Need a parallel switching solution for battery bank

You probably are going to need circuit breakers anyway (for safety). Just use A circuit breaker to Bank A, and a second circuit breaker to Bank B, and so on... You do need do make sure you get DC rated breakers for your system. The behaviour of DC current is quite different than AC current, and using AC breakers on a DC system can spell big trouble.

There are interlocks (that bolt onto standard breaker panels) you can use to make sure only A or B is on (transfer switch), and not both (if that is an issue). And/or you can use lockouts (keyed locks to breaker(s)) so that only you or others trained to transfer power can throw the switches when needed.

Is this the "system" you are planning on using?

https://www.plxdevices.com/product-p/897346002580.htm

You mentioned you want to time shift your energy usage (charge during the day, use battery power/assistance during the night?). Have you checked out the price of doing this? So far, I have not seen any utility power that is more expensive than using solar+inverter+battery systems... Typically, a battery based solar power system (from my back of the envelope calculations) costs something like $0.50 to $1.00 per kWH minimum, and even in California, at this point, we are pushing around $0.40 per kWH in summer afternoons and $0.20 per kWH at night--So, for those of us on Time of Use power plans, it makes even less sense to run battery power at night (although "afternoon runs until something like 8-9pm these days--So running batteries in the evening makes a little more sense).

Figure out the cost of hardware/installation, new batteries every 5 years, and new electronics every ~10 years, then divide by the number of kWH over that 10-20 year time period that the system will produce, and see how much it costs/saves you.

-Bill

What is the primary reason for your system? Is it to 1) save money on your utility bill, or 2) to have full solar backup for frequent power outages and save a bit of money on the utility bill when the grid is up and running?

1) Just do a standard Grid Tied Inverter system (central or micro inverters). If your utility prevents GT Solar connections and back feeding to the grid--That is their (and the state public utility commissions') power system/their rules. There are utilities that have biling plans that make GT Solar less economic (typically high monthly connection fees, low $/kWH power costs, etc.). And some actually "outlaw" the connection of GT+Battery systems to the grid (they don't want people "buying" cheap power at night, and selling it back to the utility during summer afternoons at peak retail rate).

2) Backup power systems are almost always uneconomic vs utility power. They only make sense if you need backup power (personal choice). A full blown solar+battery bank+hybrid inverter system--When you include initial install, new batteries every 5-7 years, and new electronics (inverter, charge controller, monthly minimum power connection fee, etc.) every 10+ years... Typically you are looking at $0.40 to $0.50 per kWH minimum "all in costs" for your "free solar power" (and can run $1.00-$2.00+ per kWH for full off grid systems--where you have to over design the system to provide winter power in poor sun, vs the lots of excess unused power in the summer). Whereas a typically GT Solar+Inverter only system, probably costs something like $0.10-$0.15 per kWH, has no maintenance costs (other than inverter failure every 10+ year, possible solar panel failures in 20+ years).

A backup genset... Assume $4.00 per gallon for gas, ~4kWH per gallon, or ~$1.00 per kWH that only takes money from your wallet when the power is out for a couple hours to a couple days per year... + the $X.xxx for the initial generator cost and installation+transfer switch+protected power sub-panel, if needed.

And if you have natural gas or propane on site--That is much better than storing gasoline/diesel on site.

I store 20 gallons of gasoline (with preservative) and recycle once a year into the vehicles. And have an Honda eu2000i that will keep me happy on something like 2 gallons of gasoline per day--20 Gallons = 10 days of no power--Plus I have fuel for a gasoline cook stove (if I am out of 20 lb bottles of propane), and fuel for the car if needed. $8.00 per day (our gasoline price went from $3.00 per gallon to $4.00 per gallon in the last few months) if I don't have utility power... And I have had a genset and fuel since 2000 (y2k--wife wanted backup power) and never needed the genset yet. Probably about time to go out and buy a second Honda eu2000i (or whatever is current) for a second (actually third) spare genset.

For folks in California, the picture is a bit muddled. Because of the last few years of "electric utility related" massive brush fires (there is lots of blame to pass around in my humble opinion), our local utility has been threatening to shut of power to large areas of the state (mostly rural) for hours to days at a time during the summer/fall. I am not sure they have actually done this yet (perhaps a couple times so far), but that does put the ball into the (especially) rural consumer's court and make having a full off grid solar power system + generator back (if needed) more useful. And simply disconnect permanently from the grid if the utility is "not reliable" any more, and the State of California keeps escalating power bills.

The PLX system look way too complex (too many bits and pieces that can fail, and operator error) to be a good deal for the long term (you will probably responsible for maintenance, and what if a BC fails 5 years from now and you cannot get a replacement for the custom product)...

Regarding the battery connections... I think you are going to need double pole breakers (like 240 VAC breakers) and open/transfer both B+ and B- lines... I don't think the system was designed with a common ground (negative) line between each BC unit. That leaves out most DC standard transfer A/B marine switches--They only switch the positive lead.

I highly suggest you go through some paper designs first, and run a little math/spreadsheet on your loads and equipment costs, to calculate the actual cost per kWH for the various solutions. And all that goes back to measuring your loads (Watt*Hours per day) and basing your design on supporting those loads.

And look at your location too... Solar panels only work in full sun. Any shading (trees, overhead wires, roof stacks and chimneys, neighboring buildings, hills, valleys, local weather conditions, marine layer, non-southerly facing panels, etc.) only serve to dramatically reduce your harvest.

A Kill-a-Watt type meter is a great way to start, and not bad for figuring out conservation measures (what to turn off when not used, or replace if consuming more power than a modern energy star replacement would use... Using a laptop computer (at 30-60 Watts) vs a desktop at ~300 Watts, etc...

https://www.amazon.com/s?k=kill+a+watt+meter

It is almost always cheaper to conserve power than to generate it.

-Bill