Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Service says $22k for new battery on 2012 Model S

TwistedGray

Ludicrous > Ludacris
Mar 12, 2021
295
258
Monterey Bay, CA
All correct.

My main point is that what Gruber claims is a repair isn't actually a repair. It's nonsense. If theres a problem at the module level related to cells or cell fuses, that module is trash and by extension the rest of the pack is useless as a pack.

You can't just remove individual cells from the mix or otherwise replace modules or sub-components of a module and except the car to live happily ever after. The cell fuses are a safety mechanism designed to protect the module and pack from a cell failure causing a catastrophic failure (read: fire). They successfully do this. HOWEVER, once this safety is tripped and a fuse blown (or in the case of ignorant "repairs", cut) the module is shot. It may function for a bit before throwing a "Contact Tesla Service" error and even a bit after that before shutting down the vehicle with a hidden SoC imbalance error... but it will inevitably fail because it was never designed to function after such a failure happens.

Only Tesla _may_ have enough stock of recovered modules to possibly match one for use in a pack that needs a replacement. I have data on thousands of individual modules I've tested to-date, and none of them match closely enough that the BMS would be happy indefinitely if I mixed one into another group. Some are close enough that they'd last months, but none close enough to run error free forever.

So, what are your thoughts on my comments. Curious
 

cduzz

Member
Jun 6, 2019
419
519
boston ma
Doesn't "repairing" a pack prevent the owner from being able to use that pack as a core in the future if/when they want a brand new pack from/at Tesla? There's speculation...whether it's true or not, I don't know.

I imagine to tesla a pack is a pack, so long as it is unopened. Maybe they'd comment if you show up with a car with a different pack than what it was born with, but that implies they've got immaculate record keeping.

Were I tesla, I'd have the logic of: if you gruber a pack, you've broken the warranty sticker and the pack is probably not a viable core. They'll do the swap, charge you extra for not having a viable core, and keep the grubered pack.

If you've pulled a pack from a 2017 90D and put it into a 2014 75, it is weird and surprising but a pack is a pack, so long as it passes inspection, but there is certainly some risk in them saying "this is not the correct pack for this car, we don't know what's going on with the replacement pack." and they may charge you extra depending on if the person you're working with at the SC is having a good or bad morning.

The Grubering makes much more sense on roadsters where there isn't nearly the same supply of replacement packs. But hey, everyone's gotta make a living.

As far as having 20k removed from your posterior by tesla for the service... It is a huge amount of money. But, in going to tesla you get a warranty, you get a loaner while the car is having the battery swapped, you have the (potential, depending on your location) luxury of being able to go to the service center down the street rather than several states away.

I'm hoping that they're going to come up with both lower cost battery packs (4860 / LFE / ???) as well as provide support for 3rd party repair shops. They'll need to as they get larger. I also hope for world peace and dogs that speak english without a translator box that makes their cute little necks itch.
 

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
Personally, I think spending 20k+ to fix what's effectively the ICE equivalent of a major mechanical problem on a Tesla is probably a pretty terrible idea. Easily half the value of the car or more. $5k or so, sure. $20k? Not so much.

Most likely this is an older car that probably wouldn't get $40k on the market anyway. Just sell the car as-is (I'll buy such cars any day of the week at a reasonable rate, just shoot me a note), take the sale payout and/or the $20k you were going to spend on a battery, and just put that down on a new Model S. You'll likely be much happier with the result.

Not that I'm advocating Tesla's price gouging, but the latest batteries really just don't have the problems that are causing issues in the older out of warranty packs.

Aside from that, if you're still looking to keep and fix your car, shoot me a note and we'll see what we can work out that makes sense for you.

---

As for Tesla, there's only ever an issue with upgraded vehicles. If you take a car in that was "born" as a 75 that we've upgraded to a 90, and want Tesla to replace the pack, they're only going to sell you a 75 because that's the part linked to the VIN. If it's an out of warranty purchase, they couldn't care less what the issue is with the pack. The service centers just ship these off. There's only so much they can inspect on site. Granted, if you bring your car in with a battery pack case full of concrete and expect them to replace the pack or something... you're probably going to run into some issues.

