Does the Model S battery use active balancing?

[Kwillscherer1026]

This is really interesting. Does anyone know if the Model S uses active or passive battery balancing? Seems like this active system could add a lot more efficiency.


Linear Technologys new active cell balancer | ChargedEVs

 

[Doug_G]

What is the cost of implementing your active system?

MV:
In terms of cost per cell, we think our solution cost - which includes all of the external components required - is in the two to three dollars per cell range.

This would cost $14,000 to $21,000 for a Tesla battery. Non-starter. Maybe for prismatic cells...

 

[CroDriver]

This would cost $14,000 to $21,000 for a Tesla battery. Non-starter. Maybe for prismatic cells...

No - a BMS treats a string of any number of paralleled cells as one cell. The Roadster has 99 cells in series which means that the BMS monitors 99 individual voltages, no matter how many cells are in parallel.

 

[ggr]

No - a BMS treats a string of any number of paralleled cells as one cell. The Roadster has 99 cells in series which means that the BMS monitors 99 individual voltages, no matter how many cells are in parallel.

I don't think this applies to the Model S battery and I'm quite sure it doesn't apply to the Roadster. Talking here about the Roadster: First, it depends on just which way the cells are all connected; is there one chain with 99 sets of cells in parallel, or is there some number of chains (each with 99 cells) wired in parallel? (I don't know, but I imagine there are people who do.) But even then, there are a number of independent sheets in the battery, and they would certainly each need their own balancers.

 

[Doug_G]

No - a BMS treats a string of any number of paralleled cells as one cell. The Roadster has 99 cells in series which means that the BMS monitors 99 individual voltages, no matter how many cells are in parallel.

Did you look at the article? It's showing an architecture that requires connections to every cell.

 

[Dave EV]

I'm fairly certain that the Tesla must be able to shunt/bleed off current from any single cell - can anyone verify? Or are they only able to balance "sheets"? If the latter - I wonder how they are able to keep cells in a sheet from going out of balance - 99 cells is a lot of cells in series and eventually even the slightest difference in cell characteristics will result in individual cells going out of balance...

 

[jerry33]

I'm fairly certain that the Tesla must be able to shunt/bleed off current from any single cell - can anyone verify? Or are they only able to balance "sheets"? If the latter - I wonder how they are able to keep cells in a sheet from going out of balance - 99 cells is a lot of cells in series and eventually even the slightest difference in cell characteristics will result in individual cells going out of balance...

We know that each cell is individually fused. My thought, from what I've read, is that they can't turn off a single cell but they can turn off a small group of cells (way less than a sheet).

 

[strider]

We know that each cell is individually fused. My thought, from what I've read, is that they can't turn off a single cell but they can turn off a small group of cells (way less than a sheet).

I'm no battery engineer but from what I've read on the Roadster, they balance to the "brick" level, each of which has 69 18650 cells. 9 of these bricks are rolled up into a "sheet", of which there are 11.

 

[Dave EV]

I'm no battery engineer but from what I've read on the Roadster, they balance to the "brick" level, each of which has 69 18650 cells. 9 or these bricks are rolled up into a "sheet", of which there are 11.

Thanks, that makes perfect sense!

In this configuration each "brick" has 69 cells wired in parallel. Voltage range of each brick ~3.0-4.1V.
9 bricks are then wired into series to form a sheet. Voltage range each sheet is ~27.0-36.9V.
11 sheets are then wired into series to form a pack. Voltage range each pack is ~297-406V.

The 85 kWh Model S probably has more cells in each brick. The 60 kWh Model S appears to have a lower voltage by looking at 60 kWh pack SuperCharger voltage reports - I'd bet they use fewer sheets than the 85 kWh Model S.

 

[Engineer5]

Which cell balancing technique is Tesla using?