Just got BMS_u029 Issue on a 4 year old battery, after being gone for 10 days.

[JaeTee]

I have a 2013 Model S P85 with a four year old warranty replacement battery. I've put 75k miles on that battery in my ownership and have had no issues at all. Had battery checked about four months ago and was showing less than 0.0004v difference between best and weakest cells.

I use this car every day and do a mix of supercharging when out and about, and plug in for level 1 charging whenever I'm at home.

I just went on a 10 day trip out of town to visit family and came back to find "maximum charging level reduced" on the dash screen indicator, and found BMS_u029 code under the error code list. Car is sitting at 53% charge right now, and when I try to charge it the car just tells me that the charge level is above the maximum charge level and it will not charge.

Prior to my departure, everything was working just fine. Since coming home I installed one pending update and steering wheel reset (foot on brake + both jog wheels on steering wheel for approx 10 seconds) and there was no change.

Seems a bit more than coincidental that this happened after prolonged period of non-use??? Any ideas?

Trip to SC is imminent. I've searched web and forums and have found lots of discussion about this error code, but none with the same type of lead-up to the error as I've experienced. Hoping to gain some insight from others in the hopes that it makes a bit smarter before handing the car off for what will likely be an expensive fix.

Thanks in advance for any info.

 

[JaeTee]

Added detail.... I flew out of town and the car was parked in hot Florida garage while I was gone.

 

[islandbayy]

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[Rocky_H]

I've searched web and forums and have found lots of discussion about this error code, but none with the same type of lead-up to the error as I've experienced.

There is nothing special or indicative about any "lead-up" to when this happened. It's just coincidental. These things happen to a lot of the older batteries with those design flaws.

 

[JaeTee]

Added info to my statements above, which may have contributed to my issue: While I was gone for those 10 days, my roommate unplugged my car in order to charge his, then he plugged mine back up again although it had not been driven... That is also out of the ordinary as I never unplug, then replug my car without significantly depleting the battery.

I took the car to a local third party Tesla shop and had my battery checked and was told that one of my cells is very slightly imbalanced. It was recommended that I only supercharge up to 75 or 80% (I had been going to 90%) and then follow that with level 1 or level 2 charge at home up to 100% as much as I can. The technician added that my imbalance was so slight, that the Battery Management System should be able to bringing that weak cell back up to better levels. He also said that, despite what is recommended by Tesla, based on his own extensive testing the BMS is most effective near 100% charge level on L1 or L2 charging. Supercharging generally throws way too much amperage at the car and overpowers the BMS. The cables are simply too thin at those levels for the BMS to be fully effective. He reset the code and the car has been performing just fine ever since.

 

[JaeTee]

There is nothing special or indicative about any "lead-up" to when this happened. It's just coincidental. These things happen to a lot of the older batteries with those design flaws.

My battery was replaced in 2020.

 

[LA2Naples]

Added detail.... I flew out of town and the car was parked in hot Florida garage while I was gone.

I leave my 2013 P85 sitting idle in a Florida garage for weeks, sometimes months on end. Have done so for last 2 years without issue. Dont think that is the underlying problem.

 

[Greg29]

Added info to my statements above, which may have contributed to my issue: While I was gone for those 10 days, my roommate unplugged my car in order to charge his, then he plugged mine back up again although it had not been driven... That is also out of the ordinary as I never unplug, then replug my car without significantly depleting the battery.

I took the car to a local third party Tesla shop and had my battery checked and was told that one of my cells is very slightly imbalanced. It was recommended that I only supercharge up to 75 or 80% (I had been going to 90%) and then follow that with level 1 or level 2 charge at home up to 100% as much as I can. The technician added that my imbalance was so slight, that the Battery Management System should be able to bringing that weak cell back up to better levels. He also said that, despite what is recommended by Tesla, based on his own extensive testing the BMS is most effective near 100% charge level on L1 or L2 charging. Supercharging generally throws way too much amperage at the car and overpowers the BMS. The cables are simply too thin at those levels for the BMS to be fully effective. He reset the code and the car has been performing just fine ever since.

Can you share your shop? I live near you and would like to use them.

 

[JaeTee]

Can you share your shop? I live near you and would like to use them.

Your Tesla Solution | Tesla Conversions and Repairs

They are in Odessa/Trinity area.

 

[vcor]

"Supercharging generally throws way too much amperage at the car and overpowers the BMS. The cables are simply too thin at those levels for the BMS to be fully effective."

