prove that it is the battery at all.
If less current is going into the motor as is the case in the S85 vs. P85 it will get generate less heat, so it will take longer to reach critical temperatures as well.