To-date I've had no issues with customers I've done upgrades or pack replacements for working with Tesla service. At worst, Tesla makes the customer sign a waiver about third party modifications before they'll service the car. In most cases..... they don't even notice. The people at the service center level just aren't paid enough to care quite that much.
 

TwistedGray

Ludicrous > Ludacris
Mar 12, 2021
295
258
Monterey Bay, CA
Personally, I think spending 20k+ to fix what's effectively the ICE equivalent of a major mechanical problem on a Tesla is probably a pretty terrible idea. Easily half the value of the car or more. $5k or so, sure. $20k? Not so much.

Most likely this is an older car that probably wouldn't get $40k on the market anyway. Just sell the car as-is (I'll buy such cars any day of the week at a reasonable rate, just shoot me a note), take the sale payout and/or the $20k you were going to spend on a battery, and just put that down on a new Model S. You'll likely be much happier with the result.

Sold my 2012 Sig Series P85 with a new 90 battery installed Nov 2020 (it had 36k remaining on its 4yr/50k warranty) less than a week ago. The car had 177,000 miles on it, and was otherwise on older equipment (MCU1, suspension components (some replaced), no AP/etc). I knew that the drive unit was replaced, but I didn't have a record of it. I was able to sell it quickly for $35,000. The only known issue with the car was it needing TPMS for all four wheels.
 

dark cloud

Active Member
Apr 14, 2018
2,023
2,281
BC
Aside from that, if you're still looking to keep and fix your car, shoot me a note and we'll see what we can work out that makes sense for you.
What about those with MCU2? If you are not touching these yet I am curious, are you able to revert them back to MCU1, then you do the battery swap/replacement, then the customer pays again for MCU2 again? Is that an option?
 

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
What about those with MCU2? If you are not touching these yet I am curious, are you able to revert them back to MCU1, then you do the battery swap/replacement, then the customer pays again for MCU2 again? Is that an option?
Hm. Yeah I suppose.

Working on MCU2. It's doable, but it's way more annoying at the moment and I don't have a 100% success rate with it, so not ready to offer to all customers.

Sold my 2012 Sig Series P85 with a new 90 battery installed Nov 2020 (it had 36k remaining on its 4yr/50k warranty) less than a week ago. The car had 177,000 miles on it, and was otherwise on older equipment (MCU1, suspension components (some replaced), no AP/etc). I knew that the drive unit was replaced, but I didn't have a record of it. I was able to sell it quickly for $35,000. The only known issue with the car was it needing TPMS for all four wheels.

Yeah, pre AP2 vehicles are almost all worth about $40k +/- $10k at best. They bottom out at about $22k as long as they run and drive, regardless of any other issues.
 

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
Hope you get there … I dream of a 100 battery in my MCU2 85D one day :D
The 100 packs are still ridiculously expensive on the salvage market. A lot of them don't hit the salvage yards due to some tactics by Tesla that I'm not going to get into here. Suffice it to say it artificially increases the street value of the 100 packs.
 

TheFro

Member
Feb 23, 2017
92
41
Pearland, TX
I added a link to an upgrade path compatibility spreadsheet to my previous post. Here's that link again: Tesla Model S/X Battery Upgrade Compatibility



I've run a Model S off of a Chevy Volt battery pack in the trunk. The form factory of the battery pack doesn't really matter as long as it can provide the power required. If the pack can do it, and it can be fit into the car somewhere/somehow... then there's no technical reason why it wouldn't work. Just need proper software/control hardware.

Do I think Tesla will offer an upgrade path? No.
This makes me slightly hopeful that someone may come up with an aftermarket replacement battery one day but in all honesty, this will probably not happen for these old S and X vehicles. It will be more worthwhile to develop aftermarket solutions for the Model 3/Y due to the sheer volume of vehicles being produced.

