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Resonable voltage sag in the the Tesla model S battery module.

I use 4 tesla model S modules as solar energy storage in DIY powerwall.
I have a battery managment system connected to it and. the batter nicely ballanced at low currents.

The problem I see, (if that is a problem) is that when I start heavier discharge at around 150-170A i see one cell V sag a bit more then the others by around .04V . Also this cell when charged is getting to high voltage first (by about 0.02V at 50A charge) So I suspect it is a bit week. I just do not know if that is really a big problem or such a impalance diffrences are normal.

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It is cell 6 that you can see is being balanced on the charge.

Thans for any input.