I just finished a very similar sounding installation. Started on it last June 2025 and finally got it resolved and working perfectly few weeks ago. My vendor, one of the largest Victron dealers in the US, while they have decent support staff that can answer about 90% of my questions, it was that last 10% that they got hung up on. I was asking a simple “yes” or “no” is this configuration possible question. It took them months to get back to me with a semi-sorta-answer. I had poured over every document available, on the community here and all of Victron’s docs. I was seeing conflicting information about two different ways it should be connected. I could not get a definitive answer from the dealer support, they kept telling me “yes” it SHOULD be able to be done the way I was attempting to configure it, but also in the same breath told me they had no real world examples where they knew someone that had a configuration like I was attempting. Needless to say through MUCH trial and error and testing, and re-reading.. I finally got it sorted out the way I wanted it all to work. BIG relief as my client was starting to think I was crazy. Turns out the way I initially configured everything it should have worked, but due to a bad connection and a blown fuse I was unaware of until later in the process, it was causing very strange errors that was complicating my troubleshooting steps. After giving it a rest for a few weeks I went back and tried again and pushed through all the last steps to get it running as expected.
I would say after all the missteps, errors, re-configurations etc I went through, I am probably more knowledgable than my vender tech support now, as I ended up giving them a full report of my setup and what it took to get it running.
Short version of the configuration.
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2 Multiplus IIs 48/15000 230V in parallel feeding into a 100A Autotransformer for 120v/120v split phase output, SmartSolar MPPT RS 200A, 60kW battery.
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Non-grid tied but grid fed from U.S. 240v through contactor to MPIIs A/C IN.
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Manual transfer switch to switch house from “off grid” to “on grid”.
If house is “on grid” either for maintenance or very low battery, Victron system can still charge from grid. But in reality if battery drops below 15% it closes contactor for A/C IN to MPIIs and it charges while it closes another contactor to do the A/C OUT to grid connection. So it is grid fed which acts as a generator but not grid tied, which would backfeed grid.
Adam