Backup system with SOK and DIY

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Jan 17, 2021
This question has me thinking about expanding a backup system based around an SOK, or any other similar battery such as an EG4 that has a built in BMS with communication. Most of those units can be paralleled together for increased capacity. My question is this: if you have just (1) of those units hooked up and communicating with an inverter, could you expand the capacity of that system by adding a DIY battery bank in parallel? Assume for this scenario that the added DIY unit had the same cells, and same capacity. Or, what if the added DIY unit had larger capacity, but the same type cells?

We have NEC 2020 here, and that process requires you to state in your application if a battery will be used. Our AHJ insists that everything is UL listed, or NRTL. For those of us that have a large investment in automotive cells, or just enjoy building their own batteries, this places a serious crimp on our ability to afford to expand a legal, permitted system. I don't want to get into a discussion arguing about the regulations, but that is the way it is ENFORCED here. I'm more concerned with changes taking place with home insurance companies, who are doing their best to limit claims, or just find any possible excuse to deny a claim.

So, could a person purchase a code compliant battery, install it, and then add an "expansion" unit that is DIY? Set aside the issue of the second unit being DIY, is certainly not UL listed. I'm asking for the simple reason that I don't know if it would work properly.

I'm selling off my batteries to people who are not regulated by NEC 2020, or are building EV conversions. My sister has a remote cabin, and she will be getting a nice bank for free...

Inquiring minds want to know....
I know a bit of international standards (not specifically NEC 2020) and I'd say that adding a DIY battery to a certified system quite surely invalidates system's compliance.
italianuser: yes, it would certainly not be compliant. No question about that. My curiosity is if it would work properly or not. For the example of my sister's remote cabin, we could buy a unit ready to go, no hassle, and that would work with the Victron inverter she is going to buy from me. Later, I could build a DIY battery if it needed to be expanded. Rather than purchasing more units from a vendor like Signature Solar. The 2020 NEC is not in force where she lives.
Oh yes, it should work with an extra battery at the same voltage; if the battery you add is the same capacity (and same charge/dicharge curves) as the original the system will be balanced and charging time will be doubled unless you can adjust charging current; if the batteries are same voltage and different capacity (and/or different charge/discharge curves) the system will be unbalanced. Obviously a balanced system is better because stress during battery's life cycle is equally divided (between batteries, and in each battery between cells)