Investor Engineering Discussions

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@MP3Mike
From main thread:

In the short term, yes.
In the long term: if you blow one cell fuse, the remaining cells in that group go through a larger SOC swing compared to all other groups each charge/discharge cycle along with experiencing higher currents. This means more degradation which eventually gets to the point that the BMS can't compensate (assuming it were able to initially).

With a 92s9p 4680 pack, losing one cell drops energy capacity 11%.

Good point that losing a single cell is far worse both in terms of immediate capacity loss and stress on the remaining cells in the parallel group when there are many fewer cells in parallel. When Tesla first announced they were going to the 4680 form factor I wondered whether it was such a good idea just from a manufacturing yield perspective. But it also has potential for some significant impacts to overall reliability of the pack. I always thought that one of the big Achilles' heel issues with most other OEM's batteries was their use of huge prismatic cells. Not just the asymmetries in the prismatic design, but also just the size of the individual cells.

 

[MP3Mike]

in the short term, yes.
In the long term: if you blow one cell fuse, the remaining cells in that group go through a larger SOC swing compared to all other groups each charge/discharge cycle along with experiencing higher currents. This means more degredation which eventually gets to the point that the BMS can't compensate (assuming it were able to initially).

Correct. And wk057 has said that in less than a year the module with the bad cell would get so far out of balance that the BMS would give up. But even more than that one long trip with a lot of Supercharging would cycle the module so many times that the BMS couldn't keep up in trying to balance the pack and cause it to give up. So in reality it could take as little as one or two days of driving with a disconnected cell before the pack fails.

Removing just one cell from a brick means every other brick's bleeders need to run for about a full 32 hrs per charge cycle to keep the pack in balance. (We'll ignore that this means wasting ~1-2% SoC just for balancing...) So every ~1 hr of use the car needs at least ~8 hours rest for balancing to stay in check. And that's just with one cell missing!

That is for the 18650 based packs. Obviously it would be much worse for the 2170, or 4680, based packs.

 

[MP3Mike]

All of the older packs had individually fused cells so that if one cell shorted ("went bad") the fuse would blow and the rest of the pack would function just fine.

No, the point of the fuses is to prevent the cell from overheating and catching the pack on fire. It isn't to allow the pack to continue to function normally after a cell fails.

With an 18650 pack one cell doesn't remove a lot of capacity, in a 2170 or 4680 based pack losing one cell would cause the lose of a lot of capacity and cause the pack to go out of balance and fail very quickly.

 

[mongo]

Correct. And wk057 has said that in less than a year the module with the bad cell would get so far out of balance that the BMS would give up. But even more than that one long trip with a lot of Supercharging would cycle the module so many times that the BMS couldn't keep up in trying to balance the pack and cause it to give up. So in reality it could take as little as one or two days of driving with a disconnected cell before the pack fails.


That is for the 18650 based packs. Obviously it would be much worse for the 2170, or 4680, based packs.

My viewpoint differs from wk057 in terms of the pack balancing load. Note: this it theoretical, not necessarily how Tesla does it.

There is no requirement for all parallel groups (bricks) to stay at the same SOC all the time. The only two critical points are the max and min voltage limits. If the BMS targets all bricks to reach max voltage/ 100% SOC simultaneously, that maximizes pack capacity.
If they all reach min voltage at the same time instead, that is less total total energy due to lower average voltage, but still functional. As long as the undersized brick hits the high and low limit first (meaning all its energy is available), the pack will approximate the reduced brick's capacity.
If the bricks are still fairly close in self discharge, minimal balancing is needed, even though the spread may be 100%-100% when full and 22%-11% when drained. It just requires smarter BMS logic (or dumber since it can only focus on highest voltage, rather than full range).

Packs ultimately fail when the leakage differential of any two bricks is more that the BMS balance capacity. In this situation, drift will increase until one brick is critically overdischarged.

Recovery may take a looong time though. Following a cell short with brick now at 80% capacity.
80%-20% -> inital
100%-45% -> charge stopped on nominal brick limit
45%SOC*80%capacity=36% pack effective energy
If differential leakage is acceptable, it will bleed the nominal bricks down:
100%-100% charged
20%-0% discharged

 

[MP3Mike]

My viewpoint differs from wk057 in terms of the pack balancing load. Note: this it theoretical, not necessarily how Tesla does it.

Ok, what he has been saying is about how the Tesla BMS actually works.