Until then I will save my money, wait until my battery warranty runs out on my P85 in 6 months, then contact you to get on a waiting list for a replacement 90kWh battery. If you're still doing this in 4 years and my wife's X 75D holds up, maybe do the same with her car and a 100kWh battery. 🤔
 

aerodyne

Active Member
Nov 19, 2018
2,748
2,808
Los Angeles
Correct. Except in very rare cases (contactor failure, fuse failure, obvious single BMB failure) I never repair packs. The customer gets a pack that's 100% in working order, fully tested and inspected. We get their pack that has some kind of issue and resell the working bits to recover costs for the customer. The bits with issues are usually sold to a few vendors we work with that break them down even further than a module to recover individual cells.

There is no repairing a bad module that has a bad cell, cell group, cell fuse, etc. It's just not possible, despite claims to the contrary on YouTube. If a module has a problem that isn't related to the electronic hardware (BMS, BMB, sense wiring, etc) then it's just no good and shouldn't be used in a vehicle. It should be recycled, and the customer needs a full new pack since replacing modules with random ones (without a massive tested pool of thousands like Tesla would have) is not possible as this would throw the pack out of balance beyond the ability of the BMS to handle, resulting in an unusable vehicle within a few cycles.

If a company has the money and resources to make these decently edited and produced videos trying to sell you on some "simple" "repair" to a battery... they're probably full of crap and just milking customers that don't know any better with that flashy marketing. Same seems to be the case with the popular EV conversion shops... they all seem to completely suck, and we get tons of customers coming to us with horror stories about their experiences. Great marketing teams, horrible companies.
So some simple math and assumptions here...

The second string in module two in my '15 85D is consistantly low about 20mv at rest.

Let's say a bad cell is involved.

If the string is under a light load, and the voltage of said string is say, 3.5v on average, then the bad cell craps out, not open circuit, but a low voltage like say 1V, then my 74 cell string is now at 3.47V.

That 30 mv plus other factors could cause the pack to go out of balance.

It seems that Grubering the bad cell would be even worse as far as imbalance goes...
 

C7R35

Member
May 14, 2021
11
2
Salt Lake City
Sold my 2012 Sig Series P85 with a new 90 battery installed Nov 2020 (it had 36k remaining on its 4yr/50k warranty) less than a week ago. The car had 177,000 miles on it, and was otherwise on older equipment (MCU1, suspension components (some replaced), no AP/etc). I knew that the drive unit was replaced, but I didn't have a record of it. I was able to sell it quickly for $35,000. The only known issue with the car was it needing TPMS for all four wheels.
Jeez! I'm beginning to think I got a good deal on my 2015 P90DL with 100k miles. Paid $36k and has every option (near $150k sticker).
 

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
So some simple math and assumptions here...

The second string in module two in my '15 85D is consistantly low about 20mv at rest.

Let's say a bad cell is involved.

If the string is under a light load, and the voltage of said string is say, 3.5v on average, then the bad cell craps out, not open circuit, but a low voltage like say 1V, then my 74 cell string is now at 3.47V.

That 30 mv plus other factors could cause the pack to go out of balance.

It seems that Grubering the bad cell would be even worse as far as imbalance goes...

The TL;DR for the quote above: Due to electrical rules, the weak cell that supposedly went to 1V would instead actually only drop to the voltage of the other cells in the brick, but likely would be providing close to no current. Then when the load is removed, to recover it would snag energy from the rest of the parallel brick and remain at the brick voltage as expected.

Bit of complexity here that's not immediately obvious.

Basic rules:
  • Cells in parallel always have an equal voltage. Current can vary.
  • Cells (or cell groups) in series have an equal current. Voltage can vary.
So first, every cell in the brick of 74 cells is at the same voltage. That's what being in parallel does. It doesn't matter which ones are weak or strong or whatever, they are forced to the same voltage. Your above example doesn't work because a single cell in the group can't be at 1V because its + and - are in parallel with other cells. Might seam counterintuitive, but this is how it is. The voltage at the cell sense for a brick of cells is the voltage of every cell in that brick.

