Battery imbalance cell reading changes

[nicog]

Tesla told me that the vehicles are supposed to automatically be balancing the batteries, just charge to 80% - 90%. They said that they don’t see vehicles in the service center with imbalanced batteries. May be your case is so minor that the BMS is ignoring it for now?

My battery was perfectly balanced during the past 6 month (I monitor the pack this time frame). But now somehow BMS is not able or will not to balance this one brick.
Here Acceptable cell imbalance range? I have got some interesting answers about the imbalance. Maybe this imbalance is ok or maybe it is because of cold weather or maybe one cell in this brick is weak or dead or the contacts from the cell are not holding any more.
Any way I will continue to monitor the pack and will see in the next month, if the imbalance will go down, stay in this range or go up.
I will write it here.

 

[howardc64]

Hello.

first of all marry Christmas and happy new year to you.

@wk057
I have red many threads you wrote about Tesla HV batteries and I know you are expert in this segment.
because of this I have a question.

My 2015 70D model S 135k KM began for about a month to increase imbalance. the problem according to SMT has module 4's last brick. The imbalance varies between 18-30 mv.
I already charged it fully and discharged to 15-20% 2 times, but no decrease in imbalance.
I'm now charging it with 5 amp on 220v socket to help to balance it. but until now I see no improvement.
The 100% range dropped from 353km to 345km in this one month period.
Now dais it is colder here: 1 to 10 Celsius.
How can I balance it or check if the pack begins to failure?

Thank you in advance.

Regards

Nico

Could be 3 possibilities

- short cell in that brick
- corrosion on BMB board in module 4
- detaching voltage sensor wire on that brick

But can't know without opening up the battery pack which require handling deadly dangerous HV pack.

Since in UK, might try to sneak a peek at the fuse cover (2015 pack likely still upward facing) and battery front hump seam to see the corrosion conditions (probably need a borescope on dual motor car?). All near traditional firewall area.

 

[nicog]

Could be 3 possibilities

- short cell in that brick
- corrosion on BMB board in module 4
- detaching voltage sensor wire on that brick

But can't know without opening up the battery pack which require handling deadly dangerous HV pack.

Since in UK, might try to sneak a peek at the fuse cover (2015 pack likely still upward facing) and battery front hump seam to see the corrosion conditions (probably need a borescope on dual motor car?). All near traditional firewall area.

Hi.
- if the cell has short, the brick would discharge continuously, wouldn't it?
- BMB corrosion could also be there, but why does the brick always has 20 - 30 mv imbalance? I don't understand this.
- Same is with voltage sensor - if it would be detached, how can I get the readings in SMT?

If the HV battery pack would have the corrosion at the front side, the first and last bricks must have some problems, but not 4th, which is located in the middle of the pack.
I'm testing now charging the pack to 100% 3-5 times to give it a chance to top balance the pack. I already charged it 2 times and at 96% and above I always have more than 40mv imbalance. after charge is completed, it settles down between 20-30mv

 

[howardc64]

Hi.
- if the cell has short, the brick would discharge continuously, wouldn't it?
- BMB corrosion could also be there, but why does the brick always has 20 - 30 mv imbalance? I don't understand this.
- Same is with voltage sensor - if it would be detached, how can I get the readings in SMT?

If the HV battery pack would have the corrosion at the front side, the first and last bricks must have some problems, but not 4th, which is located in the middle of the pack.
I'm testing now charging the pack to 100% 3-5 times to give it a chance to top balance the pack. I already charged it 2 times and at 96% and above I always have more than 40mv imbalance. after charge is completed, it settles down between 20-30mv

Reasonable guess. There aren't much post mortem analysis on packs that have bricks down multiple 10s of mV. I'm battling similar issue but all 6 bricks of 2 frontal modules are down which strongly suggest BMB board issue ( link ). No amount of rebalancing effort helped. Did see a 4mV improvement once after a lengthly rebalance procedure but quickly reverted ( link and link )

Been in touch with a Roadster battery repair expert. A couple of notes

- Working on a disabled MS battery with 029 (weak short) error. Trying to find the bad cell in the module but not easy. Discharge rate doesn't seem like a complete short and need to find it with a very sensitive volt meter and check every cell's connection junction and depends on SOC.

- He said the board's calibration sometime have issues. There is obviously an Analog to Digital converter on these boards to take the brick voltage readings. I don't know the latest A/D designs but historically these require calibration at manufacturing and I suppose any resistance/capacitance change on the sensing circuit would change the voltage reading.

My car also drives fine but debating whether to open up to find cause and take care of any preventive moisture issue on a 10yo pack.

Regarding middle of the pack board corrosion. Saw an 8 year old pack recently without any pack seal failure with BMB corrosion towards about middle of the pack in front of module 4 and 13 so definitely make its way towards middle of the pack ( link )

Anyhow lots of unknowns. And just noting again, opening the pack is deadly dangerous.