I've just reread the whole thread. Think it would be useful for anyone considering lithium.
This is where CAN or a smart BMS is a bit of a differentiator IMO.
In my choice to go cheap drop-in, I lose visibility into individual cell health. It occurs to me that the ultimate demise of my (individual) battery is not going to be predictable. If I had cell-level monitoring I'd be able to recognize a problem long before it manifested itself in a BMS shutdown.
But if you have a bank of them you have almost complete redundancy, and can swap out when they fail.
Exactly. The BMS makes disconnect decisions based on individual cell status, but outside the battery you can't see that. All you see is the sum voltage of all the cells. So a high terminal voltage on the battery is not predictive of when a high voltage disconnect will occur, and why you can't simply turn off your alternator based on battery voltage.
All it takes is a couple of wires coming out of the battery to give the rest of the system all the info it needs. A CANbus is definitely not needed, nor is bluetooth, an app, or anything else.
"Allow to Charge" (ATC) says it's OK to charge the battery. The installer uses that to enable/disable the alternator. Problem solved. It can (should) also be used to control other chargers, and protects against improperly configured chargers, charging when temps are too cold, etc. It's just one wire, and all of these integration problems are solved.
"Allow to Discharge" (ATD) says it's OK to discharge the battery, and can be used to shed loads.
And both of these signals serve as warnings of a possible approaching disconnect.
Regarding a parallel bank of drop-ins and the resulting redundancy, keep in mind that without any of the above signals, you won't know that one of your batteries disconnected. The system will keep running which is good, but you will be none the wiser that there has been a failure, and have no way to check or test without breaking up the battery bank to see if they are working individually. The only observable change will be a drop in battery bank capacity, and that might not be observable at all depending on your use pattern.
So I get the attraction to Drop-ins, but they do create a bunch of issues that have to be worked around like adding alternator protection, creating hybrid battery banks, or running all your alternator current through a DC/DC converter. And you lose fault indication and troubleshooting capability. So they seem easy, but actually create a lot of work.
On the other hand, if you use batteries and/or a BMS that has these simple signals, all of these integration problems become trivial.