More than 12 batteries per BMS: RS485 Repeater/Hub Required
For any larger deployment where there are more than 12 batteries connected per BMS (and sometimes even less), RS485 MODBUS signal amplification can be required.
This is due to the presence of electronic interface protection hardware on the ZBM2's MODBUS ports. This hardware draws enough power that the maximum un-assisted MODBUS signal chain length, for reliable operation, is typically to a maximum 12 x ZBM2 batteries.
The failure mode
The symptom that you have an issue of this nature - that is likely to be solved with the addition of an RS485 repeater/hub - is that batteries intermittently appear and disappear from the BMS status page, showing 'communication errors'. That intermittent loss of battery access is a sure sign that you need an RS485 signal boost.
You can prove the point by de-configuring and unplugging some batteries to make the total system size smaller. If the fault disappears, then an RS485 signal boost is the likely solution.
Importantly, the issue here is primarily with ZBM2 battery count on the MODBUS daisy-chain, and not so much about RS485 cable length, but on some systems with long cable lengths, even 12 batteries (un-repeated) can be 1 or 2 too many.
Deploying The Solution
The brand or type of the RS485 repeater is non-critical, and it is sufficient to obtain and run a simple repeater (one 'in' and one 'out' port) somewhere in the middle of the end-to-end chain of MODBUS cabling to the battery string.
In large systems, it can be inconvenient to locate (and to separately power) an RS485 repeater located 'in amongst the batteries'. It is far more convenient (and it is recommended) to use a multiport RS485 hub located beside the BMS cabinet and to multiple MODBUS daisy-chain strings from that location out to the batteries in a 'star' pattern.
The reference unit Redflow uses for this purpose is a "UT-5204" RS485 multi-port hub. This device is readily obtainable 'new in box' via eBay - as per the photographs below. See also the UT-5204 manual (attached at the end of this article).
The UT-5204 has one 'input' side (which goes to the BMS) and four independent output daisy-chain paths (that you can then use to connect up shorter daisy-chains to your batteries).
While the unit does require a DC power supply, it is an otherwise passive device, requiring no configuration, and it will work work independently of RS485 signal protocol or baud rate.
It does not matter in which order you connect batteries to the output legs of the UT-5204, nor does it matter whether you use some or all of the four output connections... just as long as the total number of batteries per 'leg' is below 12, you will have achieved the required aim.
The next limitation (16 batteries per BMS) even with an RS485 signal strength boost, is a limitation derived from the rate at which the BMS can retrieve data from all connected batteries while still being sufficiently responsive to the upstream energy management system.
It is not an absolute limit, but response speed to changes in battery status would become progressively slower beyond this size, and systems are not routinely built with more than 16 batteries per BMS as a result.
Instead, for systems requiring a total of more than 16 batteries on a single DC bus, a BMS Master/Slave configuration is used.
Please see this article for details on how to set up a Master/Slave BMS structure.