Pics/Info: Inside the battery pack

[WarpedOne]

Is that if you integrate for power delivered ("area under the curve") while discharging a battery the area=total power delivered will vary depending on the slope/shape of the curve i.e. the use case (faster/slower discharge, average discharge rate, bursts etc.)?

Yes.
But industry standard is not to integrate but to cut shortcuts using 3,6V or 3,7V as a magic number.

If so, aren't Tesla just adhering to industry standards as they rate their big pack at 85kWh.

Nope.
85.000Wh / 7104 cells = 11,965 Wh/cell
wk measured those cells a bit under 3000mAh. To reach 11,9Wh at 3Ah, the nominal voltage would need to be 3,99V. According to the picture above also this is clearly not so.
One could probably stretch those cells a bit higher and lower but I just cannot see how can you stretch them to reach 11,965 / 3,6 = 3300 mAh or even 11,965/3,7 = 3230 mAh

 

[JRP3]

Worth considering is the steep initial capacity loss. A cell that tested 3.2ah at the factory at 100% might drop a percent or two after a few initial cycles.

 

[lolachampcar]

Slightly off topic but relevant to an earlier post and comments in other threads-
I am curious as to the source of all the angst for Tesla not providing an "accurate" spec. when something like battery capacity or motor horsepower is concerned. Are the nominal values of much if any importance? Is it not how the system performs in the wild that matters?

I really do not care if the battery is an 85 by one person's definition and a 75 by another nor do I care if the motor(s) are 500 hp or 800 hp. What matters to me is how far can I go in the car and how does the car perform. If I get 240 miles out of my PD on a trip leg and it gets to 60 mph in 3.3(ish) seconds then I believe Tesla has delivered. Any focus on numbers or specs to paint the company in one light or another seems simply to be politicking. Am I missing something here?

 

[WarpedOne]

A few percent yes but over 10? And then in the next few years only few percent more?
I find this hard to 'believe' without actually seeing the data that proves it.

My bet is still on intermittent loads that somehow get a bit more energy out of the cells.
There is a bit of "infant mortality" and I also bet a touch of marketing - tesla never said those battery is 85.000 Wh, they only said 85kWh.
It might be marketing that rounded those new 82,6kWh to the nearest multiple of 5.

I am curious as to the source of all the angst

Some people can drive it, others can only discuss it

 

[Johan]

Slightly off topic but relevant to an earlier post and comments in other threads-
I am curious as to the source of all the angst for Tesla not providing an "accurate" spec. when something like battery capacity or motor horsepower is concerned. Are the nominal values of much if any importance? Is it not how the system performs in the wild that matters?

I really do not care if the battery is an 85 by one person's definition and a 75 by another nor do I care if the motor(s) are 500 hp or 800 hp. What matters to me is how far can I go in the car and how does the car perform. If I get 240 miles out of my PD on a trip leg and it gets to 60 mph in 3.3(ish) seconds then I believe Tesla has delivered. Any focus on numbers or specs to paint the company in one light or another seems simply to be politicking. Am I missing something here?

I agree with this in general. Performance and range is all that really counts, in the end.

However, you will create a lot of confusion by letting the user see very detailed Wh/mile numbers in the car, yet when you do simple math the numbers don't add up. Remeber, it's Tesla who have designed the UI that show these detailed values.

Let's say I'm an averge Joe consumer. I got me an 85kWh car. I charged it to 100%. I'm driving at an average of 300 Wh/mile (the car tells me so). So I can go 85000/300=283,33 miles until I run out of charge, right? Or not? See the problem?

 

[JRP3]

My bet is still on intermittent loads that somehow get a bit more energy out of the cells.

That would be counter to all known capacity rating tests.

 

[lolachampcar]

Johan,
I see your point. I actually faced that conundrum when I first started driving MS but then realized the numbers did not add up and moved to believing the range I had experienced. From that point on, I simply used the WHr/mile number as a gauge of my performance as a driver and to spot environmental issues that could affect range.