That said, let's say there is a weak cell. Under load, because its in parallel with the rest, it will reach a lower voltage while outputting less current than the other cells at the same voltage. So if we load the cell group at X amps, and this causes the brick to drop from Y volts to Y - 1 volts, then we're getting X watts from that brick of 74 cells in parallel. Within that group, however, the distribution of current does not have to be constant like the voltage does. One of the cells may only be putting out 1/148th of an amp (because its weak and has half capacity of the rest of the cells) and that means the rest as a whole are putting out 147/148th of an amp, or slightly more than 1/74th of an amp per cell. The end result is the same, though, that the cell group drains each cell's SoC equally regardless of which cells are weak or strong. In parallel this doesn't generally matter as long as the connection making them parallel is low resistance (in this case, it's a solid aluminum bus plate) and the cells are of the same general type (same voltage curve).

This is generalized, but should get the idea across.

Here's some potential issues:

A problem comes in when a cell is so disproportionately matched with the group such that at some point under load or charge it attempts to output or draw more than the rating of the cell level fuse (~25A instant pop, ~20A for 15s, etc). This is where we get into the weeds a bit, but suffice it to say a weak cell will lose its "surface charge" more quickly than a strong cell under load. So let's say under a load, a weak cell quickly drops its voltage to line up with the group... then the load is released. The stronger cells will more quickly recover to a higher resting voltage because they may or may not have released their surface charge, while the weak cell may have. In this case, the weak cell wouldn't recover as quickly and charge would be shuffled through the bus plate to equalize it with the rest of the group. This inrush of current can be very high if the cell is very weak, enough to pop a cell level fuse in extreme cases.

A far less common, but more severe issue is that a cell becomes parasitic in some way. In this case, a cell is self discharging at some rate due to some defect. As it discharges, its voltage attempts to drop. But since its in parallel with other cells, it can't drop its voltage alone, so the other cells attempt to balance out with it. This effectively makes this one parasitic cell parasitic to the entire brick of 74 cells, and the voltage of the entire brick slowly drops as a result (at 1/74th the rate that it would had we been measuring just a single parasitic cell in isolation outside a brick). This becomes an issue when the parasitic load can handle high currents that are possible from many cells in parallel like this. When it gets to the point where it weakens the cell level fuse, it pops and this cell is pulled out of the brick as a result and then just self discharges by itself down to 0V, rendering it pretty inert.

Suffice it to say, there is actually a great bit of variation among the cells that are in parallel. In my testing of cells taking from completely stripped modules, they can vary in double digit percentages on capacity and still function perfectly fine as a brick. It's mostly these differences that tend to cause an imbalance under normal use and why the BMS has to balance at all. If all the cells were perfect, you wouldn't need a BMS to do balancing. Instead, what happens is during normal use some amount of energy is lost as weaker cells snag energy from stronger cells during load changes. This process is not 100% efficient, so this tiny amount of energy loss adds up to an imbalance over time. Bricks that are very well matched actually end up being out of balance by positive deltas, and bricks that are very mismatched tend to be the ones out of balance by negative deltas. With 74 cells in parallel, most bricks are pretty on par with the average capacities for its neighboring bricks in the same pack and tend towards a negative delta that is pretty even and thus not really noticeable. Then the most evenly matched groups that lose the least amount of energy end up a higher voltages over time and need to be actively balanced. There's also the scenario where a group is significantly weaker and ends up at a higher voltage due to reaching a charge peak faster, but that's another story. I'm mainly referring to the average pack here.

