As I found out the battery pack module has 74p6s 444 cells (
Tesla 5.3 kWh Module (85 kWh Pack) | Batteries | Stealth EV). 85d has 16 modules in all and my 70d has 14 modules.
I my problematic module all 5 brick voltages are 3.924-3.925v and only one has 3.905v. in this case the module has 20mv imbalance.
If I calculate its percentage diff between 3.925v(good brick) and 3.905v(problematic brick) it would be 100-(3.905*100/3.925)=100 - 99.49044586=0.50955414%
after that, if I take the whole brick cells (in my case 74 cells) and calculate from this we will get 74*0.50955414/100=0.377070064 cell
So, from this calculation I'm getting 0.38 dead cell. Because of this, I think, that the cell is weak or hes weak contact.
Based on this I can translate it in this: about 53mv imbalance will give us a picture of one dead cell.
BUT.... I'm not Electrician, so I'm not sure if this calculations are correct for this situation. I'm not sure if dead cell(s) can be determined with such calculations. It' could be that in case of 53mv imbalance 3 cells are weak, but not dead.
Please, correct me if this all is wrong, what I wrote here!
Thanks and Happy New Year
Have you tried charging to 100% yet? While the BMS now days always works on balancing out the pack, going to 100% forces a more "Thorough" job of it. It can take a long time depending on how bad. However, it does kind of "force" the BMS to bring all bricks to the same or as close of voltage as it can. It may not happen all in one charge though, it may take 3-5 100% charges in a row. I do find it takes longer and does a better job doing so on a supercharger, and cuts off charging & balancing much sooner on AC charging.
After doing it 3-5 times in a row, letting it go all the way until "Charge Complete", then drive it until it's lower SOC (10-20%) and let the car sit for 15-30 minutes after you are done driving. After it sits, connect TM-SPY or SMT again, and check voltages. If you see bricks that are lower than the rest, those either have a dead cell/blown cell level fuse, or are becoming weaker/less capacity.
If at 100%, and left for a couple hours after charging, those weak looking bricks are still holding voltage, you are fine, if after charging to 100% and checking after a couple hours, those "weak" looking bricks are starting to drop in voltage, then you have a parasitic cell, that is self discharging and slowly bringing the voltage of that brick down. Either way, not much you can do about it other than keep the battery in a charged condition and don't store it unplugged (such as leaving it at the airport) for any period of time. Doing so will eventually trigger the BMS and an error limiting max charge, basically forcing either a battery pack replacement or third party repair. Even with a parasitic cell, you can keep the pack in operation quite a long time as long as it doesn't get over-discharged.
One not best option with a parasitic cell is to "snip" the cell level fuse. This still costs money, time, or both depending on if you do it or have someone else do it. It's not a walk in the park for most people. I personally do not believe you have a parasitic cell though based on your numbers, just throwing out more information.
In all honesty, your pack is older and used. It's not just mileage but also age with lithium. You just happen to have a brick or two that are starting to show it's age. It really only takes one weak cell to start bringing down a brick. Heck, one dead cell in one single brick can reduce the entire pack capacity by 1-2%. Though additional single cell failures in other bricks will have little to no effect. Pack is only as strong as it's weakest brick.
In the end, keep an eye on it, not much you can do right now, drive it and enjoy. Don't stress over it. If/when it's ready to fail, it will fail. Not much you can do to change that. Just keep the pack charged. Charge to 100% on a regular basis, at least once/month, to help keep everything in balance and happy, and just drive on.
