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Creating a parallel string battery, which BMS?

5K views 13 replies 3 participants last post by  rishimaze 
No, Tesla puts them all together, treats them as one big cell in terms of monitoring. But it has a fuse wire to each cell, if it burns the cell is no longer part of the circuit, no longer used. A very different, but also safe approach.
I'm sure that's what rishimaze meant: Tesla "does this", which is to monitor at the level of a parallel group of cells. Indeed, it is not possible to monitor parallel cells separately without dozens of current measurement devices (shunts, whatever), since by definition parallel cells have the same voltage across them, so a bunch of voltage measurements would just be redundant.

Of course every EV manufacturer does the same thing; the only difference is that a large number of small cells in parallel are individually fused (by Tesla, the only major EV manufacturer using small cells), while a small number of large cells are fused only at the pack level. It is rare to parallel large (prismatic or pouch) cells in groups of more than three, but small (cylindrical) cells are paralleled in the dozens.
 
Yes, agree, but it is not about parallel cells. It is about parallel strings.
Yes, and no EV manufacturer uses parallel strings. That includes Tesla - the discussion of what EV manufacturers do is all about batteries configured with parallel connections only at the lowest level, and that's what I was commenting on.

Well, there is an exception: GM's Ultium system in the Hummer EV configures the battery in two sections, which are operated in series (~720V nominal) for vehicle operation and for charging from 800 V DC chargers, but are reconfigured as two parallel or independent sections (~360 V nominal) to charge from the more common 400 V DC chargers. It would be interesting to see how they manage 400 volt charging, but since the BMS obviously must have separate monitoring for each section, I'm sure there is are no connections between the cells of one section and those of the other section; they presumably just charge the two sections as if they were in different vehicles and charging to the same target state of charge.

And if you put thin balance wires between these cells of the parallel strings, I still don't see them as parallel cells. And the same voltage will not be on these parallel cells unser heavy load. If one cell drops to 3v under heavy load (because it is bad) and the adjacent cell only drops to 3.7v or something, lots of amps will want to go through balance wire.
And the balance wire being so thin cannot keep the voltage at the same level of the cells. That 0.7v difference can happen at cell level, being scattered over the complete string.
Yes, that all makes sense, and I agree that this is why paralleling strings via BMS wiring is undesirable. They're in parallel, but with additional accidental resistors (the balance wires) in the network as well, and that's a bad thing. And the voltage difference between a cell in one string and the corresponding cell in the other string isn't just the result of the state of charge of those cells, and the voltage drops due to their (different) internal resistances and string currents, but the accumulated difference of those factors for all of the cells in the string up to that point.

Not being a manufacturer and reusing EV cells, I find nice modules that only have balancing/sensing wires coming out of them per cell. Which limits my possibilities.
If it were just seperate cells like a Mitsubishi or Nissan, you could just parallel them.
Right now I have worked with the Ipace modules, which are 3s and closed.
Yes, that's a really nice feature (for reconfiguration purposes) of the traditional Leaf modules (with their big balancing tap terminals) and the Mitsubishi Outlander PHEV modules (with the individual prismatic cells with accessible bolted terminals).
 
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