Multiplus with CerboGX, ESS and Grid-Inverter at AC-out1: How charge Battery with excess Power

Really interested in this topic as I've been wrestling with something similar on my static caravan setup.

Running a Multiplus-II with a CerboGX and ESS, and I've got a small grid-tied inverter (Enphase microinverter on the caravan roof) connected to AC-out1. On sunny days with low load, the excess generation just seems to... disappear rather than pushing into the battery. Feels like such a waste.

From what I've read, this is actually a known quirk with ESS when a grid inverter sits on AC-out1 — the Multiplus can struggle to "see" that excess power properly and prioritise battery charging accordingly.

A few things I'm wondering:

• Does setting a lower AC input current limit in VEConfig actually help coax the system into pulling more from the AC-out1 inverter first?
• Is there a specific ESS Assistant setting that handles this, or do you need to fiddle with the DVCC current limiters in the Cerbo?
• Has anyone used the grid setpoint adjustment in the ESS menu on VRM to effectively trick the system into charging harder?

I've also got an EV (Nissan Leaf) I occasionally charge from the same setup, so getting battery state right before evening draws matters quite a bit.

Would love to hear from anyone who's cracked this — particularly if you've done it without needing a CT clamp on AC-out1, as mine isn't currently fitted that way.

@IvyWalker the classic AC-coupled headache — your grid-tied inverter on AC-out1 needs the Multiplus to do frequency shifting (raise Hz above 50 to throttle the inverter down) when the battery's full, but the real trick is making sure your ESS assistant is configured to allow charging from AC-coupled sources, not just block backfeed.

Check your ESS Grid feed-in settings in VictronConnect — specifically AC-coupled PV feed-in excess — and ensure the Multiplus firmware is current, as older versions had quirks with frequency-shifting response times.

Also worth noting: Enphase micros can be stubborn about responding to frequency shifts compared to, say, a Fronius, so you may need to lower your Maximum inverter power threshold in ESS to force earlier throttling before the battery hits absorption.

What's your current DVCC status on the Cerbo?

@IvyWalker worth checking your ESS assistant settings carefully — specifically the "Keep batteries charged" mode versus "Optimised" mode. In optimised mode, the Multiplus should automatically absorb excess AC-coupled generation via frequency shifting (as @OldSailor touched on), but you need to ensure your grid-tied inverter actually responds correctly to that frequency rise.

In VictronConnect, verify your AC-input current limit isn't artificially capping things. Also check the ESS relay settings — sometimes the feed-in behaviour gets confused when the grid-tied unit is on AC-out1 rather than AC-in.

One thing I'd genuinely recommend: enable DVCC in the Cerbo if you haven't already. It coordinates charging between sources far more reliably. Had similar issues on my motorhome build before enabling it, and it transformed how the system managed excess generation.

What firmware version are you running on the Multiplus-II?

Really useful thread — I went through exactly this on my boat setup with AC coupling.

One thing worth adding: make sure your AC-in limit isn't throttling what the Multiplus can actually pull in to charge the batteries. I had mine set too low and couldn't figure out why excess power wasn't being stored properly.

Also, with the Cerbo, check the VRM portal — the energy flow diagram makes it immediately obvious whether your grid-tied inverter's output is actually being seen correctly by the system. Saved me hours of guesswork.

@SimonKelly makes a good point about ESS modes — "Optimised (with BatteryLife)" behaves quite differently to "Keep batteries charged" and catches people out regularly.

What battery chemistry are you running @IvyWalker? That can affect how aggressively ESS will accept charge.

Went through this exact rabbit hole when I set up my garden office last summer. The piece that clicked it for me was understanding the AC-coupled PV feed-in offset — in ESS you can set a small feed-in value (even 1-2W) which essentially tells the Multiplus to stop trying to push everything back to grid and instead allow the frequency shifting @OldSailor mentioned to do its job properly.

Also worth checking your AC-out1 inverter's frequency response curve — some cheaper units don't respond until 51Hz or higher, meaning there's a dead zone where your Multiplus is waiting for the inverter to throttle but nothing happens. My Fogstar battery was getting hammered before I sorted that.

Victron's ESS design guide PDF is genuinely worth reading cover to cover on this one.