Looks like you got lots of answers to this question!
This is a snag with using multiple LIFEPO4 in series or parallel as unlike flooded or AGM they have their own BMS that can sometimes throw a battery out of balance and messes with the entire bank.
To prevent that I suggest using fewer batteries and a larger battery capacity. May be more expensive but less headaches.
I recently replaced three 125AH AGM and considered using two 200AH LIFEPO4 but considering I live off grid full time and can't have my system go down I decided to go with a single Enjoybot 400Ah LIFEPO4 for that reason and I get more than double the amp output for a bigger inverter.
Good luck!
I live alone off-grid and very remote. Absolutely wonderful in the summer, pretty stressful in the winter. I've been there for 8 years now and feel very exposed if something should fail. Winters in Sweden can be harsh. So I'm slowly building up several systems for redundancy. One thing I decided this year was to invest in an over capacity in solar panels. I went from 1.9kW to 5.5 kW of panels on my roof. I can easily manage on 1kW, if the sun shines all the time. So the idea is that they will give me enough power even when it is over cast or raining. Unlike batteries, solar panels never wear out (30+ year life span). That way I can have lower capacity batteries, because they don't need to cover my use for more than a day or two.
My lead acid battery system (8x 12V 100Ah = 9.6 kWh) is starting to fail now after 8 years of use. Two batteries already dead. So now I'm investing in LiFePO4 instead. I decided to go the DIY route. I know, it might sound stupid for something I need to rely on, but I have an electronics engineering background, so I think I can pull it off. Plus I'm low on savings. Today I ordered 16 x EVE 280 Ah grade B cells and two BMS though Luyuan on Alibaba. I will build two 24 V units of 8 cells each, so in total 13 kWh. If play nice with those units and not top charge or bottom discharge I might have 10 kWh of useful capacity, which is HUGE improvement over my current system where I rarely go below 80% SOC to extend lifetime, so that's only a useful capacity of 2 kWh. I understand your reasoning with having few batteries so that fewer things can fail. But I prefer to have two parallel units. If one fails hopefully the other will still work.
Multiple units in parallel is normally not problem, and will not make the batteries unbalanced. But multiple units in series without a BMS between them might lead to imbalances. In theory. I have no practical experience of how common that problem is though for LiFePO4. But I can tell that my lead acid system (2s4p) have batteries that are seriously out of balance, even though lead acid is said to balance itself if you equalize charge every now and then. But then my batteries is way past their due date, so maybe that's not strange.
I have looked at A LOT of energy storage systems. At one point I decide to learn how to make my own lead acid batteries, but eventually gave it up. Toxic mess. I was into compressed air for a while. And cogen (combining heating and electric generation) through a Stirling engine or Peltier modules. Pumped storage. Fly wheel. But in the end it is hard to beat batteries.
I also keep two petrol generators for worst case scenarios. I heat my home with firewood, so that part is no stress at all. Cold inside? Light a fire!
Good luck to you too! Hope to stay in touch and share ideas.
