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Battery defect for IT brainstorming BMS_u029 - P85 MS 2013
- Thread starter BIZTEam
- Start date
Pending any official word from Tesla, I've temporarily removed "Reset of 029/18 alert by 3rd party" as a recommended BMS_u029/018 alert solution from these FAQs - https://docs.google.com/document/d/...ouid=101367879314455822316&rtpof=true&sd=true
Moisture ingress and rust causing fuse wires breaking.
Tale of Two Errors and Why Not to Reset BMS_u029
Here’s something Recell has been working on that we thought was really important to share
Attached is a graphic showing four modules - two from separate battery packs that came to us with a BMS_u029 / Weak Short error and two, again from separate battery packs, that came to us with a BMS_u018 / Capacity Imbalance error
View attachment 936017
What they illustrate is something quite remarkable. each of the red dots is a cell that is self-discharging, in fact self-discharging at a quite a rapid rate, as much as 20 mV per HOUR or more, to the point that when isolated from the rest of the brick, these cells are completely discharged within 16 hours, and not just 0% SoC, but 0V period. (a more typical discharge rate for a healthy cell would be <0.5 mV per DAY, literally a thousand times slower)
You’ll notice of course that the red dots are only on the left-hand side in the BMS_u029 column, while there are none on the right-hand side in the BMS_u018 column. that’s not a coincidence.
(we should note for the record that these modules are not unique. we have seen this EXACT same behavior across dozens and dozens of modules, just like these)
What’s curious is that the both the BMS_u029 and BMS_u018 modules have noticeable capacity imbalances, as much as 15 Ah or more in the case of the two BMS_u018 modules, but they have two VERY different failure modes.
In the case of the BMS_u018 modules, the cells themselves work perfectly fine with very very little self discharge and can soldier on - they are just tired and worn with less capacity than they once had, but otherwise perfectly operational. in fact, they can be repurposed/recycled for ‘secondary use’, just not in their current configuration as a Model S battery module.
In the case of the BMS__u029 modules, however, each one of these red dots represents a potential runaway thermal event
quite aside from looking like it has chicken pox, the top left module is actually quite shocking to realize - you would never ever want this module sitting in your pack. and the last thing you should be doing is resetting BMS errors on it.
but even the lower left module has the one cell, lying in wait. all it takes is one of these cells to short and you’ve got a potential runaway thermal event.
perhaps what’s more impressive is that the BMS is able to identify even this one cell and flag it as a potential short, and then latch the error to prevent the condition from inadvertently being reset or overlooked during some unrelated firmware update, system reboot, etc.
What’s more, the BMS is able to differentiate cells with potential for a weak short from worn and tired cells with a capacity imbalance, and identify them as just that, worn and tired, but really nothing more. in fact, should the charge balance happen to improve between charging cycles, the BMS will clear the BMS_u018 error and happily return the battery to its normal, albeit diminished, operation.
so…. a couple of key takeaways
1) Tesla is NOT bricking these packs with an OTA software update, slipping something in while upgrading your MCU, or arbitrarily increasing some threshold limit. these BMS_u029 weak short conditions in the latest BMS firmware are real and they are for YOUR safety — we’ve seen this behavior in ALL of the BMS_u029 packs we get in. we literally have stacks of these modules that have have been reprocessed and decommissioned - each with identical results. The BMS_u029 modules have self-discharging cells, and the BMS_u018 cells are worn and tired, but nothing more, just worn and tired.
* we start the decommissioning process by ripping off the collector plates, as we have in the attached photos, to isolate each cell, and we then dismantle the module as appropriate to repurpose any good cells for secondary use.
2) it’s really really really important that these BMS_u029 errors NOT be reset without addressing the underlying root cause (by completely REMOVING and decommissioning the module in question). Even just one of these cells has the potential to cause a thermal runaway event if charged above 50%.
We can’t say it enough, and we hope this illustrates why, these BMS_u029 errors should NOT be reset.
While I'm certain that this will just be ignored by people selling such a reset, nice visual and writeup.
Hopefully helps at least some people make a more sane decision on this.I've been trying to find ways to gracefully repair these types of modules for ages now, and many modules have been sacrificed to the thermal runaway gods as a result. Have tried all sorts of methods, most of which either just didn't work or persuaded the cell's inner fire to show itself. At some point I need to show off the chamber of battery death we built for potentially destructive testing...
At this point I'm pretty convinced there's no way to handle these besides breaking them down, same as ReCell noted, in 99% of cases.
We have run into a few cases where external factors were the cause of the "weak short" condition and ended up being repairable... but we're talking sub-1% to-date.
I find it interesting that the u029 error reads: "Maximum battery charge level reduced. OK to drive - Schedule service." The "OK to drive" part is starting to seem highly questionable to me. I can see that message being appropriate for the u18 imbalance error, where a brick is just losing capacity and wearing out, reducing charge level. But, you would think the law-folk at Tesla would recommend that any warning for potential electrical short and chance of fire, no matter how small or large, read something like "pull over safely, stop driving and service immediately."
Questionable based on what? All of the reports of vehicles that reported a U209 error that burst into flames? (I don't think I have seen any reported.)
Recell mentioned that the danger occurred when charging one of these modules past 50%. (Which I don't think you can do with the U209 error active. The amount it reduces the maximum charge level to is variable, but I think it is always below 50%.)
charge levels below 50% are an order of magnitude (perhaps several orders?) less dangerous. for example, the Uniform Fire Code (UFC) considers lithium ion to be effectively inert below 50%. (the aviation industry requires 30%, but that’s understandable given the environment)
it’s when you get above 50% that it can get hairy.
thus the 50% limit on the highest brick in the pack - per the UFC.
it’s when you get above 50% that it can get hairy.
thus the 50% limit on the highest brick in the pack - per the UFC.
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Makes sense. I didn't realize that the u029 error reduced charging capability that much in all cases.
Again, seems like with a chance of thermal runaway/fire, no matter how small that chance, that some level of urgency might be relayed by the warning. I've seen "pull over safely and tow vehicle" warnings for much less, such as a catalytic converter/secondary air injection system error in an old Audi of mine.
Again, seems like with a chance of thermal runaway/fire, no matter how small that chance, that some level of urgency might be relayed by the warning. I've seen "pull over safely and tow vehicle" warnings for much less, such as a catalytic converter/secondary air injection system error in an old Audi of mine.
ResHacker
Member
Can I ask, on approx how high mileage can I expect those errors to occur? What lifetime should I expect from 2017 100kWh battery if treated well? What maintenance should I provide to it, just to check umbrella valves from time to time? As I expect the condensate hose problem is solved on a 2015 car. Thanks for the feedback and sorry for a little OT 
ResHacker
Member
I have seen it on some video, they're from the under side of the battery on both sides. And if they don't seal as they should, water can come through an hold there. Some guy in the video was trying to open them a bit using a needle hook and from a part of them water poured out - meaning they're failed.
ResHacker
Member
No, these valves have nothing to do with the AC drain hose. They're situated under the battery so whe water going from wheel can get into them under some circumstances while the AC drain hose is only one situated above the front battery penthouse. Nevertheless on the newer vehicles there's an "updated" prolonged version of the hose so the water is leaking aside of the battery instead on the top of it which caused also many water ingress cases.
I'm aware of the moister ingress issue and the short hose above the battery on some older cars, but I was not aware that these so-called "umbrella valves" are under the pack. I did ask @chrisro the question but he never replied (Post#46). Thank you for the response.
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