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Keeping all those cells balanced

Discussion in 'Model S: Battery & Charging' started by Bataleon, Jan 11, 2015.

  1. Bataleon

    Bataleon Member

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    Hey all,

    Long time Tesla fan and TMC lurker here.

    I was wondering, how does the Model S keep all 7000+ li-ion cells in its pack balanced? Does it happen on a per-cell basis or per-module basis? (I understand that pack cells are grouped into separate modules)

    Is there a 'balancing' mode which one can use when charging?

    Cheers,
    Ted
     
  2. stopcrazypp

    stopcrazypp Well-Known Member

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    It's neither per module or per cell, but rather per group (with all cells in that group in parallel). Cells held in parallel are essentially self balanced because they are held at the same voltage and connected in such a way that charge can move between cells in the same group (the layers within a cell follow the same principle: they are connected in parallel).

    For the 85kWh pack, there are 16 modules with each module having 6 such groups (making up 96 groups total). Each group has 74 cells (by educated guesses made in other threads), for a total of 74 * 96 = 7104 cells.
    These groups are wired in series in order to give a pack nominal voltage of ~3.6V*96=345.6V. It is these series groups that need balancing.

    The Model S does not have an explicit balancing mode, but there are circumstances that can trigger balancing. It usually involves charging the car to 100% and keeping it plugged in for a significant period even after that (using a slow charging rate).
     
  3. Bataleon

    Bataleon Member

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    Great answer—thanks for the detailed information.

    - - - Updated - - -

    I forgot to ask. What happens if a single cell within a group fails?
     
  4. tom66

    tom66 Member

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    The capacity would reduce by about 1/96 or slightly over 1%.
     
  5. Bataleon

    Bataleon Member

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    Makes sense. Thanks :smile:
     
  6. kennybobby

    kennybobby Member

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    Shouldn't that be a 1/74 reduction--the pack voltage remains the same, only the current capacity (Amp-hrs) has been affected.
     
  7. scaesare

    scaesare Active Member

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    Actually, it would be 1/74th, or about 1.4%

    (on edit: I was away from desk for a while before committing my post... kennybobby is correct)
     
  8. rdrcrmatt

    rdrcrmatt Member

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    If that happens I really hope the car would pop up a notification and that the pack would be replaced.
     
  9. David99

    David99 Active Member

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    Everything you read on this forum about balancing is guessing. There is no official info from Tesla about it or how it works. There is one forum member who has taken apart two Model S battery packs and found a battery managing board that has a few resistors that could be used to bleed off some extra charge, allowing lower modules to catch up, thus balancing them, but again, that's a guess. No one here knows for a fact how Tesla balances the batteries or when.
     
  10. stopcrazypp

    stopcrazypp Well-Known Member

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    There is no question that it is these groups that are being balanced. When people still had access to the diagnostic screen of the Model S, there was a screen that showed all 96 groups with voltage monitored individually (it was visually separated out to 6 per module). And no matter how complex the battery design is, it's a fundamental characteristic that series strings are what need balancing. And all the math checks out in terms of charging voltage (which is visible during charging).

    There has been some argument over the mechanism of balancing: active or passive. But I think with the opening of the pack, it has been shown there is no active balancing circuitry, so passive (bleeding through resistors) is the most likely.
     

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