[brian_]

Can you monitor all packs with one set of BMS that are tied with small wire jumpers into same level cells on other packs? Only 12 BMS ties but each connected to 3 cells.

I don't think so - any pair of cells which are not electrically connected at their positive and negative terminals are not really in parallel, and cannot be expected to be at the same voltage, even if they are nominally the same because they are the same number of cells up the stack. If the cumulative voltage of all cells "below" that level in each stack is different, the little BMS monitoring wires would be taking substantial current to equalize those voltages.

 

[brian_]

Having each cell (actually two parallel cells) on its own BMS would be safer.

Yes, or in the case of a Leaf re-configure the packs to parallel at the module level (instead of parallel high voltage packs), which involves electrical connections using threaded studs. This is not so easy for the Volt modules, which would need their welded cell-level connections to be modified.

 

[brian_]

... So each pack is is series with heavy cables at negative and positive terminals going to controller. I could connect all cells at same level with connecting wires- any size and thus parallel the cells on that level. The question is how much current could/would flow through these wires? It seems that due to same level of voltage there would be minimum....

If the centre tap terminals of paralleled Leaf modules are connected, the only current between them would be due to imbalance of cells which are at the same potential. That must be a small fraction of the current drawn through the main terminals, or the cells would be severely out of balance at the end of discharging or charging. With a couple hundred amps (per module if in parallel) on the intended path, it's hard to imagine more than a few amps through the ties between the centre taps. But that's with the cells paralleled.

Connecting just BMS taps across packs is essentially paralleling them through BMS wires.

 

[brian_]

I thought that Tesla does something like this with fused wires in case a cell goes out.

This Tesla configuration is simpler than any of the scenarios which we are discussing.

All current Tesla batteries have many cells parallel at the lowest level (number depending on battery capacity), then purely series beyond that. The fuse wires (just wire, so fine that it acts as a fuse) connect each cell to the bus plate for it's level of the module, and there is one BMS connection to that bus plate. If a cell passes too much current, the wire melts and that group of parallel cells just has one less cell; it makes no difference to the voltage seen by the BMS.

 

[brian_]

There would be three wires to the first module - neg,center tap, positive and two to each following module (-,+,-,+,-,+,-,+) same as a BMS would do.

You need to connect to every centre tap, so the BMS can manage all modules at the cell level. Whether you tie the centre taps of modules directly together (with the bus bars as you've shown before) or run separate BMS leads to them, they all need to be connected. Have I missed something?