With all of the above as a bit of base knowledge, if you manually cut the fuse on a parasitic cell, you've stopped the drain, but now you've assured that that brick is 1/74th out of balance with the rest of the pack forever. This will manifest as being weaker under loads and normal use, and will reach charge voltages sooner than the rest of the series cell groups in the pack every single time. The BMS will only tolerate temporary gross imbalances like this. If they persist it throws a series of errors, culminating in complete shutdown for pack safety. How quickly this happens depends on actual usage (with more parked/idle time helping the BMS correct the gross imbalance each time), but the BMS can not keep up with a completely missing cell from a group for the long haul. It was never designed to. It was, however, designed to detect this issue, report it, and eventually shut things down for safety. If you use the car for a 30 mile commute once a week or something, it might last a while with a missing cell.... but daily normal use for a bit, or even one relatively long supercharger trip where the BMS has no time to tackle the imbalance sufficiently for the next charge and your "grubered" pack is now a rather large and expensive paperweight.

In theory if you could find and cut the fuse on the weakest cell in every single one of the 84 to 96 cell bricks, this "fix" would work indefinitely.... but the aforementioned common voltage in parallel makes this impossible (you can not test the individual cells to find the weak and strong), so, don't try it because you'll only make things worse (you'll remove the strongest cell in some, weakest in others, and randoms in the rest and cause even more imbalance).

If you follow through in more detail with the application of our basic rules above, some of this may seem self-correcting (higher voltage bricks will provide more power under load because of I*V coming out higher due to constant current in series, and vice verse for lower voltage bricks). However, that only works with "ideal" batteries. (Think spherical cows in a vacuum.) Since our real world batteries have varying internal resistance, vary amounts of surface charge as that resistance varies, internal resistance that varies with temperature variations between cells, etc... the self-correcting aspects of cell groups in series are almost completely negated............. that's why we have a BMS.

This is not an extensive an in depth explanation of all of this, but should make some of this make sense.

Anyway... hope this is informative. If you like this video I hope you'll like, share, and subscribe..... er, wait. :p
 
Last edited:

aerodyne

Active Member
Nov 19, 2018
2,748
2,808
Los Angeles
The TL;DR for the quote above: Due to electrical rules, the weak cell that supposedly went to 1V would instead actually only drop to the voltage of the other cells in the brick, but likely would be providing close to no current. Then when the load is removed, to recover it would snag energy from the rest of the parallel brick and remain at the brick voltage as expected.

Bit of complexity here that's not immediately obvious.

Basic rules:
  • Cells in parallel always have an equal voltage. Current can vary.
  • Cells (or cell groups) in series have an equal current. Voltage can vary.
So first, every cell in the brick of 74 cells is at the same voltage. That's what being in parallel does. It doesn't matter which ones are weak or strong or whatever, they are forced to the same voltage. Your above example doesn't work because a single cell in the group can't be at 1V because its + and - are in parallel with other cells. Might seam counterintuitive, but this is how it is. The voltage at the cell sense for a brick of cells is the voltage of every cell in that brick.

That said, let's say there is a weak cell. Under load, because its in parallel with the rest, it will reach a lower voltage while outputting less current than the other cells at the same voltage. So if we load the cell group at X amps, and this causes the brick to drop from Y volts to Y - 1 volts, then we're getting X watts from that brick of 74 cells in parallel. Within that group, however, the distribution of current does not have to be constant like the voltage does. One of the cells may only be putting out 1/148th of an amp (because its weak and has half capacity of the rest of the cells) and that means the rest as a whole are putting out 147/148th of an amp, or slightly more than 1/74th of an amp per cell. The end result is the same, though, that the cell group drains each cell's SoC equally regardless of which cells are weak or strong. In parallel this doesn't generally matter as long as the connection making them parallel is low resistance (in this case, it's a solid aluminum bus plate) and the cells are of the same general type (same voltage curve).

This is generalized, but should get the idea across.

Here's some potential issues:

A problem comes in when a cell is so disproportionately matched with the group such that at some point under load or charge it attempts to output or draw more than the rating of the cell level fuse (~25A instant pop, ~20A for 15s, etc). This is where we get into the weeds a bit, but suffice it to say a weak cell will lose its "surface charge" more quickly than a strong cell under load. So let's say under a load, a weak cell quickly drops its voltage to line up with the group... then the load is released. The stronger cells will more quickly recover to a higher resting voltage because they may or may not have released their surface charge, while the weak cell may have. In this case, the weak cell wouldn't recover as quickly and charge would be shuffled through the bus plate to equalize it with the rest of the group. This inrush of current can be very high if the cell is very weak, enough to pop a cell level fuse in extreme cases.

