This is about expectation, design and every product has pluses and minuses. This is not the OP fault, they thought someone who makes their living by selling would provide the design that met the needs. It's never that way, they ( sales ) live and die by tuning over inventory, nothing else.
I concur, every product has pluses and minuses. You're right, it's not her fault. Sales agents are frequently clueless at best and scum at worst. The nature of the job itself can tend to attract some of the worst of us.
If a product is used in a manner that is not optimal, such as 10, 5 kwh batteries, each with its own bms, that isn't an optimal configuration. If one looks at what is done in the HV battery market, you can stack up to 32 kwh into a single BMS ( BYD Batterybox HVL ) Pylontech and others have similar configurations. Is the Fortress the best available choice I don't know, but not being the best doesn't mean it is broken and throw it out or sell for a loss.
In a SHTF type situation, having multiple points of failure (10 batteries) might be better than one big high voltage battery. At the very least, with one large battery, she would need to have back up parts on hand and know how to swap them without internet assistance. A large battery can certainly have advantages over a smaller battery, if designed well it wouldn't need to waste as much power, but it can be a challenge to extract maximum output from a single large battery due to cable sizing restraints and cost considerations unless the voltage is increased to potentially deadly levels.
The reason there are trades, such as Electrical, Plumbing, HVAC and many others is there is a knowledge set needed to provide expertise and optimal solutions. The idea that anyone can build a power plant seems to leave out before your spend 10's of thousands of your hard earned dollars that you need to be an expert yourself, or things many not turn out well.
Sure, that's one reason why "trades" are around. Another is because they have regulatory captured themselves into having a potentially unnecessary barrier to entry that prevents any sort of fair competition. There are many situations where a certified professional with years of experience isn't really necessary, a simple tutorial would suffice, and the only reason the experts are used is because of laws requiring them to be used. Not every situation requires an "expert", if that's what we want to call the average tradesman. The ones I've encountered on here are pretty smart, and I respect their opinion, but I've met some pretty questionable ones out and about.
While a home based solar system is technically a "power plant", it's of nowhere near the complexity of an actual industrial "power plant" and calling it one is kind of silly. As it scales up, and gets into dangerous voltages it certainly needs to be treated with respect, and fire is always a possibility, but people are allowed to work on their own cars, operate powered wood saws at home, weld at home, climb on and fix their own roofs, and countless other activities that are equally dangerous to themselves and their community. Many of those activities don't have any real licensing or regulation around them either.
Sales has been and will always be a race to the bottom, they should NEVER be relied on for designing, say it again Never.
No argument from me on that.
So the path with the least cost is keep what you have and adjust to make it as good as it can be, in this case, keep most of it in storage and rotate as the capacity is mismatched to the need.
That has been suggested to her many times. Theoretically, according to the Fortress guy, they can handle voltage variations of up to 2vdc upon connection to another Fortress battery. That's one of the reasons they command such a high price of double what a similarly sized competitor battery would cost. It should be relatively easy, if that's really the case, to turn batteries on and off as needed. The only potential problem is likely in the Sol-Arks as they probably expect some consistency in the AH size of the battery array and might not appreciate batteries coming in and out. Also, while cost is a concern of hers, I think she also wants a certain level of efficiency while not having to babysit them all day long. Lower idle consumption batteries would require less babysitting, but would certainly cost her more money at this point.
Finally, I've been in and out of this industry for over 20 years, it is constant flux of new vendors, little to no testing and stranded customer's. Until there are standards of required performance, it will never changes and the manufacture's seem to be destin to repeat the sames mistakes. My personal view is the mobile market set the bar ( customer is the tester ) and that doesn't work well for quality of hardware.
Unfortunately it's not just this industry, as you are obviously aware. There are pluses and minuses to our economic systems, quality is often a casualty of it as it's weighed against cost of failure/litigation.
