Seems like a ModBus-RS485 PLC could interface directly with the XW Pro Gateway.
Please excuse my ignorance but I am stil coming up to speed with all this Modbus and PLC technology.
Are you just saying that a PLC-based ‘brain’ / controller will have an easier time interfacing with the XW Pro gateway than an RPi-based brain / controller?
Or are you saying there are some smart shunts supporting Modbys-RS485 communication that could directly interface with the XW Pro Gateway?
I’m still trying to understand whether a PLC can directly communicate with an XW Pro or (Victron) MPII or a gateway is required…
As far as accuracy it would depend on the amp rating of the shunt and the resolution of the analog input on the PLC.
This I understand - PLC will have a resolution limit on analog input and shunt will have an analog noise level that needs to be accounted for in that context. But that makes it sound as though it’s a workable idea (as long as accuracy is sufficient).
One of the problems I have is not being able to monitor small standby currents (150mA or less) with a 100A, 75mV shunt connected to a ZEVA BMS.
What shunt are yo using and is it connected directly to the BMS or through a PLC? (In which case I’d also appreciate understanding which PLC you are using).
The ADC, probably 8 or 10 bits only, does not have a small enough least significant bit to capture those small mV readings.
In your case DC only flows one way from the SCC to the battery, so you don't have to worry about reserving half the ADC range for negative values.
Out of the SCC, yes, but I am hoping to absorb the majority of that current in an inverter before it gets to used to charge the battery).
I’m looking at max charge current of 100A so an 8-bit ADC should provide accuracy of ~0.4A / 10W if I can find a shunt that expounds the full analog range from 0-100A.
I’d be happy with any accuracy below 50W, so this should not be a problem…
Also depends on how small of a current the SCC can deliver before it shuts off either with a full battery or low sun light.
This is actually not a problem. The whole goal of this approach is to exploit the simple and natural throttling / curtailing capability DC-coupling through an SCC has to offer. The inverter will invert all of the incoming DC-power whenever it can, so the battery will not be charging and the SCC will remain in CC / Boost mode.
Only in the circumstance that the inverter cannot export all incoming DC power (because my export cap of 3.5kW has been reached) AND the battery is full enough to cause the SCCs t enter CV mode will DC power be getting curtailed (in which case I had no choice but to dump / waste it and n which case the incoming DC power measured by the SCC remains accurate).
The RPi alternative is questionable, can it even be used to capture mV readings from the shunt and communicate via ModBus RS485 to the Gateway at the same time.
I don’t know (which is why I asked).
Several members have told me on other threads that RPis have the advantage of being faster the PLCs, in case that provides an answer.
But as far as the code / library to translate desired Modbus commands into the correct output string, my sense is that PLCs make that more turnkey / foolproof than figuring out how to do so on an RPi…
Especially if you need to add a high resolution ADC hat and an RS485 hat.
Not sure whet a ‘hat’ is - it that an add-on board for an RPi?
Assuming it is, I suppose I should start looking into what RPi ‘hats’ are typically being used for Energy Monitors…
Appreciate your inputs and would love to understand in more detail what you are found with your shunt…