Hi Peter,
Many members here have off-grid systems with part RE and part fuel-driven supplies, so you've picked a great place to share your experience and ask questions. Welcome!
I don't think the Victron Multi-Plus inverter chargers have the ability to see the other inputs and throttle back. What I'm hoping is that the Outback MPPTs via a MATE and shunt can regulate themselves so that the generator becomes the primary charging source and the solar gets regulated to the battery's maximum input.
Yes some of these problems take care of themselves. As pointed out before, your Outbacks will observe the voltage of the batteries as they are being charged, but have no way to know if that voltage is being maintained by a power source or not, or their state of charge. In this case the Outbacks will supply only such current as will suit the apparent need of the batteries - probably a current to maintain float voltage, somewhere around 26.8 to 27.4 V depending on your settings. This is one of the basic jobs of a charge controller - to regulate the current so that the batteries are not overcharged. I do a similar thing with wind/solar on my batteries. The wind can be going full tilt, the batteries are at float charge, and I don't worry about the solar charge controller adding more current. It measures the voltage and figures out the solar isn't needed today.
I don't actually know anything about the Victron inverter. If you have a device like a battery management systems (BMS) measuring the current from all charging sources and going out to all loads, then it is possible for an estimation of the SOC in the battery. Then the BMS could decide whether the generator turns on or not. Not a lot of folks go for that kind of device because needless to say it's pretty complex! And I don't think the simpler types of BMS could properly manage a system such as yours with 3 charge controllers and 2 inverters...
BUT... if the conditions are right, say I hit my low SOC or voltage setpoint overnight and my generator starts at 6am. Now my inverter charger will put in 70A (100 if I feed the generator into both inverters) and as soon as the sun comes up I'm going to exceed the max input of the batteries.
This won't really happen. The Outbacks will measure the voltage of the batteries as "at float" or "at bulk" because the generator is charging them. Their voltage will be well above 26V, probably above 27V too if it's still running, so the charge controllers will know they don't have any work to do.
Unless on that same morning your wife is already out of the shower, using the hair dryer, curling iron, and toasting a bagel. In that case all three of the Outbacks will be going full-tilt to keep up!
The ideal system, of course, is as smart as a person and can see the future - having some way of knowing if it will be sunny today, if there is plenty of diesel in the tank or not, choosing if today will be a laundry/washing day or a conservation day, how low the battery charge is, and so on. In reality, that's a lot to ask for unless your budget is infinite. We all make do with compromises, to get the system working first. Then we work on those things that increase our satisfaction the most.