There is no requirement for all parallel groups (bricks) to stay at the same SOC all the time. The only two critical points are the max and min voltage limits. If the BMS targets all bricks to reach max voltage/ 100% SOC simultaneously, that maximizes pack capacity.
If they all reach min voltage at the same time instead, that is less total total energy due to lower average voltage, but still functional. As long as the undersized brick hits the high and low limit first (meaning all its energy is available), the pack will approximate the reduced brick's capacity.

It does work that way, the problem is that the brick with the missing cell goes through more charge "cycles" than the rest of the pack causing it to degrade faster. And that is worsened by the fact the it hits 0%, and 100%, before the rest of the pack, putting it in the territory of even faster degradation than normal cycling. Wk057 has reported that he has seen weak bricks get "burned out" by this in packs where people have attempted module replacements.

There is a reason that Tesla has set such tight balancing requirements on their packs...

 

[mongo]

Ok, what he has been saying is about how the Tesla BMS actually works.


It does work that way, the problem is that the brick with the missing cell goes through more charge "cycles" than the rest of the pack causing it to degrade faster. And that is worsened by the fact the it hits 0%, and 100%, before the rest of the pack, putting it in the territory of even faster degradation than normal cycling. Wk057 has reported that he has seen weak bricks get "burned out" by this in packs where people have attempted module replacements.

There is a reason that Tesla has set such tight balancing requirements on their packs...

Yah, agree with that. I'm just pointing out the BMS doesn't need to work more due directly to charging (but does due to induced asymmetric degradation). Tesla could make it more adaptable via software, which might let packs limp along longer. Useful for out of warranty owners.

 

[JRP3]

Nor do I know of any legacies using 2170.

Non EV products are using 2170's as well, e-bikes and power tools, but I don't know the volumes involved.

 

[wk057]

The main issue is that any imbalance greater than the capability of the BMS gets exponentially exaggerated over time in a bit of a feedback loop. That's generally what causes these "repaired" packs to fail pretty suddenly.

Edit: Single cell losses, and mismatched modules are both outside the ability of the Tesla Model S/3/X/Y BMS's.

 

[MC3OZ]

Correct. And wk057 has said that in less than a year the module with the bad cell would get so far out of balance that the BMS would give up. But even more than that one long trip with a lot of Supercharging would cycle the module so many times that the BMS couldn't keep up in trying to balance the pack and cause it to give up. So in reality it could take as little as one or two days of driving with a disconnected cell before the pack fails.


That is for the 18650 based packs. Obviously it would be much worse for the 2170, or 4680, based packs.

The surprising thing here is that those Gruber Motors roadster repairs were claimed to be operating well after a period of time.

The factors may be:-

• 18650 based, and the particular architecture.
• Low mileage when driving a repaired pack
• Owners cycle repaired packs very conservatively
• Gruber Motors claims are exaggerated

I would be very surprised if anyone is driving an original roadster as their primary means of transport, and flogging a repaired pack on a daily basis.

As you mentioned fusing is mostly to prevent thermal runaway, when one cell has had a thermal runaway event, replacing the whole pack seems sensible.

 

[MP3Mike]

New Munro video that shows the evolution of the Tesla Body in white:

 

[Cosmacelf]

Patreon subscribers can listen to The Limiting Factor's latest video dissecting the last quarterly conference call WRT all battery related answers. Nothing new (if you've listened to the CC), but you might pick up tidbits here and there. Of interest is that he notices that 4680 cell development is quite a bit behind schedule.

 

[Doggydogworld]

Of interest is that he notices that 4680 cell development is quite a bit behind schedule.

Who could have ever predicted such a thing?

Elon started backing away from DBE almost immediately, but he's been more optimistic lately. I think they've made some real progress the past few months, after a couple years of banging their heads against the wall with little to show for it. I was surprised they bought so much cell-making equipment for Austin and Berlin before working the bugs out on Kato Road. But most of that equipment would still be perfectly serviceable even if they scrapped DBE completely. So it's not really high risk.

The rumors that Tesla wouldn't share their 4680 work with Panasonic are interesting. Two ways to interpret that, haha.

Did Jordan give an update on the cell he bought from Sandy?

 

[Cosmacelf]

Did Jordan give an update on the cell he bought from Sandy?

Not that I heard. I doubt Sandy has even shipped them out yet?

