Keeping all those cells balanced

[Bataleon]

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

 

[stopcrazypp]

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).

 

[Bataleon]

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).

Great answer—thanks for the detailed information.

- - - Updated - - -

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

 

[tom66]

Great answer—thanks for the detailed information.

- - - Updated - - -

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

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

 

[Bataleon]

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

Makes sense. Thanks :smile:

 

[kennybobby]

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

 

[scaesare]

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

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)

 

[rdrcrmatt]

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

 

[David99]

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.

 

[stopcrazypp]

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.

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.