john61ct said:
amberwolf said:
If every cell in a parallel group is also connected via it's own series tab to the next and previous groups, then the parallel connection doesn't carry pack current, it just maintains "balance" between cells and allows a BMS to monitor the whole group.
If there are only series tabs every few cells, then the parallel connection must carry all the current from every cell not attached to a series tab thru the pack. The more cells without series connections, the more current hte parallel connections must carry.
But if you want **each cell** to be individually protected / monitored, then you can only have one +/- pair connecting each parallel group in series to the next,
right?
I don't understand what you mean by "one +/- pair".
If you mean only one series connection, then I don't see what that has to do with protection.
If you are actually going to monitor each cell individually, then you cannot even connect them in parallel *at all*.
Otherwise the paralleling means all the paralleled cells are one voltage, and you are automatically monitoring all of them at once
If for some reason you really really wanted to monitor each cell's voltage, you can ONLY connect them in series strings, and then parallel ONLY the ends of the series strings. In practice, this is not done. It means you need as many cell balance / sense wires as you have cells, and you need electronics with that many input channels (or an expensive and complex (and thus vulnerable) analog multiplexer for them. More wires is more failure points, more places for opens or shorts and fire potentials.
Imagine a car-sized (tesla) pack, with thousands of cells. You would then need thousands of wires to monitor each cell.
At best, it's impractical and expensive.
At worse, if not designed and built properly, it's a fire waiting to happen.
So...packs that are individually-cell-protected are done by fusing each cell, like Tesla, but still connecting cells in parallel groups, and monitoring each parallel group.
If a fuse blows, the monitoring (if advanced enough) will count the parallel group capacity as less than the rest of the groups, even when balanced, and advise whoever looks at the logs or monitoring equipment that there is a pack problem.
If (like just about every common BMS out there) it's not advanced, and just monitors realtime voltage of each group, then it just stops discharge when the group reaches LVC, and stops charge when it reaches HVC (each of which will happen earlier than other groups that have not lost a cell), and the only way a person ahs to know that there is a problem is to see the pack appears to ahve less capacity than it used to (by the amount of 1P x Ah), and/or that it has less current capability than it used to (by the amount of 1P x Ah x C-rate).
