Tesla Model 3/Y Battery Cell voltage variation REPAIR? Balacing

[SamPop07]

Hello Great Community , i have curently installed 4 X tesla model 3 and model Y batteries for my offgrid setup and am about to add another 4 batteries to the system, all batteries have their own DEYE 20kw inverters, after using the system for about 6 months i noticed one of the batteries health has been reduced to just over 70% . now when i check the cell voltage there is significant variation. I seeinf if anybody can tell me if these cells can be balanced and brought back to normal? or this is not repairable? the variation is about 300mV and only occurs when batteries are charging. Also a note that on the events page of the battery emulator it says cell overvoltage, charging stopped, Any help would be appriciated

[mohammadrezwankhan]

[SamPop07]

Thank you for your detailed response, what do you think could have caused this imbalance or overvoltage to occur? I can get the tesla battery's at a reasonable price , I'm thinking of scrapping this battery and replacing it rather than trying to repair the internals , what are your thoughts?

[v0van]

Good afternoon. Only an active balancer can handle this kind of imbalance. A balancer built into the battery will maintain balance for the rest of its life.

[SamPop07]

What would be involved in putting an active balancer ?

[v0van]

1. Perseverance
2. 5 balancers
3. 200 m of cable

[v0van]

This is all because this battery doesn't balance itself during operation. It needs to be disconnected from the system and allowed to balance, which is unacceptable in solar energy. Therefore, this will happen to every Tesla battery you own, sooner or later.

[retho1]

I have the same issue with a model 3 LFP pack and from everything i have read so far, this seems to be an issue with these batteries with no real soulution in sight yet. You only need 1 cell to go out of balance and you lose capacity as they are all in series.
I have tried the balance method in the wiki a few times over multiple days and didn't notice any gains.
I don't think this is a faulty pack or connection issue as mine was from a low Km car with 100% SOH that also fell to around 70% within 8 months use. I will proberbly be going down the external active balancer path to correct this as replaceing it every year or so - even with cheap packs is not practical. Keep in mind this can be a serious saftey issue if not done correctlly as it involves removing the PCS and then the top cover of pack to access the 108 cells to make connections for the BMS. A little work but my thoughts are this pack should last 10+ years once done and provide the full or close to 60Kwh again.
I would be interested if anyone elese with these tesla LFP packs are having more success with staying in balance after using them for 6+ months.

[v0van]

I also tried balancing using the standard method (charging the battery to 95%, turning off the contactors, and leaving it for a few days), but it didn't help. I decided to install active balancers. I’ve already run AWG18 wiring to each cell and am waiting for five 24S BMS units.

[SamPop07]

Based on some of the responses , looks like everyone using tesla batteries are having the same issue, I would love to see a solution without opening the pack, I'm wondering why this is happening in the first place? And seems to be consistent with 6-8 months of use.
Has anyone done the BMS balance and been successful? Any information would help

[v0van]

I currently have a battery pack with a 170mV imbalance. For over a month, I tried to balance it using the stock BMS—charging to 95%, leaving the pack powered on but with the contactors open, and charging at a low current of 5A—but nothing worked. Now I'm installing active balancers.