A far less common, but more severe issue is that a cell becomes parasitic in some way. In this case, a cell is self discharging at some rate due to some defect. As it discharges, its voltage attempts to drop. But since its in parallel with other cells, it can't drop its voltage alone, so the other cells attempt to balance out with it. This effectively makes this one parasitic cell parasitic to the entire brick of 74 cells, and the voltage of the entire brick slowly drops as a result (at 1/74th the rate that it would had we been measuring just a single parasitic cell in isolation outside a brick). This becomes an issue when the parasitic load can handle high currents that are possible from many cells in parallel like this. When it gets to the point where it weakens the cell level fuse, it pops and this cell is pulled out of the brick as a result and then just self discharges by itself down to 0V, rendering it pretty inert.

Suffice it to say, there is actually a great bit of variation among the cells that are in parallel. In my testing of cells taking from completely stripped modules, they can vary in double digit percentages on capacity and still function perfectly fine as a brick. It's mostly these differences that tend to cause an imbalance under normal use and why the BMS has to balance at all. If all the cells were perfect, you wouldn't need a BMS to do balancing. Instead, what happens is during normal use some amount of energy is lost as weaker cells snag energy from stronger cells during load changes. This process is not 100% efficient, so this tiny amount of energy loss adds up to an imbalance over time. Bricks that are very well matched actually end up being out of balance by positive deltas, and bricks that are very mismatched tend to be the ones out of balance by negative deltas. With 74 cells in parallel, most bricks are pretty on par with the average capacities for its neighboring bricks in the same pack and tend towards a negative delta that is pretty even and thus not really noticeable. Then the most evenly matched groups that lose the least amount of energy end up a higher voltages over time and need to be actively balanced. There's also the scenario where a group is significantly weaker and ends up at a higher voltage due to reaching a charge peak faster, but that's another story. I'm mainly referring to the average pack here.

With all of the above as a bit of base knowledge, if you manually cut the fuse on a parasitic cell, you've stopped the drain, but now you've assured that that brick is 1/74th out of balance with the rest of the pack forever. This will manifest as being weaker under loads and normal use, and will reach charge voltages sooner than the rest of the series cell groups in the pack every single time. The BMS will only tolerate temporary gross imbalances like this. If they persist it throws a series of errors, culminating in complete shutdown for pack safety. How quickly this happens depends on actual usage (with more parked/idle time helping the BMS correct the gross imbalance each time), but the BMS can not keep up with a completely missing cell from a group for the long haul. It was never designed to. It was, however, designed to detect this issue, report it, and eventually shut things down for safety. If you use the car for a 30 mile commute once a week or something, it might last a while with a missing cell.... but daily normal use for a bit, or even one relatively long supercharger trip where the BMS has no time to tackle the imbalance sufficiently for the next charge and your "grubered" pack is now a rather large and expensive paperweight.

In theory if you could find and cut the fuse on the weakest cell in every single one of the 84 to 96 cell bricks, this "fix" would work indefinitely.... but the aforementioned common voltage in parallel makes this impossible (you can not test the individual cells to find the weak and strong), so, don't try it because you'll only make things worse (you'll remove the strongest cell in some, weakest in others, and randoms in the rest and cause even more imbalance).

If you follow through in more detail with the application of our basic rules above, some of this may seem self-correcting (higher voltage bricks will provide more power under load because of I*V coming out higher due to constant current in series, and vice verse for lower voltage bricks). However, that only works with "ideal" batteries. (Think spherical cows in a vacuum.) Since our real world batteries have varying internal resistance, vary amounts of surface charge as that resistance varies, internal resistance that varies with temperature variations between cells, etc... the self-correcting aspects of cell groups in series are almost completely negated............. that's why we have a BMS.

