Disclaimer: I don't actually own a Victron BMV myself, all knowledge I have about it comes strictly from my reading through the manual a few times in the course of doing research for my own future build.
2nd disclaimer: I'm generally familiar with what an ACR does, but had never heard of the Cyrix-Li-CT specifically until 5 minutes ago when I read your post
Therefore, treat this post with the appropriate amount of dismissability
That being said...
It seems to me you don't need the second device, the BSS ML-RBS, and will be just fine with just the Cyrix-Li-CT and the BMV. Looking at the Cyrix manual, I think you have the right idea in your post#15, to just send a generic +12V signal to the Cyrix as your control input. You just need to use the D+ as this +12V signal, and use the BMV's relay to control when to send it.
The Cyrix has 4 terminals:
van battery +ve
lithium battery +ve
a Control Input
a battery -ve for grounding
In the Cyrix manual, it says it will engage when the Control Input is "high" (it sees ~12V at the control terminal) and disengage when the Control Input is "free-floating or pulled to ground". So it seems to charge through the Cyrix-Li-CT, it needs to see BOTH a ~12V signal at the control input AND a voltage greater than ~13V at either the starter battery terminal or the lithium battery terminal.
Note:
It looks like the Cyrix is operating by sensing voltage on both batteries, meaning in theory you have the option of wiring things such that, when the van is off, the Cyrix will automatically charge the van battery from the lithium battery when V_lithium > ~13 V
If you go with wiring the D+ signal into the Cyrix relay controls, you will lose this feature. Does that matter? I'm guessing the answer is no...
(Of course, you would still have the ability to manually connect them using the start assist switch)
So here's what I would do:
- wire the D+ into one of the relay terminals on the BMV
- wire the other BMV relay terminal to the Control Input port of the Cyrix
- program the relay control settings in the BMV appropriately
- For manual start assist, run a wire from the lithium battery +ve to one terminal of the cab switch, the other cab switch terminal goes to the Control Input on the Cyrix. I think you can even make this a momentary switch (so she can't forget to turn it off), as it seems like the Cyrix programming will read a momentary +12V at the Control Input as a command to close the relay for 30sec
There are a large number of BMV settings, especially if you're adding temperature monitoring and control, but I think you want to carefully set these ones as far as lithium battery voltage/SOC (names of settings taken from the BMV manual):
- SOC relay: will close the BMV relay terminals (i.e. send ~+12V to the Cyrix Control Input, enabling charging) when SOC is less than this value. I would pick something like 85% here. If you only want to enable charging when the battery is very low, and rely on solar or shore power most of the time, pick something like 20% here.
- Clear SOC relay: will open the BMV relay terminals (i.e. disable charging on the Cyrix) when SOC is above this value. I'd pick like ~95% here to be on the safe side.
- Low voltage relay: will close BMV relay when V_lithium is below this value. Pick something safe based on your battery's datasheet.
- Clear Low voltage relay: opens BMV relay when V_lithium is above this value - again pick something safe
- High voltage relay: closes the BMV relay when V_lithium is above this value. Probably want to set this value to something outrageously high like 50V. You never want to connect the van and lithium batteries when V_lithium is at high voltage
- Clear high voltage relay: see above
- other settings like temperature control, minimum relay engagement times, etc. are at your discretion
Hope this helps.
