islandbayy
Active Member
Those 6 cells being seen as one are already in parallel with each other so they are always balanced between each other, and the only way they could fall out of balance is if one of those six cells completely fails.
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Johan
Ex got M3 in the divorce, waiting for EU Model Y!
How can they be in both parallell and serial mode at the same time??? The way I understood the previous posts is that you have 6 cells serially connected, the OP writes: "There are six groups of cells in series with each group having all cells in parallel." Maybe I'm misunderstanding this? But at some level there has to be serial connections since otherwise all you would have would be a giant ~4V battery pack with 85kW of capacity, right?
Very interesting thread! Thank you for posting this.
Isn't it possible to switch serial to parallel, or vice versa, to balance the module? If you can "move" the module then you might be able to place the out-of-balance module to the correcting position.
Isn't it possible to switch serial to parallel, or vice versa, to balance the module? If you can "move" the module then you might be able to place the out-of-balance module to the correcting position.
islandbayy
Active Member
Each smaller group of cells, in this case, I believe it is 6 cells or 6x18560 style lithium cells, are in parallel. That group of 6 cells makes up one module. Each module is then connected in series, to make up the nominal whole pack voltage. So example, you have 96 bricks of 6 cells. So 96 bricks, times a nominal fully charged voltage of 4.2v per brick would be ~403v for the complete pack.
I'll draw a picture, it should help for those that cant visualize and not familiar with how battery packs are setup.
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I believe this to be incorrect.
To be clear: There are 96 groups of 74 cells each. Those 74 cells are in parallel, and thus those groups will all be balanced within themselves. Each group in essence acts as one large ~4.1v cell with greater current capacity (~230Ah).
Logically, each of those groups are connected in series, rendering a pack output of ~400v@230Ah
Physically, each sheet (Tesla's parlance I believe), contains 6 of those groups, all connected in series, rendering a sheet output of ~25v@230A. Each of those 16 sheets is then connected in series for the final output.
I make the distinction of the physical layout, because any inter-group balancing would have to take into account the series connections either within the sheets themselves, or between entire sheets. Or both.
In other words, They might be able to bleed current back & forth between only the 6 groups on an individual sheet. Or they may be only able to balance from one entire sheet to another. Or they may be able to balance both, in a sort of dual-level hierarchy.
What I don't' expect is possible, is that any one arbitrary group of of cells could "cross sheet boundaries" to be balanced to any of the other 96 groups. But This would not seem necessary if they can do the above.
islandbayy
Active Member
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I'm trying to keep it more simple to help others understand. Id imagine the final setup is inter "sheet" balancing. Balancing each group of cells within the sheet, then balancing the sheets between other sheets. It would be much less complicated. As long as no one is taking any sheets, the balancing should be quicker in that manor.
It would also be much less costly to balance the cells within a sheet, then balance each sheet to each other yet still have the same outcome.
Like I said though, I'm just trying to keep it simpler for others to understand. I have a feeling this will turn into another one of my youtube videos
djp
Model 3 Performance
The pack architecture has 74 cells wired in parallel to create a "brick" (in Roadster terms). Six of these bricks are wired in series to create a "module". The sixteen modules are wired in series to create the pack.
Overall it's 96 bricks in series, each brick having 74 cells in parallel. Balancing and monitoring happens at the brick level.
Johan
Ex got M3 in the divorce, waiting for EU Model Y!
Thanks scaesare and islandbayy for making it a lot clearer!
It would be very interesting then to see if and how each of the 16 sheets (what I used to think of as modules) may be able to bleed energy between them for (to use scaesare's terminology) "second hierarchy level" total pack balancing.
It would be very interesting then to see if and how each of the 16 sheets (what I used to think of as modules) may be able to bleed energy between them for (to use scaesare's terminology) "second hierarchy level" total pack balancing.
islandbayy said:I'm trying to keep it more simple to help others understand. Id imagine the final setup is inter "sheet" balancing. Balancing each group of cells within the sheet, then balancing the sheets between other sheets. It would be much less complicated. As long as no one is taking any sheets, the balancing should be quicker in that manor.
It would also be much less costly to balance the cells within a sheet, then balance each sheet to each other yet still have the same outcome.
Like I said though, I'm just trying to keep it simpler for others to understand. I have a feeling this will turn into another one of my youtube videos
My point is you said: "Those 6 cells being seen as one are already in parallel with each other".
Assuming you are talking about the 6 groups of cells in a sheet, that's incorrect. The groups are in series, not parallel.
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Can you expand? Each brick (the Tesla seems to use "sheet" for the S pack) is balanced internally? That would seem to be what is shown in the BMS Service Screen
Do you know for a fact that the individual sheets are not also somehow balanced?
Thanks...
islandbayy
Active Member
Im talking about the cells within each brick (as I show in the drawing). The cells within the brick are in parallel with each other, each brick is connected to the next in series, which is where you get the higher voltage/voltage increase from.