This is not an extensive an in depth explanation of all of this, but should make some of this make sense.

Anyway... hope this is informative. If you like this video I hope you'll like, share, and subscribe..... er, wait. :p
Very helpful, thanks. My example was overly simplistic. I agree that cells in parallel connected to a bus plate would show the same voltage. I was trying to say if the cells were separated, and one had little or no voltage, when connected to the rest of the string current would flow into the bad cell which would drop the voltage of the string slightly.

A parasitic cell as you describe could be a bigger problem, and could be stopped by a cell fuse blowing or Grubering. However, I can visualize that the BMS would not tolerate such an imbalance for long, so might make the errors go away only temporarily.

I gather from your write up that avoiding high power draws or SuC rates might reduce the chance of a premature shutdown. I am always in chill mode, but do see imbalances as high as 300 mv under load.

Conversely, if I see bad things happening on the CAN bus, I could try to cause a failure by doing the opposite while under warranty. Risky approach, though.

Interesting that after a drive, esp if I have SuCed, I see less range and greater imbalance. Then after a day or so of sitting, get a few miles RR back and the imbalance goes to low double digits. Must be the BMS doing its thing.
 

Veggen

Member
Dec 22, 2011
136
109
Snip...
With all of the above as a bit of base knowledge, if you manually cut the fuse on a parasitic cell, you've stopped the drain, but now you've assured that that brick is 1/74th out of balance with the rest of the pack forever. This will manifest as being weaker under loads and normal use, and will reach charge voltages sooner than the rest of the series cell groups in the pack every single time. The BMS will only tolerate temporary gross imbalances like this. If they persist it throws a series of errors, culminating in complete shutdown for pack safety. How quickly this happens depends on actual usage (with more parked/idle time helping the BMS correct the gross imbalance each time), but the BMS can not keep up with a completely missing cell from a group for the long haul. It was never designed to. It was, however, designed to detect this issue, report it, and eventually shut things down for safety.
Thanks for the insight.
As an EE, I must admit I find it a bit disappointing that the BMS is not capable of handling even a single blown cell fuse.
You would think that would be a reasonable design requirement.
Well, I guess they had other priorities.
 
  • Like
Reactions: Jason Bloomberg

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
Interesting that after a drive, esp if I have SuCed, I see less range and greater imbalance. Then after a day or so of sitting, get a few miles RR back and the imbalance goes to low double digits. Must be the BMS doing its thing.
The BMS's balanacing ability, and by extension its ability to correct for a missing cell, is limited to a 100mA discharge rate on a full cell group. It can not charge a cell group, it can only discharge groups of its choosing at 100mA.

Allowing the car to sit idle gives the BMS time to correct imbalances, which it can only do very slowly.

In the case of a missing cell, let's assume the rest of the pack is 100% perfect and no other imbalance that requires the BMS's attention ever happens (highly unlikely, but let's go with it). One cell in an 85 pack is about 3.2 Ah. The 10,000 ft view of this is that for the BMS to compensate for this missing cell, it has to discharge every other cell group for a total of about 32 hours to bring them in line with the now-weak cell group. Utilizing the pack while an imbalance is present can exaggerate the imbalance... I won't get into the weeds here, but using and/or charging the car effectively resets the clock.

This is overly simplified, but a pack with a lost cell can only function indefinitely if the car sits idle for about 32 hours after every discharge/charge cycle (generally more, since normal imbalances still require time to correct). Every time it's used before the BMS is able to complete its work, you push this back and the imbalance grows. As soon as it outpaces the BMS's ability to compensate beyond a safety threshold, the BMS stops you from using the pack.

That said, this is almost never an issue with normal use of a normal unmodified pack. Gross imbalances during use are generally much faster to correct than entire missing cells (usually a couple hours vs over a day). The BMS keeps track of the health of every cell group, and it calculates what the anticipated imbalance will be during normal use. When that imbalance becomes uncorrectable in reasonable time frames is when you have a problem.

