question

@Edgargw22, just to be clear, the Cyrix -by itself- isn't cycling on and off unless something that it's connected to is telling it to do that.

Under most circumstances, if your engine is running the Cyrix will be closed and will be passing charge current from your start battery through to your house battery, and a short period of time after you switch your engine off, the Cyrix will open and disconnect the two batteries. There isn't a cycling on/off operation that naturally occurs unless the AMS or BMS is telling it to do that, which is certainly possible.

Many of our customers run these while using drop-in LFP banks that don't have external signalling capability, so simply jumper the POS control wire to the BAT2 POS connection, and then run a small on/off switch that disconnects the Cyrix's GND connection should they ever wish to manually disconnect the Cyrix; this is a viable solution for those who don't have external signalling devices and/or prefer a degree of manual control over its operation.

Nope, it even mentions the control signal on the data sheet that is posted. i even have a smaller Cyril-Li-ct in my van controlled by a bmv712 (12/24 120 variant)

Its in the manual /datasheet:

Pay attention to the 'Charge Disconnect' statements as this is what connects to the control terminals. The IF statement is the important one. If its high, if its not it does nothing.

As it says, if the control output is free floating, it disengates.

Cyrix-Li -ct The functionality of the Cyrix-Li -ct is analogous to the Cyrix-ct.The Cyrix-Li -ct will parallel connect a lead acid starter battery and a LiFePO4 battery:-if the Charge Disconnect output of the VE.Bus BMS is high, and-if it senses 13,4V (resp. 26,8V) or more on one of its power terminals.The Cyrix will disengage immediately:-when its control output becomes free floating, signalling cell over voltage or cell over temperature, and/or-when battery voltage drops below 13,2V.Start assist function: a short positive pulse will close the relay during 30 seconds (see figure on page 2).A built-in transient voltage suppressor will limit the voltage spike that may occur when the Cyrix suddenly disengages due to cell overvoltage or over temperature.

@abogaard, I can see why that might be confusing - part of it is because I was replying specifically to behavior that the OP was describing, and part of it is simply non-standard terminology, so to be clear: "cycling" on and off is not normal behavior for the Cyrix; I draw a line of differentiation between normal operation, where it closes at one voltage and opens at another, and "cycling", where it's repeatedly opening and closing... it doesn't "cycle" unless it's being told to (in fact it has intelligent timers built in that are designed to specifically prevent "cycling").

All that being said, let me try to rephrase the Cyrix-li-ct operation (and I may well go back and add all this to my original answer to avoid future confusion): Yes, vbatt must be present on pin 85 for the device to function.

The Cyrix uses a microprocessor to control its operation, so I'm going to give its operating conditions as such: the processor uses IF/AND logic to close the relay (joining the batteries) and it uses EITHER/OR logic to open the relay (disconnecting the batteries).

IF vbatt is present on pin 85 AND voltage is above the thresholds described in the manual on either 87 or 30, the Cyrix closes and connects the two batteries together.

If EITHER vbatt on pin 85 OR voltage on 87/30 falls below the thresholds described in the manual, the Cyrix opens and disconnects the two batteries.

Now, vbatt can be supplied a number of ways depending on your system and what you're trying to accomplish. For purely automatic v-based operation, one can simply jumper pin 85 to 87 or 30, and the Cyrix will open and close based purely on the voltages present at 87 and 30.

Alternatively, on a system with an external BMS that has an "allow to charge" port, pin 85 of the Cyrix could be connected to the BMS's allow to charge port, thus allowing the Cyrix operation to be controlled by the BMS (this configuration is reflected in the diagrams shown in the manual).

Alternatively, on a system with, say, drop-in batteries like Battle Borns that don't allow you to interface with the BMS, you could wire pin 85 to an ignition switch (or a manual switch, for that matter) that would allow it to close only when the ignition is on, or only when your switch is in the "On" position.

Alternatively, pin 85 could be connected to the relay-out of a BMV-7xx with the BMV configured to provide vbatt to the Cyrix based on SOC or other factors.

...and plenty of other options. These are highly versatile devices, and so long as you understand the basic operating principle, you can use them in a wide array of applications, too numerous to be included in the manual. But yes, one way or another there must be vbatt present on pin 85 for the device to be able to close.