This makes it highly unlikely that the balancing is done by any of the more complex schemes suggested of temporarily re-ordering how the pack is wired. Each of those bricks is taking the entire pack current (hundreds of amps). There's definitely no switching of that going on - any means of switching that current would be huge, expensive, and visible. So the bricks are permanently wired in series into modules and the modules wired in series to create the pack.
It would still be theoretically possible to build a circuit to draw power from one brick, pass it through some isolation device like a transformer, and deliver it to another brick - but that's huge complexity to achieve a really tiny saving in overall efficiency, and one that only occurs when already plugged in so of no benefit to range. Maybe they've done it, but doesn't seem likely.
Vastly more likely that they've just done it the same way everybody else does - simple circuit to bypass each brick with a resistor and burn the excess energy.
djp
Model 3 Performance
Internally in a "brick" all cells are in parallel so are guaranteed to be balanced, the individual cells will be at the same voltage and act like a single large cell.
The BMS Service Screen has 96 voltage measurements (one for each brick) and highlights the high bricks and low bricks. On the Roadster the high bricks are bled off during balancing - the Model S probably does the same, the pack architecture is very similar between the two (Roadster having 99 bricks of 69 cells).
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Agreed, the 74 parallel cells in a group/brick will always be at the same potential.
I was speaking of each of the 6 groups/bricks making up a sheet being balanced with each other.
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There are 74 cells in parallel in each brick/group. There are 6 brick/groups in series to a sheet.
There are not 6 of anything in parallel.
djp
Model 3 Performance
I'm not sure that's even necessary, overall it's 96 bricks in series. The grouping of six bricks is just for convenience of packaging. As long as the 96 bricks are balanced relative to each other, you wouldn't need more balancing within the groups of six.
OK, so, let me hopefully clear some things up...
Definitions I've been using myself in real life conversation so far are as follows:
Pack - 16 modules wired in series. 403V fully charged.
Module - One of the 16 liquid cooled sets of cells that has a BMS board attached to it with a fully charged output of 25.2V from six sub-groups wired in series.
Sub-group - One of the six sets of cells within a module. These are all connected in parallel to form a high capacity 4.2V battery (fully charged). These appear to consist of about 74 individual cells. There are 96 total sub-groups.
Now, the BMS boards on each module are wired to be able to monitor each of its respective sub-groups. So, the smallest group of cells that can be monitored by the BMS would be a sub-group. The sub-group has all cells in parallel, so they should self balance at this level.
Each module's BMS board's low voltage connection starts with a connection from the main BMS board (in the back of the pack by the contactors) and connected to the first, then is daisy chained with the next until it reaches the last module (top one up front). There are roughly 10 small low-voltage conductors in this chain/bus. They are actually labeled "low voltage" on the main BMS board.
I'll venture to guess that some kind of active balancing could be done within a module, but not between modules. So, the BMS may have to resort to passive balancing (dumping power into a resistor) for modules or sub-groups that are far out of balance.
Hope this helps.
Definitions I've been using myself in real life conversation so far are as follows:
Pack - 16 modules wired in series. 403V fully charged.
Module - One of the 16 liquid cooled sets of cells that has a BMS board attached to it with a fully charged output of 25.2V from six sub-groups wired in series.
Sub-group - One of the six sets of cells within a module. These are all connected in parallel to form a high capacity 4.2V battery (fully charged). These appear to consist of about 74 individual cells. There are 96 total sub-groups.
Now, the BMS boards on each module are wired to be able to monitor each of its respective sub-groups. So, the smallest group of cells that can be monitored by the BMS would be a sub-group. The sub-group has all cells in parallel, so they should self balance at this level.
Each module's BMS board's low voltage connection starts with a connection from the main BMS board (in the back of the pack by the contactors) and connected to the first, then is daisy chained with the next until it reaches the last module (top one up front). There are roughly 10 small low-voltage conductors in this chain/bus. They are actually labeled "low voltage" on the main BMS board.
I'll venture to guess that some kind of active balancing could be done within a module, but not between modules. So, the BMS may have to resort to passive balancing (dumping power into a resistor) for modules or sub-groups that are far out of balance.
Hope this helps.
However the pics show the wires within the sheet/module attached to each of the 6 bricks. Assuming the BMS module is local to the sheet/module, that would only seem to allow balancing of those 6 with respect to each other, no?
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Thanks wk057.wk057 said:
So intra-module balancing seems to be present.
Any evidence of energy sinks (resistors, etc...) for a whole module to dump in to?
As I mentioned previously, I didn't immediately see anything visible. But, I was also thinking about higher currents than probably needed when I was looking. I won't have time to tinker with this again until maybe Monday.
djp
Model 3 Performance
From wk057's description, the BMS boards on each module tie back to a master BMS for the pack, so it's possible that the master BMS can balance all subgroups relative to each other, similar to the Roadster.
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