 

[scaesare]

Slightly off topic but relevant to an earlier post and comments in other threads-
I am curious as to the source of all the angst for Tesla not providing an "accurate" spec. when something like battery capacity or motor horsepower is concerned. Are the nominal values of much if any importance? Is it not how the system performs in the wild that matters?

I really do not care if the battery is an 85 by one person's definition and a 75 by another nor do I care if the motor(s) are 500 hp or 800 hp. What matters to me is how far can I go in the car and how does the car perform. If I get 240 miles out of my PD on a trip leg and it gets to 60 mph in 3.3(ish) seconds then I believe Tesla has delivered. Any focus on numbers or specs to paint the company in one light or another seems simply to be politicking. Am I missing something here?

I always wanna know how stuff works. Doesn't matter if it's a clock, an operating system, or a car.

- - - Updated - - -

So, finished my first 500mA discharge curve for a single cell from a Model S module. Charged to 4.2V @ 1A constant-current, then constant-voltage until current dropped to 120mA, 5 minute delay, then discharge at 500mA until voltage read 2.85V. Test was done using FMA Powerlab 8 and the custom settings above. After charging the cell settled to a resting voltage of about 4.16V.

Was able to draw 2,963mAh. Using 10 second voltage averages this came out to 10.605 Wh. That'd be about 4.7kWh for a module, or about 75kWh for a full 85kWh pack using this method. I think there is room for improvement on the extreme ends, however.

My 120mA charge current during CV stage could probably be dropped to squeeze some more juice into the cell, and I could probably discharge it lower than 2.85V. There was ~100mV voltage sag at the begining of the discharge, also, and this seemed to increase quite a bit during the cycle as evident by a resting voltage of ~3.1V after removal of the 500mA load. I may try soldering heavier gauge wire to my 18650 cell holder later and retesting to see if that improves the voltage sag.

I believe ~50ma might be more like the CV cut point on the top end.

I wonder what the low-discharge threshold is that Tesla uses. I thought I've seen some of the diagnostic screen voltages that implied it wasn't much below 3V (which would be right given a Vnom. of 3.6v).

Okashira's test went down to something like 2.5v....

 

[apacheguy]

I wonder what the low-discharge threshold is that Tesla uses. I thought I've seen some of the diagnostic screen voltages that implied it wasn't much below 3V (which would be right given a Vnom. of 3.6v).

Cant we calculate that from SpC sessions where the owner connects with rock bottom SOC?

 

[Johan]

You've already answered that from your test results and from the battery pack label picture in your first post:

"So, finished my first 500mA discharge curve for a single cell from a Model S module. Charged to 4.2V @ 1A constant-current, then constant-voltage until current dropped to 120mA, 5 minute delay, then discharge at 500mA until voltage read 2.85V. Test was done using FMA Powerlab 8 and the custom settings above. After charging the cell settled to a resting voltage of about 4.16V.

Was able to draw 2,963mAh."

And from the picture, 85kWh, 400VDC => 212.5 A-hr /74 parallel cells = 2.87 A-hr per cell, which is basically what you measured and verified. You probably took the cells lower (2.85) than what Tesla uses (3.1?) so you measured slightly more capacity.

Seems that i saw a video posted on tmc of supercharging from empty and it didn't really ramp up the current until the pack voltage was around 320 (3.3 vpc), before then it was cycling on-off at low current and gradually bringing the voltage up.

Excellent testing and results on your part, thanks for sharing.

I'm I just plain stupid?

2.87 Ah x 3.6 V nominal x 7104 cells = 73.4 kWh (kVAh)

 

[scaesare]

I'm I just plain stupid?

2.87 Ah x 3.6 V nominal x 7104 cells = 73.4 kWh (kVAh)

I think there's some likelihood that although the cells may be rated to discharge down to something lower, and thus "really" have ~85KW of total capacity, Tesla conservatively operates in a shallower voltage envelope to provide greater battery cycle life, protection from bricking, etc...

- - - Updated - - -

Cant we calculate that from SpC sessions where the owner connects with rock bottom SOC?

Certainly should be able too... now to find some of those pics/videos...