So yes, after a multi-charge supercharger trip, you're going to see decent imbalances on even the best packs... but they'll be corrected relatively quickly because this is normal and expected use within the specifications of the BMS design. A lost cell is not. Since the reasoning behind a lost cell can be something that is or can lead to a safety issue, the best course of action is to prevent use until the pack is replaced, which is why the BMS will lock it down eventually.
 

wk057

Senior Tinkerer
Feb 23, 2014
5,692
11,812
Hickory, NC, USA
@wk057 I really enjoy your posts and to me you come across as someone who likes to share his knowledge. Why don't you transform your articles into a wiki? That we don't have to follow you around all the time on different topics ;) In any case: Thanks again for your efforts, your articles are worth the chase.
Perhaps. I've been debating on what exactly to do with my public info sharing and contributions for a while now since my posts here are restricted. Some posts that violate no rules get discarded or otherwise get ignored and never approved, which is an issue. It's kind of difficult to contribute under those conditions. My last post that even mentioned my being on moderation here was removed with no note whatsoever, complete with the other content in that post. So I'm guessing this one will also disappear before becoming public, despite violating no rules. (Edit: Seems I was incorrect. Thanks.) It seems staff just wants to avoid public notes on their actions because it causes them to have to deal with all of the other people confused as to why they've restricted me in the first place. I wish they would just communicate such things to me, but this seems to be beyond the ability of the folks in charge. So instead I sit in limbo, posting occasionally, fingers crossed that the post doesn't get nixed and be wasting my time.

The convenience factor of just popping in and sharing things I share here without needing to run a site myself is definitely a plus, but it's probably not enough to keep me around here. Unless something changes here my postings will be pretty limited and likely restricted to whatever somewhat interesting topics people link me to from elsewhere (like this thread).

Definitely unfortunate, but it is what it is, I have no control over what the staff here does or doesn't do, and my pleas to resolve whatever issue they have with me continue to be ignored.

Anyways, thanks for the support!
 
Last edited:
Anyways, thanks for the support!

I hope to never need a pack repair/replacement, but I appreciate the depth of your understanding of these issues and I'm glad you're potentially an option (at least, I assume so without knowing the actual cost comparison).

In the absence of a widespread third-party shops to do this kind of work-- just a few "boutique" services like yours, no offense intended (!)-- I hope that in the event you decide to not scale up/out or, even worse, stop doing this kind of work that you will consider sharing your expertise/processes with other shops that might want to pick up the baton and continue supporting these early cars.

Thank you for your analysis and discussion -- your time and patience with the community is greatly appreciated.
 

cousin_IT

Face provided by boredhumans.com
Oct 27, 2020
293
212
Netherlands
With all of the above as a bit of base knowledge, if you manually cut the fuse on a parasitic cell, you've stopped the drain, but now you've assured that that brick is 1/74th out of balance with the rest of the pack forever. This will manifest as being weaker under loads and normal use, and will reach charge voltages sooner than the rest of the series cell groups in the pack every single time. The BMS will only tolerate temporary gross imbalances like this. If they persist it throws a series of errors, culminating in complete shutdown for pack safety. How quickly this happens depends on actual usage (with more parked/idle time helping the BMS correct the gross imbalance each time), but the BMS can not keep up with a completely missing cell from a group for the long haul. It was never designed to. It was, however, designed to detect this issue, report it, and eventually shut things down for safety. If you use the car for a 30 mile commute once a week or something, it might last a while with a missing cell.... but daily normal use for a bit, or even one relatively long supercharger trip where the BMS has no time to tackle the imbalance sufficiently for the next charge and your "grubered" pack is now a rather large and expensive paperweight.
To make sure I understand this correctly... If you have a pack that you suspect is unbalanced but doesn't throw an error the best way to know for sure is to do 3 days of full range driving and then the pack will detect this and shut down?
 

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top