I'd disagree with what you were told. The BMS monitors the cell groups (temperature and voltage) and the software then regulates the charging rate and temperatures (cooling or heating as needed). The BMS will infrequently make slight adjustments by draining cell groups to equalize the groups after charging is complete. My understanding this used to be to be done only at close to 100% SOC, but software changes in the last few years made the adjustments a lower SOC levels (90%?). It is not done while driving, so if you charge to 100% and immediately drive (as you are more often to do at a Supercharger) the BMS will not run its equalization step. Not a big deal and it will get to it at a later time.

The statement "cables too thin.." makes no sense at all. The wire sizes of the BMS are fine and they are not part of the charging L2 or Supercharging. Now one could argue the BMS cannot compensate for large voltage discrepancies quickly - but at that point, there is something wrong with a cell group. I suspect the problem you had will return. Resetting the BMS is usually a very short-term fix, as the BMS takes quite a while (weeks?) to rebuild its database of values and it will try to make adjustments to compensate for a declining cell group. Until it tries, it has to assume everything is good, when it is not. So you might be very lucky but be prepared for the problems to reoccur.

 

[islandbayy]

"Supercharging generally throws way too much amperage at the car and overpowers the BMS. The cables are simply too thin at those levels for the BMS to be fully effective."
Quite the contrary. The BMS will balance out the pack more so at a supercharger (Or any DC Charger) than on AC Charging.

 

[aerodyne]

Is quite possible being in FL, moisture or corrosion got into to the pack and failed a BMS board.
Agree with other posts charging habits had nothing to do with the failure, and will not fix a bad pack.

 

[tesluv108]

This is why no one trusts the out of warranty used Tesla market. People resetting codes out there on battery failures so buying one out of warranty is scary to the common buyer who doesn’t have the technical tools or knowledge to see if the codes have been reset

 

[vcor]

It's similar to the ICE market where unscrupulous dealers/sellers roll back the odometer, use special chemicals and thickeners to hide coolant and/or oil leaks, and I'm sure many other tricks to hide eminent engine or transmission failures.

Hopefully, Tesla will change the BMS "reset" to record a permanent log entry "BMS was reset on 7/26/2024". Another nice feature would be a periodic battery test that can't be erased and is saved for a year. For example, the test runs in the background every 3 months or so and records "Battery health 94% on 3/30/2024".

 

[MP3Mike]

Hopefully, Tesla will change the BMS "reset" to record a permanent log entry "BMS was reset on 7/26/2024". Another nice feature would be a periodic battery test that can't be erased and is saved for a year. For example, the test runs in the background every 3 months or so and records "Battery health 94% on 3/30/2024".

California is actually mandating stuff like that starting for 2026 model year EVs: https://ww2.arb.ca.gov/sites/default/files/barcu/regact/2022/accii/acciifro1962.5.pdf
(I think it will apply to all CARB states.)

Here are some highlights:

2. Additionally, for all off-board charge capable vehicles: a. High voltage battery pack: state of health (SOH), distance traveled since SOH last updated or reset, quantity of battery energy remaining in reserve (only required for vehicles designed to initially hold some battery capacity or energy in reserve and open up access as the vehicle or battery ages)

The manufacturer may limit calculation of an updated battery SOH parameter to certain usage conditions of the vehicle (e.g., only when a sequence of sufficient depth of discharge and subsequent charge event occurs) if necessary to maintain the accuracy of the data parameter within the requirements of subsection (c)(4)(A)4.c. above. However, a manufacturer may only use conditions which are technically necessary to ensure robust calculation of the battery SOH parameter, designed to ensure calculation of an updated value will occur under conditions which may reasonably be expected to been countered in normal urban vehicle operation and use, and designed to ensure calculation of an updated value will occur for vehicles in a test group ,on average, at least once every 4,000 miles.

(6) Display of Data to the Vehicle User

(A) Each vehicle shall also be able to display the battery SOH parameters in subsection (c)(4)(A)2.a. above, in vehicle, to the vehicle user without the use of any tools.

(C) The display in vehicle of the parameters identified in this subsection (c)(6) shall be:1. readable by the user with no more than 5 selectable screens or submenu selections needed to access the parameter from the home or default display/screen;

I'm hoping that Tesla will implement at least the user facing portion and SOH calculation on all vehicles, not just model year 2026 and newer. (And hopefully sooner than required.)