 

[JRP3]

What about the cell he already had taken apart? Supposedly he did testing on it before the tear down but I haven't seen any results.

 

[mongo]

What about the cell he already had taken apart? Supposedly he did testing on it before the tear down but I haven't seen any results.

Jordan did not do any preteardown testing because the cell can was damaged. Based on teardown, it never went through formation step so testing wouldn't have worked anyway.
They did take punch outs from the the electrodes to make test coin cells for capacity/ power testing.

 

[unk45]

Here is the casting machine that Tesla allegedly has ordered:
Carat+610+die+casting+machine.jpg
There is reliable rumor that Bühler, after having declined the Tesla request of a few years ago has not only produced and delivered 610-series machines but is now working on larger ones.

 

[ZsoZso]

John Goodenough 100th Birthday Symposium​

Jeff Dahn had a talk on this symposium about Li-ion batteries lasting 100 years, starting at 2:50 on the video:

Some interesting points:

• He explains why such lifetime is crucial if we want to have enough stationary batteries to replace fossil fuels with wind/solar + daily storage batteries.
• NMC811 (which is what Tesla uses currently) can avoid major deterioration of voltage kept below 4.06V instead of full charge 4.2V for the specific cells they tested.
• He makes a claim NMC can actually have longer lifetime than LFP despite "conventional wisdom"
• Instead of focusing $/kWh initial cost, better to measure cost spread over lifetime, if you can make a a bit more expensive cell last a lot longer, it is better overall value
• they estimate lifetime by testing cycling at higher temperature (70C instead of 30C or 40C) to simulate higher degradation without needing to cycle so many times
• they even tested at 85C cycling over 2600 hours
• need more research to improve LFP lifetime, key seems to be to figure out how to stop Fe+ ions migrating to the anode and "polluting" it

 

[mongo]

It also sounds like you are under the mistaken impression that the A/C charge controller is not part of the BMS.

Depends what you mean by controller...
The BMS tells the charger the max voltage and current. At that level, the BMS is the controller. Wheras the brains of the AC/DC hardware, its controller, are not part of the BMS.
Level 2 has clear differentiation between targets (from BMS) and hardware (Supercharger).

 

[ggies07]

John Goodenough 100th Birthday Symposium​

Jeff Dahn had a talk on this symposium about Li-ion batteries lasting 100 years, starting at 2:50 on the video:

Some interesting points:

• He explains why such lifetime is crucial if we want to have enough stationary batteries to replace fossil fuels with wind/solar + daily storage batteries.
• NMC811 (which is what Tesla uses currently) can avoid major deterioration of voltage kept below 4.06V instead of full charge 4.2V for the specific cells they tested.
• He makes a claim NMC can actually have longer lifetime than LFP despite "conventional wisdom"
• Instead of focusing $/kWh initial cost, better to measure cost spread over lifetime, if you can make a a bit more expensive cell last a lot longer, it is better overall value
• they estimate lifetime by testing cycling at higher temperature (70C instead of 30C or 40C) to simulate higher degradation without needing to cycle so many times
• they even tested at 85C cycling over 2600 hours
• need more research to improve LFP lifetime, key seems to be to figure out how to stop Fe+ ions migrating to the anode and "polluting" it

What do you guys think about the fact that Tesla is trying to use DBE (which is supposed to be better and cutting edge) in their cells when Jeff's group just found a liquid electrode that even beats their newly designed NMC352 cell? Will Tesla take Jeff's work and move it into storage only, even though Elon said that will be using LFP? Trying to figure out why Jeff and his team work for Tesla if they are not going to use his new findings....

Or will DBE be used only for transportation to keep costs down?

 

[Cosmacelf]

What do you guys think about the fact that Tesla is trying to use DBE (which is supposed to be better and cutting edge) in their cells when Jeff's group just found a liquid electrode that even beats their newly designed NMC352 cell? Will Tesla take Jeff's work and move it into storage only, even though Elon said that will be using LFP? Trying to figure out why Jeff and his team work for Tesla if they are not going to use his new findings....

View attachment 835877

Edit. Sorry, my brain isn’t working today. What I wrote below isn’t quite on point. Read the rest of the answers.

Because DBE is all about cost reduction. Jeff’s group is working on longevity. At some point, when they have DBE working well, they can merge the two advances. Jeff would probably be working on a dry longevity process, but he can’t as long as DBE technology is still being developed itself.