So, the fortunate thing about Valence U27s and likely all Valence batteries is that EACH module has its own built-in BMS for cell balancing. My inverter/controller (Xantrex Freedom 2000) has a high/low voltage and high/low temp cutoff, so that's taken care of. The only thing I would not have would be some kind of balancing between the modules (batteries). I don't know how critical that is and if it's worth the $250 for the BMS. I will have a low voltage cutoff on the 12V side (Victron Smart Battery Connect) so both sides (120V/12V) are protected. So to me, it seems like the Valence BMS would be pretty huge overkill in a parallel setup. But I just might be missing something.
What the BMS does, and what the inverter does in terms of high/low voltage disconnect is different. The inverter can only 'see' pack level voltage (i.e. 12v, 24v, etc). The importance of the BMS is cell level high and low voltage protection. Damage can occur at the cell level, while voltages are still in the normal range at the pack level. Its likely (almost certain I would think) if your batteries have a builtin BMS this will handle cell level HVD/LVD.
My understanding is that batteries in parallel should balance themselves and need no additional components to do so. Again I have no direct knowledge of Valence, and if there is anything quirky/specific to them.
His statement is correct and the latter part of yours is also true. For the internal balancing (and SOC counter apparently) to be active, there has to be ongoing communication to the battery either by the Valence Diagnostic software (alternatives exist) or the Valence external BMS controller. If the green LED is flashing at a 20 second rate, the internal BMS is sleeping and will not balance. There's a thread around here with thermal images of the internal BMS PCB that illustrates this in action.
