question

@Jafa Russell, I will add something here that I can add because I'm not Victron staff so I can -perhaps- play a little bit more with non-supported connections, although I will re-emphasize the not-supported part, and state right up front that I do not recommend doing this:

The MPPT does not need to see battery voltage to turn on; it will turn on based on PV voltage, and shortly after it turns on it will -on its own- apply enough voltage through its battery-out terminals to wake up the ArgoFET (the ArgoFET needs ~8v at the alternator input to turn on) and will then, after ~45 seconds delay, start charging through the alternator input to the batteries connected to the outputs. Thus, you do not actually need to use the energize alternator input at all for this to technically work; the positive "battery" connection from the MPPT can be connected to the "alternator" input of the ArgoFET and the negative battery connection can be connected to the common NEG shared by the ArgoFET, the batteries, and etc.

I know this only because I was curious enough to go replicate and document the behavior on my test bench.

Two caveats that I can immediately identify:

1. The MPPT history and metrics will be messed up because it's shutting all the way off, and starting back up from 0v at the "battery", which normally is a condition that it would never see. Feeding the energize input with 12v will keep the MPPT "alive", which would solve that issue, but it also keeps the ArgoFET alive, thus adding a certain amount of self-consumption that you would need to figure into your calculations. On my test bench, the ArgoFET 200-2 and the BlueSolar 75/15 together draw ~0.05A in total to keep themselves awake.

2. This is, as pointed out, not an approved connection, and therefore you'd be on your own if something doesn't work right or goes wrong and drains your batteries or damages the device itself. As soon as you start connecting things in other-than-approved manners, your warranty and service options pretty much go out the window, so -again- I do not recommend this connection even though in testing it does appear to work perfectly well.