​(And I'm now preparing to be annoyed that the forum will coalesce my posts)

 

[billarnett]

So, finished my first 500mA discharge curve for a single cell from a Model S module. Charged to 4.2V @ 1A constant-current, then constant-voltage until current dropped to 120mA, 5 minute delay, then discharge at 500mA until voltage read 2.85V. Test was done using FMA Powerlab 8 and the custom settings above. After charging the cell settled to a resting voltage of about 4.16V...

View attachment 77512

Did the cell get hot during the discharge? Perhaps Tesla's careful thermal management makes a difference to the total measured capacity?

 

[wk057]

Did the cell get hot during the discharge? Perhaps Tesla's careful thermal management makes a difference to the total measured capacity?

No noticeable change according to my FLIR

 

[austinEV]

This reminds me of harddrive "megabytes" that are 1,000,000 bytes instead of 1,048,576.

 

[Johan]

This reminds me of harddrive "megabytes" that are 1,000,000 bytes instead of 1,048,576.

This is a good analogy for how this issue can be confusing to the consumers as I described above, and could tarnish Tesla's rep. in the future when people start realizing the math doesn't add up.

(I think this is an interesting discussion but that it is derailing wk's great thread, so I hope an administrator would "carve out" this discussion to a separate thread).

 

[AnxietyRanger]

Just wanted to extend my thanks again to wk057 and others paying detailed attention to this stuff. Magnificent contribution(s) and much appreciated.

 

[lolachampcar]

++++

 

[redi]

Just wanted to extend my thanks again to wk057 and others paying detailed attention to this stuff. Magnificent contribution(s) and much appreciated.

Yep, they are pioneering and passing the info for all of our benefit - info that is unavailable elsewhere.

Thanks.

 

[Filb]

So, finished my first 500mA discharge curve for a single cell from a Model S module. Charged to 4.2V @ 1A constant-current, then constant-voltage until current dropped to 120mA, 5 minute delay, then discharge at 500mA until voltage read 2.85V. Test was done using FMA Powerlab 8 and the custom settings above. After charging the cell settled to a resting voltage of about 4.16V.

Was able to draw 2,963mAh. Using 10 second voltage averages this came out to 10.605 Wh. That'd be about 4.7kWh for a module, or about 75kWh for a full 85kWh pack using this method. I think there is room for improvement on the extreme ends, however.

My 120mA charge current during CV stage could probably be dropped to squeeze some more juice into the cell, and I could probably discharge it lower than 2.85V. There was ~100mV voltage sag at the begining of the discharge, also, and this seemed to increase quite a bit during the cycle as evident by a resting voltage of ~3.1V after removal of the 500mA load. I may try soldering heavier gauge wire to my 18650 cell holder later and retesting to see if that improves the voltage sag.

View attachment 77512

A possible (although unlikely) explanation for the slightly lower capacity that you found was that maybe Tesla measured full pack capacity from the pack itself, or from one of the 74 in parallel blocks to calculate total pack capacity.

When batteries are wired in parallel, their internal capacity decreases (similar to the way resistors in parallel have a lower measured resistance). Since some of the energy in a battery is lost to heat because of internal resistance when being discharged, the significant drop in internal resistance from being paralleled may make up the difference in capacity that you measured.

Even though the 74 in parallel are then wired in series which increases internal resistance, the decrease in internal resistance from being paralleled is greater than the increase from being put in series.

This will make a difference to the measured capacity of the pack, especially at higher discharge rates.

However, from what I have read, I think that Tesla likely discharged the cell to a lower voltage - possibly 2.5 volts for capacity measurements and a better connection to the battery might give you a higher capacity measurement.

This thread has been an incredible source of information - thank you for sharing!

 

[LargeHamCollider]

Just wanted to extend my thanks again to wk057 and others paying detailed attention to this stuff. Magnificent contribution(s) and much appreciated.

+1,
I'm interested to know the exact mass of one of the 16 battery modules from an 85, then we'd have independent measurements of capacity and mass and could get a very solid energy density number that isn't affected by the weight of the heavy shield on the bottom of the pack.