Community members, we can't express enough how important it is to avoid the number of oh-so-tantalizing, yet risky BMS_u029 reset options that are out there.
We had a BMS_u018 pack in this week that was significantly, but not wildly out of balance - 200 mV. Pretty standard fare for a BMS_u018 - wouldn't charge past 50% due to the pack imbalance, but otherwise unremarkable.
What made this pack DIFFERENT however is that the car kept discharging WHILE IN TRANSIT. We of course get lots of INOP (inoperative) cars, and certainly some arrive very close to empty by the time they get to our Service Center, but NONE that have ever discharged twice while sitting on a transport, in just a matter of days. Lithium ion just doesn't self-discharge that fast. Head scratcher, for us, and our always dutiful driver, since it was coming in as a BMS_u018 with a module failure and typical pack imbalance.
When we pulled the diagnostics however, they showed a SEVERE weak short on one of the bricks - more that 100 times (!) beyond the trigger threshold
the dSOCAlpha value on one of the bricks was below -11 (the threshold for triggering a weak short alert is -0.1). The lowest we've ever seen dSOCAlpha is around -2, never below -11. This brick was literally shedding electrons 100 times faster than the trigger threshold!
quick background: dSOCAlpha is tracked by the BMS to measure a weak short and the percent change in SOC of the brick with the greatest drop in SOC, as measured after the contactors are opened (OCV - open circuit voltage)
What's more, weak short data on this brick was well established throughout the logs, BUT there was no BMS_u029 alert anywhere in sight.
We're still unclear exactly why the BMS_u029 wasn't triggered or present - had it been repeatedly reset? was the BMS somehow missing the trigger? had the MCU been rooted?, etc.
Regardless, there's little doubt that the BMS_u018 limit on charging prevented this from being a much much different outcome for the owner of this Model S.
These charge limits are in place for you and your family's safety. Please, please don't override them without thoroughly addressing the underlying root cause of the issue.
We had a BMS_u018 pack in this week that was significantly, but not wildly out of balance - 200 mV. Pretty standard fare for a BMS_u018 - wouldn't charge past 50% due to the pack imbalance, but otherwise unremarkable.
What made this pack DIFFERENT however is that the car kept discharging WHILE IN TRANSIT. We of course get lots of INOP (inoperative) cars, and certainly some arrive very close to empty by the time they get to our Service Center, but NONE that have ever discharged twice while sitting on a transport, in just a matter of days. Lithium ion just doesn't self-discharge that fast. Head scratcher, for us, and our always dutiful driver, since it was coming in as a BMS_u018 with a module failure and typical pack imbalance.
When we pulled the diagnostics however, they showed a SEVERE weak short on one of the bricks - more that 100 times (!) beyond the trigger threshold
the dSOCAlpha value on one of the bricks was below -11 (the threshold for triggering a weak short alert is -0.1). The lowest we've ever seen dSOCAlpha is around -2, never below -11. This brick was literally shedding electrons 100 times faster than the trigger threshold!
quick background: dSOCAlpha is tracked by the BMS to measure a weak short and the percent change in SOC of the brick with the greatest drop in SOC, as measured after the contactors are opened (OCV - open circuit voltage)
What's more, weak short data on this brick was well established throughout the logs, BUT there was no BMS_u029 alert anywhere in sight.
We're still unclear exactly why the BMS_u029 wasn't triggered or present - had it been repeatedly reset? was the BMS somehow missing the trigger? had the MCU been rooted?, etc.
Regardless, there's little doubt that the BMS_u018 limit on charging prevented this from being a much much different outcome for the owner of this Model S.
These charge limits are in place for you and your family's safety. Please, please don't override them without thoroughly addressing the underlying root cause of the issue.
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