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
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 current 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.
