First I need to make sure we are using the same language, because I am not sure we are. 3p4s is understood to mean 3 cells are paralleled together, most often just by connecting them with bus bars, then treating that as a single cell. 4 of those in series would be 3p4s, and would use a standard 4s BMS. (3p4s means first parallel the cells, then series them)Without a diagram of your 3p4s system, it is hard to assess failure modes. While you might disagree with the author of the ORION BMS white paper referenced above (I will find it and post it here),
Parallel Cell Capacity Balancing (PCCB) Procedure.
INTRODUCTION I’m going to describe my setup and the initial electrical performance results as well as a balancing technique that I think is pretty effective at matching parallel cells (e.g an 8 cell 4S battery or a 16 cell 8S battery). I’m a semi-retired Electrical Engineer, but this is my...diysolarforum.com
the primary concern with parallel strings is the energy from one entire string dumping into the other due to a single failed cell. In a parallel cell configuration, this simply can't happen. Yes, one cell in parallel configuration will dump into the mate if that mate fails (assuming no other protections), but that is much less energy than in say a 2P 16S configuration for example. So if there is any possibility this needs to be avoided. At the very least one would need separate breakers and BMSs per parallel string.
There are two separate issues in BMS monitoring in comparing serial vs parallel configurations; the first is whether separate parallel string BMSs per string can detect faults that a single BMS can not and the second is what happens worst-case. The first question is really beyond the scope of what I would want to get into here. But suffice to say, that even with separate voltage measurements per cell, I could envision cell faults that remain undetected within the BMS cell and array level voltage ranges (2.5 to 3.65V). With regard to the second, there is still potentially a whole lot of fault current and more importantly total energy that could be dissipated in a parallel string failure mode within the BMS fault limits. In contrast, the worst case for parallel cells is one dumping into the other. It can't get worse than that. This is 1/16 the energy for a 16S configuration comparing parallel cells vs parallel strings.
If we are really concerned about diagnostics (fault detection) and prognostics (detecting onset of failure of accelerated end-of-life) of a LiFePO battery array, I think it is far more effective to monitor crosscurrents in a parallel cell configuration. Voltage just does not tell you that much. But this means a different topology than a standard BLS which is probably beyond most people's capability to implement. That said, with an early warning, corrective action can be taken earlier and the resulting downtime may in fact be expected to be not only less but also planned.
Most grid-tied battery backup systems have separate critical loads panels. We plan on a transfer switch that has the critical loads being run as effectively off-grid with battery backup. By means of the transfer switch, the critical circuits can be switched to grid-tied and allows for plenty of time to perform any maintenance under scheduled planning allowed by the prognostics look-ahead times.
We can think of cell balance both in series (vertically) and in parallel (horizontally). Parallel strings automatically balance the string average SOC but not at the cell level. With passive or active balancing, some level of string SOC balance will tend to bring all cells in parallel balance but that is indirect (by balancing all cells in all strings toward the same average voltage and SOC). In contrast, parallel cells will automatically balance in parallel without any BMS balancing. I don't have any experience or real data on the subject, but in my mind, it remains to be seen if even after only 5 years, matched cells remain so well balanced as to justify no form of active or passive balancing.
In summary, most of your points are quite valid, but I would counter with there are many ways to "skin a cat" (as I have proposed above), and so my main focus is on safety under an absolute worst-case scenario. At this point, I would consider worst-case to entail limiting the maximum possible energy loss under a single-point cell short scenario. To my mind parallel cells and good fire protection in a metal box is inherently the safest.
4s3p, would mean making a battery of 4 series cells, with a normal BMS. Then taking 3 of those(with 3 BMS's), and paralleling them together. First series them, then parallel them.
I am not sure which of those you mean when you say "parallel strings" Anyway, in a 4s3p configuration, one battery (a single 4s battery) cannot empty into another because of both the fuse on that battery, and because if any cell goes out of spec the BMS will disconnect.
In the 3p4s configuration, there is no way to protect the cells in a parallel group. The BMS only sees the whole group as a single cell. If one cell shorts, there will be massive current, but the voltage could possibly still be in spec for a short time. And if it went out of spec, the cells are connected directly with buss bars, with no way to disconnect them. The whole battery would be turned off, but current between cells within the battery would still flow.
Let's get into it, as it is a key issue, and really quite simple. In a 4s3p configuration, no two cells are paralleled together, and every single cells has its own connection to a BMS. If any single cell goes out of spec, the BMS turns off the battery, and all current through the battery and defective cell stops. This would happen if a cell shorts, if a cell goes open, or if voltage goes out of spec. In a series circuit, current is the same throughout. So disconnecting the battery, stops everything.
in a 3p4s configuration, in fact anytime you have cells paralleled together, the BMS can not monitor the individual cells. It monitors a pair or a group of cells that are in parallel. Further, there is no capability of the BMS to stop current within that group. The BMS can disconnect the battery, but that doesn't stop current in the parallel parts, only the series part.
In the 4s3p configuration, worst case the BMS disconnects, and all current stops. In the 3p4s configuration, worst case, the BMS disconnects, there is still a short that can't be disconnected, and a fire starts.
That said, I have seen it recommended and I explored using fuses between parallel cells. That introduces a whole bunch of other issues though. I have never seen it done in practice. But if you want the absolute safest and are going to use a parallel first configuration, that is what you would do.
